[PATCH v14 0/5] overlayfs override_creds=off & nested get xattr fix

[PATCH v14 0/5] overlayfs override_creds=off & nested get xattr fix

[Mark Salyzyn]

Patch series:

Mark Salyzyn (5):
  Add flags option to get xattr method paired to __vfs_getxattr
  overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh
  overlayfs: handle XATTR_NOSECURITY flag for get xattr method
  overlayfs: internal getxattr operations without sepolicy checking
  overlayfs: override_creds=off option bypass creator_cred

The first four patches address fundamental security issues that should
be solved regardless of the override_creds=off feature.

The fifth adds the feature depends on these other fixes.

By default, all access to the upper, lower and work directories is the
recorded mounter's MAC and DAC credentials.  The incoming accesses are
checked against the caller's credentials.

If the principles of least privilege are applied for sepolicy, the
mounter's credentials might not overlap the credentials of the caller's
when accessing the overlayfs filesystem.  For example, a file that a
lower DAC privileged caller can execute, is MAC denied to the
generally higher DAC privileged mounter, to prevent an attack vector.

We add the option to turn off override_creds in the mount options; all
subsequent operations after mount on the filesystem will be only the
caller's credentials.  The module boolean parameter and mount option
override_creds is also added as a presence check for this "feature",
existence of /sys/module/overlay/parameters/overlay_creds

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

---
v14:
- Rejoin, rebase and a few adjustments.

v13:
- Pull out first patch and try to get it in alone feedback, some
  Acks, and then <crickets> because people forgot why we were doing i.

v12:
- Restore squished out patch 2 and 3 in the series,
  then change algorithm to add flags argument.
  Per-thread flag is a large security surface.

v11:
- Squish out v10 introduced patch 2 and 3 in the series,
  then and use per-thread flag instead for nesting.
- Switch name to ovl_do_vds_getxattr for __vds_getxattr wrapper.
- Add sb argument to ovl_revert_creds to match future work.

v10:
- Return NULL on CAP_DAC_READ_SEARCH
- Add __get xattr method to solve sepolicy logging issue
- Drop unnecessary sys_admin sepolicy checking for administrative
  driver internal xattr functions.

v6:
- Drop CONFIG_OVERLAY_FS_OVERRIDE_CREDS.
- Do better with the documentation, drop rationalizations.
- pr_warn message adjusted to report consequences.

v5:
- beefed up the caveats in the Documentation
- Is dependent on
  "overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh"
  "overlayfs: check CAP_MKNOD before issuing vfs_whiteout"
- Added prwarn when override_creds=off

v4:
- spelling and grammar errors in text

v3:
- Change name from caller_credentials / creator_credentials to the
  boolean override_creds.
- Changed from creator to mounter credentials.
- Updated and fortified the documentation.
- Added CONFIG_OVERLAY_FS_OVERRIDE_CREDS

v2:
- Forward port changed attr to stat, resulting in a build error.
- altered commit message.


 Documentation/filesystems/locking.rst   |  10 +--
 Documentation/filesystems/overlayfs.txt |  23 +++++
 fs/9p/acl.c                             |  51 ++++++-----
 fs/9p/xattr.c                           |  19 ++--
 fs/afs/xattr.c                          | 112 +++++++++++-------------
 fs/btrfs/xattr.c                        |  36 ++++----
 fs/ceph/xattr.c                         |  17 ++--
 fs/cifs/xattr.c                         |  72 +++++++--------
 fs/ecryptfs/crypto.c                    |  20 +++--
 fs/ecryptfs/inode.c                     |  36 +++++---
 fs/ecryptfs/mmap.c                      |  39 +++++----
 fs/erofs/xattr.c                        |   8 +-
 fs/ext2/xattr_security.c                |  16 ++--
 fs/ext2/xattr_trusted.c                 |  15 ++--
 fs/ext2/xattr_user.c                    |  19 ++--
 fs/ext4/xattr_security.c                |  15 ++--
 fs/ext4/xattr_trusted.c                 |  15 ++--
 fs/ext4/xattr_user.c                    |  19 ++--
 fs/f2fs/xattr.c                         |  42 ++++-----
 fs/fuse/xattr.c                         |  23 ++---
 fs/gfs2/xattr.c                         |  18 ++--
 fs/hfs/attr.c                           |  15 ++--
 fs/hfsplus/xattr.c                      |  17 ++--
 fs/hfsplus/xattr_security.c             |  13 ++-
 fs/hfsplus/xattr_trusted.c              |  13 ++-
 fs/hfsplus/xattr_user.c                 |  13 ++-
 fs/jffs2/security.c                     |  16 ++--
 fs/jffs2/xattr_trusted.c                |  16 ++--
 fs/jffs2/xattr_user.c                   |  16 ++--
 fs/jfs/xattr.c                          |  33 ++++---
 fs/kernfs/inode.c                       |  23 +++--
 fs/nfs/nfs4proc.c                       |  28 +++---
 fs/ocfs2/xattr.c                        |  52 +++++------
 fs/orangefs/xattr.c                     |  19 ++--
 fs/overlayfs/copy_up.c                  |   2 +-
 fs/overlayfs/dir.c                      |  17 ++--
 fs/overlayfs/file.c                     |  20 ++---
 fs/overlayfs/inode.c                    |  66 +++++++-------
 fs/overlayfs/namei.c                    |  21 +++--
 fs/overlayfs/overlayfs.h                |   9 +-
 fs/overlayfs/ovl_entry.h                |   1 +
 fs/overlayfs/readdir.c                  |   4 +-
 fs/overlayfs/super.c                    |  75 +++++++++-------
 fs/overlayfs/util.c                     |  44 +++++++---
 fs/posix_acl.c                          |  23 +++--
 fs/reiserfs/xattr.c                     |   2 +-
 fs/reiserfs/xattr_security.c            |  22 +++--
 fs/reiserfs/xattr_trusted.c             |  22 +++--
 fs/reiserfs/xattr_user.c                |  22 +++--
 fs/squashfs/xattr.c                     |  10 +--
 fs/ubifs/xattr.c                        |  33 ++++---
 fs/xattr.c                              | 112 +++++++++++++++---------
 fs/xfs/libxfs/xfs_attr.c                |   4 +-
 fs/xfs/libxfs/xfs_attr.h                |   2 +-
 fs/xfs/xfs_xattr.c                      |  35 ++++----
 include/linux/xattr.h                   |  26 ++++--
 include/uapi/linux/xattr.h              |   7 +-
 mm/shmem.c                              |  21 +++--
 net/socket.c                            |  16 ++--
 security/commoncap.c                    |  29 ++++--
 security/integrity/evm/evm_main.c       |  13 ++-
 security/selinux/hooks.c                |  28 ++++--
 security/smack/smack_lsm.c              |  38 +++++---
 63 files changed, 852 insertions(+), 771 deletions(-)

-- 
2.23.0.866.gb869b98d4c-goog

[PATCH v14 2/5] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh

[Mark Salyzyn]

Assumption never checked, should fail if the mounter creds are not
sufficient.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com

---
v11 + v12 +v13 + v14 - rebase

v10:
- return NULL rather than ERR_PTR(-EPERM)
- did _not_ add it ovl_can_decode_fh() because of changes since last
  review, suspect needs to be added to ovl_lower_uuid_ok()?

v8 + v9:
- rebase

v7:
- This time for realz

v6:
- rebase

v5:
- dependency of "overlayfs: override_creds=off option bypass creator_cred"

---
 fs/overlayfs/namei.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index e9717c2f7d45..9702f0d5309d 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -161,6 +161,9 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
 	if (!uuid_equal(&fh->uuid, &mnt->mnt_sb->s_uuid))
 		return NULL;
 
+	if (!capable(CAP_DAC_READ_SEARCH))
+		return NULL;
+
 	bytes = (fh->len - offsetof(struct ovl_fh, fid));
 	real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
 				  bytes >> 2, (int)fh->type,
-- 
2.23.0.866.gb869b98d4c-goog

[PATCH v14 3/5] overlayfs: handle XATTR_NOSECURITY flag for get xattr method

[Mark Salyzyn]

Because of the overlayfs getxattr recursion, the incoming inode fails
to update the selinux sid resulting in avc denials being reported
against a target context of u:object_r:unlabeled:s0.

Solution is to respond to the XATTR_NOSECURITY flag in get xattr
method that calls the __vfs_getxattr handler instead so that the
context can be read in, rather than being denied with an -EACCES
when vfs_getxattr handler is called.

For the use case where access is to be blocked by the security layer.

The path then would be security(dentry) ->
__vfs_getxattr({dentry...XATTR_NOSECURITY}) ->
handler->get({dentry...XATTR_NOSECURITY}) ->
__vfs_getxattr({realdentry...XATTR_NOSECURITY}) ->
lower_handler->get({realdentry...XATTR_NOSECURITY}) which
would report back through the chain data and success as expected,
the logging security layer at the top would have the data to
determine the access permissions and report back to the logs and
the caller that the target context was blocked.

For selinux this would solve the cosmetic issue of the selinux log
and allow audit2allow to correctly report the rule needed to address
the access problem.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
Cc: linux-security-module@vger.kernel.org

---
v14 - rebase to use xattr_gs_args.

v13 - rebase to use __vfs_getxattr flags option.

v12 - Added back to patch series as get xattr with flag option.

v11 - Squashed out of patch series and replaced with per-thread flag
      solution.

v10 - Added to patch series as __get xattr method.

---
 fs/overlayfs/inode.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 5fb7608647a4..2eb037c325bf 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -367,12 +367,15 @@ int ovl_xattr_get(struct xattr_gs_args *args)
 {
 	ssize_t res;
 	const struct cred *old_cred;
-	struct dentry *realdentry =
+	struct xattr_gs_args my_args = *args;
+
+	my_args.dentry =
 		ovl_i_dentry_upper(args->inode) ?:
 		ovl_dentry_lower(args->dentry);
+	my_args.inode = d_inode(my_args.dentry);
 
 	old_cred = ovl_override_creds(args->dentry->d_sb);
-	res = vfs_getxattr(realdentry, args->name, args->buffer, args->size);
+	res = __vfs_getxattr(&my_args);
 	revert_creds(old_cred);
 	return res;
 }
-- 
2.23.0.866.gb869b98d4c-goog

[PATCH v14 4/5] overlayfs: internal getxattr operations without sepolicy checking

[Mark Salyzyn]

Check impure, opaque, origin & meta xattr with no sepolicy audit
(using __vfs_getxattr) since these operations are internal to
overlayfs operations and do not disclose any data.  This became
an issue for credential override off since sys_admin would have
been required by the caller; whereas would have been inherently
present for the creator since it performed the mount.

This is a change in operations since we do not check in the new
ovl_do_vfs_getxattr function if the credential override is off or
not.  Reasoning is that the sepolicy check is unnecessary overhead,
especially since the check can be expensive.

Because for override credentials off, this affects _everyone_ that
underneath performs private xattr calls without the appropriate
sepolicy permissions and sys_admin capability.  Providing blanket
support for sys_admin would be bad for all possible callers.

For the override credentials on, this will affect only the mounter,
should it lack sepolicy permissions. Not considered a security
problem since mounting by definition has sys_admin capabilities,
but sepolicy contexts would still need to be crafted.

It should be noted that there is precedence, __vfs_getxattr is used
in other filesystems for their own internal trusted xattr management.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
Cc: linux-security-module@vger.kernel.org

---
v14 - rebase to use xattr_gs_args.

v13 - rebase to use __vfs_getxattr flags option

v12 - rebase

v11 - switch name to ovl_do_vfs_getxattr, fortify comment

v10 - added to patch series

---
 fs/overlayfs/namei.c     | 12 +++++++-----
 fs/overlayfs/overlayfs.h |  2 ++
 fs/overlayfs/util.c      | 32 +++++++++++++++++++++++---------
 3 files changed, 32 insertions(+), 14 deletions(-)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index 9702f0d5309d..a4a452c489fa 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -106,10 +106,11 @@ int ovl_check_fh_len(struct ovl_fh *fh, int fh_len)
 
 static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 {
-	int res, err;
+	ssize_t res;
+	int err;
 	struct ovl_fh *fh = NULL;
 
-	res = vfs_getxattr(dentry, name, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, name, NULL, 0);
 	if (res < 0) {
 		if (res == -ENODATA || res == -EOPNOTSUPP)
 			return NULL;
@@ -123,7 +124,7 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 	if (!fh)
 		return ERR_PTR(-ENOMEM);
 
-	res = vfs_getxattr(dentry, name, fh, res);
+	res = ovl_do_vfs_getxattr(dentry, name, fh, res);
 	if (res < 0)
 		goto fail;
 
@@ -141,10 +142,11 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 	return NULL;
 
 fail:
-	pr_warn_ratelimited("overlayfs: failed to get origin (%i)\n", res);
+	pr_warn_ratelimited("overlayfs: failed to get origin (%zi)\n", res);
 	goto out;
 invalid:
-	pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n", res, fh);
+	pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n",
+			    (int)res, fh);
 	goto out;
 }
 
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index c6a8ec049099..72762642b247 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -205,6 +205,8 @@ int ovl_want_write(struct dentry *dentry);
 void ovl_drop_write(struct dentry *dentry);
 struct dentry *ovl_workdir(struct dentry *dentry);
 const struct cred *ovl_override_creds(struct super_block *sb);
+ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
+			    size_t size);
 struct super_block *ovl_same_sb(struct super_block *sb);
 int ovl_can_decode_fh(struct super_block *sb);
 struct dentry *ovl_indexdir(struct super_block *sb);
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index f5678a3f8350..bed12aed902c 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -40,6 +40,20 @@ const struct cred *ovl_override_creds(struct super_block *sb)
 	return override_creds(ofs->creator_cred);
 }
 
+ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
+			    size_t size)
+{
+	struct xattr_gs_args args = {};
+
+	args.dentry = dentry;
+	args.inode = d_inode(dentry);
+	args.name = name;
+	args.buffer = buf;
+	args.size = size;
+	args.flags = XATTR_NOSECURITY;
+	return __vfs_getxattr(&args);
+}
+
 struct super_block *ovl_same_sb(struct super_block *sb)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
@@ -537,9 +551,9 @@ void ovl_copy_up_end(struct dentry *dentry)
 
 bool ovl_check_origin_xattr(struct dentry *dentry)
 {
-	int res;
+	ssize_t res;
 
-	res = vfs_getxattr(dentry, OVL_XATTR_ORIGIN, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, OVL_XATTR_ORIGIN, NULL, 0);
 
 	/* Zero size value means "copied up but origin unknown" */
 	if (res >= 0)
@@ -550,13 +564,13 @@ bool ovl_check_origin_xattr(struct dentry *dentry)
 
 bool ovl_check_dir_xattr(struct dentry *dentry, const char *name)
 {
-	int res;
+	ssize_t res;
 	char val;
 
 	if (!d_is_dir(dentry))
 		return false;
 
-	res = vfs_getxattr(dentry, name, &val, 1);
+	res = ovl_do_vfs_getxattr(dentry, name, &val, 1);
 	if (res == 1 && val == 'y')
 		return true;
 
@@ -837,13 +851,13 @@ int ovl_lock_rename_workdir(struct dentry *workdir, struct dentry *upperdir)
 /* err < 0, 0 if no metacopy xattr, 1 if metacopy xattr found */
 int ovl_check_metacopy_xattr(struct dentry *dentry)
 {
-	int res;
+	ssize_t res;
 
 	/* Only regular files can have metacopy xattr */
 	if (!S_ISREG(d_inode(dentry)->i_mode))
 		return 0;
 
-	res = vfs_getxattr(dentry, OVL_XATTR_METACOPY, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, OVL_XATTR_METACOPY, NULL, 0);
 	if (res < 0) {
 		if (res == -ENODATA || res == -EOPNOTSUPP)
 			return 0;
@@ -852,7 +866,7 @@ int ovl_check_metacopy_xattr(struct dentry *dentry)
 
 	return 1;
 out:
-	pr_warn_ratelimited("overlayfs: failed to get metacopy (%i)\n", res);
+	pr_warn_ratelimited("overlayfs: failed to get metacopy (%zi)\n", res);
 	return res;
 }
 
@@ -878,7 +892,7 @@ ssize_t ovl_getxattr(struct dentry *dentry, char *name, char **value,
 	ssize_t res;
 	char *buf = NULL;
 
-	res = vfs_getxattr(dentry, name, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, name, NULL, 0);
 	if (res < 0) {
 		if (res == -ENODATA || res == -EOPNOTSUPP)
 			return -ENODATA;
@@ -890,7 +904,7 @@ ssize_t ovl_getxattr(struct dentry *dentry, char *name, char **value,
 		if (!buf)
 			return -ENOMEM;
 
-		res = vfs_getxattr(dentry, name, buf, res);
+		res = ovl_do_vfs_getxattr(dentry, name, buf, res);
 		if (res < 0)
 			goto fail;
 	}
-- 
2.23.0.866.gb869b98d4c-goog

[PATCH v14 5/5] overlayfs: override_creds=off option bypass creator_cred

[Mark Salyzyn]

By default, all access to the upper, lower and work directories is the
recorded mounter's MAC and DAC credentials.  The incoming accesses are
checked against the caller's credentials.

If the principles of least privilege are applied, the mounter's
credentials might not overlap the credentials of the caller's when
accessing the overlayfs filesystem.  For example, a file that a lower
DAC privileged caller can execute, is MAC denied to the generally
higher DAC privileged mounter, to prevent an attack vector.

We add the option to turn off override_creds in the mount options; all
subsequent operations after mount on the filesystem will be only the
caller's credentials.  The module boolean parameter and mount option
override_creds is also added as a presence check for this "feature",
existence of /sys/module/overlay/parameters/override_creds.

It was not always this way.  Circa 4.6 there was no recorded mounter's
credentials, instead privileged access to upper or work directories
were temporarily increased to perform the operations.  The MAC
(selinux) policies were caller's in all cases.  override_creds=off
partially returns us to this older access model minus the insecure
temporary credential increases.  This is to permit use in a system
with non-overlapping security models for each executable including
the agent that mounts the overlayfs filesystem.  In Android
this is the case since init, which performs the mount operations,
has a minimal MAC set of privileges to reduce any attack surface,
and services that use the content have a different set of MAC
privileges (eg: read, for vendor labelled configuration, execute for
vendor libraries and modules).  The caveats are not a problem in
the Android usage model, however they should be fixed for
completeness and for general use in time.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com

---
v14:
- fix an issue in ovl_create_or_link which leaks credentials.

v12 + v13
- Rebase

v11:
- add sb argument to ovl_revert_creds to match future work

v10:
- Rebase (and expand because of increased revert_cred usage)

v9:
- Add to the caveats

v8:
- drop pr_warn message after straw poll to remove it.
- added a use case in the commit message

v7:
- change name of internal parameter to ovl_override_creds_def
- report override_creds only if different than default

v6:
- Drop CONFIG_OVERLAY_FS_OVERRIDE_CREDS.
- Do better with the documentation.
- pr_warn message adjusted to report consequences.

v5:
- beefed up the caveats in the Documentation
- Is dependent on
  "overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh"
  "overlayfs: check CAP_MKNOD before issuing vfs_whiteout"
- Added prwarn when override_creds=off

v4:
- spelling and grammar errors in text

v3:
- Change name from caller_credentials / creator_credentials to the
  boolean override_creds.
- Changed from creator to mounter credentials.
- Updated and fortified the documentation.
- Added CONFIG_OVERLAY_FS_OVERRIDE_CREDS

v2:
- Forward port changed attr to stat, resulting in a build error.
- altered commit message.

---
 Documentation/filesystems/overlayfs.txt | 23 +++++++++++++++++++++++
 fs/overlayfs/copy_up.c                  |  2 +-
 fs/overlayfs/dir.c                      | 17 ++++++++++-------
 fs/overlayfs/file.c                     | 20 ++++++++++----------
 fs/overlayfs/inode.c                    | 18 +++++++++---------
 fs/overlayfs/namei.c                    |  6 +++---
 fs/overlayfs/overlayfs.h                |  1 +
 fs/overlayfs/ovl_entry.h                |  1 +
 fs/overlayfs/readdir.c                  |  4 ++--
 fs/overlayfs/super.c                    | 22 +++++++++++++++++++++-
 fs/overlayfs/util.c                     | 12 ++++++++++--
 11 files changed, 91 insertions(+), 35 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 845d689e0fd7..c7ab389571e5 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -102,6 +102,29 @@ Only the lists of names from directories are merged.  Other content
 such as metadata and extended attributes are reported for the upper
 directory only.  These attributes of the lower directory are hidden.
 
+credentials
+-----------
+
+By default, all access to the upper, lower and work directories is the
+recorded mounter's MAC and DAC credentials.  The incoming accesses are
+checked against the caller's credentials.
+
+In the case where caller MAC or DAC credentials do not overlap, a
+use case available in older versions of the driver, the
+override_creds mount flag can be turned off and help when the use
+pattern has caller with legitimate credentials where the mounter
+does not.  Several unintended side effects will occur though.  The
+caller without certain key capabilities or lower privilege will not
+always be able to delete files or directories, create nodes, or
+search some restricted directories.  The ability to search and read
+a directory entry is spotty as a result of the cache mechanism not
+retesting the credentials because of the assumption, a privileged
+caller can fill cache, then a lower privilege can read the directory
+cache.  The uneven security model where cache, upperdir and workdir
+are opened at privilege, but accessed without creating a form of
+privilege escalation, should only be used with strict understanding
+of the side effects and of the security policies.
+
 whiteouts and opaque directories
 --------------------------------
 
diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index b801c6353100..1c1b9415e533 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -886,7 +886,7 @@ int ovl_copy_up_flags(struct dentry *dentry, int flags)
 		dput(parent);
 		dput(next);
 	}
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 
 	return err;
 }
diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 702aa63f6774..f4e6e6b6629a 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -536,7 +536,7 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 			      struct ovl_cattr *attr, bool origin)
 {
 	int err;
-	const struct cred *old_cred;
+	const struct cred *old_cred, *hold_cred = NULL;
 	struct cred *override_cred;
 	struct dentry *parent = dentry->d_parent;
 
@@ -563,14 +563,15 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 		override_cred->fsgid = inode->i_gid;
 		if (!attr->hardlink) {
 			err = security_dentry_create_files_as(dentry,
-					attr->mode, &dentry->d_name, old_cred,
+					attr->mode, &dentry->d_name,
+					old_cred ? old_cred : current_cred(),
 					override_cred);
 			if (err) {
 				put_cred(override_cred);
 				goto out_revert_creds;
 			}
 		}
-		put_cred(override_creds(override_cred));
+		hold_cred = override_creds(override_cred);
 		put_cred(override_cred);
 
 		if (!ovl_dentry_is_whiteout(dentry))
@@ -579,7 +580,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 			err = ovl_create_over_whiteout(dentry, inode, attr);
 	}
 out_revert_creds:
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred ?: hold_cred);
+	if (old_cred && hold_cred)
+		put_cred(hold_cred);
 	return err;
 }
 
@@ -655,7 +658,7 @@ static int ovl_set_link_redirect(struct dentry *dentry)
 
 	old_cred = ovl_override_creds(dentry->d_sb);
 	err = ovl_set_redirect(dentry, false);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 
 	return err;
 }
@@ -851,7 +854,7 @@ static int ovl_do_remove(struct dentry *dentry, bool is_dir)
 		err = ovl_remove_upper(dentry, is_dir, &list);
 	else
 		err = ovl_remove_and_whiteout(dentry, &list);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	if (!err) {
 		if (is_dir)
 			clear_nlink(dentry->d_inode);
@@ -1221,7 +1224,7 @@ static int ovl_rename(struct inode *olddir, struct dentry *old,
 out_unlock:
 	unlock_rename(new_upperdir, old_upperdir);
 out_revert_creds:
-	revert_creds(old_cred);
+	ovl_revert_creds(old->d_sb, old_cred);
 	if (update_nlink)
 		ovl_nlink_end(new);
 out_drop_write:
diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index e235a635d9ec..d94d6ec785a7 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -32,7 +32,7 @@ static struct file *ovl_open_realfile(const struct file *file,
 	old_cred = ovl_override_creds(inode->i_sb);
 	realfile = open_with_fake_path(&file->f_path, flags, realinode,
 				       current_cred());
-	revert_creds(old_cred);
+	ovl_revert_creds(inode->i_sb, old_cred);
 
 	pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
 		 file, file, ovl_whatisit(inode, realinode), file->f_flags,
@@ -176,7 +176,7 @@ static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
 
 	old_cred = ovl_override_creds(inode->i_sb);
 	ret = vfs_llseek(real.file, offset, whence);
-	revert_creds(old_cred);
+	ovl_revert_creds(inode->i_sb, old_cred);
 
 	file->f_pos = real.file->f_pos;
 	inode_unlock(inode);
@@ -242,7 +242,7 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
 			    ovl_iocb_to_rwf(iocb));
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	ovl_file_accessed(file);
 
@@ -278,7 +278,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
 			     ovl_iocb_to_rwf(iocb));
 	file_end_write(real.file);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	/* Update size */
 	ovl_copyattr(ovl_inode_real(inode), inode);
@@ -305,7 +305,7 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 	if (file_inode(real.file) == ovl_inode_upper(file_inode(file))) {
 		old_cred = ovl_override_creds(file_inode(file)->i_sb);
 		ret = vfs_fsync_range(real.file, start, end, datasync);
-		revert_creds(old_cred);
+		ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 	}
 
 	fdput(real);
@@ -329,7 +329,7 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = call_mmap(vma->vm_file, vma);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	if (ret) {
 		/* Drop reference count from new vm_file value */
@@ -357,7 +357,7 @@ static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = vfs_fallocate(real.file, mode, offset, len);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	/* Update size */
 	ovl_copyattr(ovl_inode_real(inode), inode);
@@ -379,7 +379,7 @@ static int ovl_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = vfs_fadvise(real.file, offset, len, advice);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	fdput(real);
 
@@ -399,7 +399,7 @@ static long ovl_real_ioctl(struct file *file, unsigned int cmd,
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = vfs_ioctl(real.file, cmd, arg);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	fdput(real);
 
@@ -589,7 +589,7 @@ static loff_t ovl_copyfile(struct file *file_in, loff_t pos_in,
 						flags);
 		break;
 	}
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file_out)->i_sb, old_cred);
 
 	/* Update size */
 	ovl_copyattr(ovl_inode_real(inode_out), inode_out);
diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 2eb037c325bf..a208d0523e62 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -61,7 +61,7 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
 		inode_lock(upperdentry->d_inode);
 		old_cred = ovl_override_creds(dentry->d_sb);
 		err = notify_change(upperdentry, attr, NULL);
-		revert_creds(old_cred);
+		ovl_revert_creds(dentry->d_sb, old_cred);
 		if (!err)
 			ovl_copyattr(upperdentry->d_inode, dentry->d_inode);
 		inode_unlock(upperdentry->d_inode);
@@ -257,7 +257,7 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 		stat->nlink = dentry->d_inode->i_nlink;
 
 out:
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 
 	return err;
 }
@@ -291,7 +291,7 @@ int ovl_permission(struct inode *inode, int mask)
 		mask |= MAY_READ;
 	}
 	err = inode_permission(realinode, mask);
-	revert_creds(old_cred);
+	ovl_revert_creds(inode->i_sb, old_cred);
 
 	return err;
 }
@@ -308,7 +308,7 @@ static const char *ovl_get_link(struct dentry *dentry,
 
 	old_cred = ovl_override_creds(dentry->d_sb);
 	p = vfs_get_link(ovl_dentry_real(dentry), done);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	return p;
 }
 
@@ -352,7 +352,7 @@ int ovl_xattr_set(struct xattr_gs_args *args)
 		WARN_ON(args->flags != XATTR_REPLACE);
 		err = vfs_removexattr(realdentry, args->name);
 	}
-	revert_creds(old_cred);
+	ovl_revert_creds(args->dentry->d_sb, old_cred);
 
 	/* copy c/mtime */
 	ovl_copyattr(d_inode(realdentry), args->inode);
@@ -376,7 +376,7 @@ int ovl_xattr_get(struct xattr_gs_args *args)
 
 	old_cred = ovl_override_creds(args->dentry->d_sb);
 	res = __vfs_getxattr(&my_args);
-	revert_creds(old_cred);
+	ovl_revert_creds(args->dentry->d_sb, old_cred);
 	return res;
 }
 
@@ -401,7 +401,7 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
 
 	old_cred = ovl_override_creds(dentry->d_sb);
 	res = vfs_listxattr(realdentry, list, size);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	if (res <= 0 || size == 0)
 		return res;
 
@@ -436,7 +436,7 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type)
 
 	old_cred = ovl_override_creds(inode->i_sb);
 	acl = get_acl(realinode, type);
-	revert_creds(old_cred);
+	ovl_revert_creds(inode->i_sb, old_cred);
 
 	return acl;
 }
@@ -474,7 +474,7 @@ static int ovl_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		filemap_write_and_wait(realinode->i_mapping);
 
 	err = realinode->i_op->fiemap(realinode, fieinfo, start, len);
-	revert_creds(old_cred);
+	ovl_revert_creds(inode->i_sb, old_cred);
 
 	return err;
 }
diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index a4a452c489fa..bab1f97dc201 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -1079,7 +1079,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			goto out_free_oe;
 	}
 
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	if (origin_path) {
 		dput(origin_path->dentry);
 		kfree(origin_path);
@@ -1106,7 +1106,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	kfree(upperredirect);
 out:
 	kfree(d.redirect);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	return ERR_PTR(err);
 }
 
@@ -1160,7 +1160,7 @@ bool ovl_lower_positive(struct dentry *dentry)
 			dput(this);
 		}
 	}
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 
 	return positive;
 }
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 72762642b247..af83b7898878 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -205,6 +205,7 @@ int ovl_want_write(struct dentry *dentry);
 void ovl_drop_write(struct dentry *dentry);
 struct dentry *ovl_workdir(struct dentry *dentry);
 const struct cred *ovl_override_creds(struct super_block *sb);
+void ovl_revert_creds(struct super_block *sb, const struct cred *oldcred);
 ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
 			    size_t size);
 struct super_block *ovl_same_sb(struct super_block *sb);
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index a8279280e88d..e847d7564999 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -17,6 +17,7 @@ struct ovl_config {
 	bool nfs_export;
 	int xino;
 	bool metacopy;
+	bool override_creds;
 };
 
 struct ovl_sb {
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 47a91c9733a5..874a1b3ff99a 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -286,7 +286,7 @@ static int ovl_check_whiteouts(struct dentry *dir, struct ovl_readdir_data *rdd)
 		}
 		inode_unlock(dir->d_inode);
 	}
-	revert_creds(old_cred);
+	ovl_revert_creds(rdd->dentry->d_sb, old_cred);
 
 	return err;
 }
@@ -918,7 +918,7 @@ int ovl_check_empty_dir(struct dentry *dentry, struct list_head *list)
 
 	old_cred = ovl_override_creds(dentry->d_sb);
 	err = ovl_dir_read_merged(dentry, list, &root);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	if (err)
 		return err;
 
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 973f9e0d7f7f..470925d96b92 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -53,6 +53,11 @@ module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644);
 MODULE_PARM_DESC(xino_auto,
 		 "Auto enable xino feature");
 
+static bool __read_mostly ovl_override_creds_def = true;
+module_param_named(override_creds, ovl_override_creds_def, bool, 0644);
+MODULE_PARM_DESC(ovl_override_creds_def,
+		 "Use mounter's credentials for accesses");
+
 static void ovl_entry_stack_free(struct ovl_entry *oe)
 {
 	unsigned int i;
@@ -363,6 +368,9 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 	if (ofs->config.metacopy != ovl_metacopy_def)
 		seq_printf(m, ",metacopy=%s",
 			   ofs->config.metacopy ? "on" : "off");
+	if (ofs->config.override_creds != ovl_override_creds_def)
+		seq_show_option(m, "override_creds",
+				ofs->config.override_creds ? "on" : "off");
 	return 0;
 }
 
@@ -403,6 +411,8 @@ enum {
 	OPT_XINO_AUTO,
 	OPT_METACOPY_ON,
 	OPT_METACOPY_OFF,
+	OPT_OVERRIDE_CREDS_ON,
+	OPT_OVERRIDE_CREDS_OFF,
 	OPT_ERR,
 };
 
@@ -421,6 +431,8 @@ static const match_table_t ovl_tokens = {
 	{OPT_XINO_AUTO,			"xino=auto"},
 	{OPT_METACOPY_ON,		"metacopy=on"},
 	{OPT_METACOPY_OFF,		"metacopy=off"},
+	{OPT_OVERRIDE_CREDS_ON,		"override_creds=on"},
+	{OPT_OVERRIDE_CREDS_OFF,	"override_creds=off"},
 	{OPT_ERR,			NULL}
 };
 
@@ -479,6 +491,7 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 	config->redirect_mode = kstrdup(ovl_redirect_mode_def(), GFP_KERNEL);
 	if (!config->redirect_mode)
 		return -ENOMEM;
+	config->override_creds = ovl_override_creds_def;
 
 	while ((p = ovl_next_opt(&opt)) != NULL) {
 		int token;
@@ -559,6 +572,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 			config->metacopy = false;
 			break;
 
+		case OPT_OVERRIDE_CREDS_ON:
+			config->override_creds = true;
+			break;
+
+		case OPT_OVERRIDE_CREDS_OFF:
+			config->override_creds = false;
+			break;
+
 		default:
 			pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p);
 			return -EINVAL;
@@ -1686,7 +1707,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		       ovl_dentry_lower(root_dentry), NULL);
 
 	sb->s_root = root_dentry;
-
 	return 0;
 
 out_free_oe:
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index bed12aed902c..e22b86980022 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -37,9 +37,17 @@ const struct cred *ovl_override_creds(struct super_block *sb)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 
+	if (!ofs->config.override_creds)
+		return NULL;
 	return override_creds(ofs->creator_cred);
 }
 
+void ovl_revert_creds(struct super_block *sb, const struct cred *old_cred)
+{
+	if (old_cred)
+		revert_creds(old_cred);
+}
+
 ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
 			    size_t size)
 {
@@ -805,7 +813,7 @@ int ovl_nlink_start(struct dentry *dentry)
 	 * value relative to the upper inode nlink in an upper inode xattr.
 	 */
 	err = ovl_set_nlink_upper(dentry);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 
 out:
 	if (err)
@@ -823,7 +831,7 @@ void ovl_nlink_end(struct dentry *dentry)
 
 		old_cred = ovl_override_creds(dentry->d_sb);
 		ovl_cleanup_index(dentry);
-		revert_creds(old_cred);
+		ovl_revert_creds(dentry->d_sb, old_cred);
 	}
 
 	ovl_inode_unlock(inode);
-- 
2.23.0.866.gb869b98d4c-goog

Re: [PATCH v14 2/5] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh

[Amir Goldstein]

On Tue, Oct 22, 2019 at 11:46 PM Mark Salyzyn <salyzyn@android.com> wrote:
>
> Assumption never checked, should fail if the mounter creds are not
> sufficient.
>
> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Stephen Smalley <sds@tycho.nsa.gov>
> Cc: linux-unionfs@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: kernel-team@android.com
>
> ---
> v11 + v12 +v13 + v14 - rebase
>
> v10:
> - return NULL rather than ERR_PTR(-EPERM)
> - did _not_ add it ovl_can_decode_fh() because of changes since last
>   review, suspect needs to be added to ovl_lower_uuid_ok()?
>
> v8 + v9:
> - rebase
>
> v7:
> - This time for realz
>
> v6:
> - rebase
>
> v5:
> - dependency of "overlayfs: override_creds=off option bypass creator_cred"
>
> ---
>  fs/overlayfs/namei.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index e9717c2f7d45..9702f0d5309d 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -161,6 +161,9 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
>         if (!uuid_equal(&fh->uuid, &mnt->mnt_sb->s_uuid))
>                 return NULL;
>
> +       if (!capable(CAP_DAC_READ_SEARCH))
> +               return NULL;
> +

Shouldn't this return EPERM?

>         bytes = (fh->len - offsetof(struct ovl_fh, fid));
>         real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
>                                   bytes >> 2, (int)fh->type,
> --
> 2.23.0.866.gb869b98d4c-goog
>

Re: [PATCH v14 4/5] overlayfs: internal getxattr operations without sepolicy checking

[Amir Goldstein]

On Tue, Oct 22, 2019 at 11:46 PM Mark Salyzyn <salyzyn@android.com> wrote:
>
> Check impure, opaque, origin & meta xattr with no sepolicy audit
> (using __vfs_getxattr) since these operations are internal to
> overlayfs operations and do not disclose any data.  This became
> an issue for credential override off since sys_admin would have
> been required by the caller; whereas would have been inherently
> present for the creator since it performed the mount.
>
> This is a change in operations since we do not check in the new
> ovl_do_vfs_getxattr function if the credential override is off or
> not.  Reasoning is that the sepolicy check is unnecessary overhead,
> especially since the check can be expensive.
>
> Because for override credentials off, this affects _everyone_ that
> underneath performs private xattr calls without the appropriate
> sepolicy permissions and sys_admin capability.  Providing blanket
> support for sys_admin would be bad for all possible callers.
>
> For the override credentials on, this will affect only the mounter,
> should it lack sepolicy permissions. Not considered a security
> problem since mounting by definition has sys_admin capabilities,
> but sepolicy contexts would still need to be crafted.
>

It sounds reasonable to me, but I am not a "security person".

> It should be noted that there is precedence, __vfs_getxattr is used
> in other filesystems for their own internal trusted xattr management.
>

Urgh! "other" filesystems meaning ecryptfs_getxattr()?
That looks like a loop hole to read any trusted xattr without any
security checks. Not sure its a good example...

> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Stephen Smalley <sds@tycho.nsa.gov>
> Cc: linux-unionfs@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: kernel-team@android.com
> Cc: linux-security-module@vger.kernel.org
>
> ---
> v14 - rebase to use xattr_gs_args.
>
> v13 - rebase to use __vfs_getxattr flags option
>
> v12 - rebase
>
> v11 - switch name to ovl_do_vfs_getxattr, fortify comment
>
> v10 - added to patch series
>
> ---
>  fs/overlayfs/namei.c     | 12 +++++++-----
>  fs/overlayfs/overlayfs.h |  2 ++
>  fs/overlayfs/util.c      | 32 +++++++++++++++++++++++---------
>  3 files changed, 32 insertions(+), 14 deletions(-)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index 9702f0d5309d..a4a452c489fa 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -106,10 +106,11 @@ int ovl_check_fh_len(struct ovl_fh *fh, int fh_len)
>
>  static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
>  {
> -       int res, err;
> +       ssize_t res;
> +       int err;
>         struct ovl_fh *fh = NULL;
>
> -       res = vfs_getxattr(dentry, name, NULL, 0);
> +       res = ovl_do_vfs_getxattr(dentry, name, NULL, 0);
>         if (res < 0) {
>                 if (res == -ENODATA || res == -EOPNOTSUPP)
>                         return NULL;
> @@ -123,7 +124,7 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
>         if (!fh)
>                 return ERR_PTR(-ENOMEM);
>
> -       res = vfs_getxattr(dentry, name, fh, res);
> +       res = ovl_do_vfs_getxattr(dentry, name, fh, res);
>         if (res < 0)
>                 goto fail;
>
> @@ -141,10 +142,11 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
>         return NULL;
>
>  fail:
> -       pr_warn_ratelimited("overlayfs: failed to get origin (%i)\n", res);
> +       pr_warn_ratelimited("overlayfs: failed to get origin (%zi)\n", res);
>         goto out;
>  invalid:
> -       pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n", res, fh);
> +       pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n",
> +                           (int)res, fh);
>         goto out;
>  }
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index c6a8ec049099..72762642b247 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -205,6 +205,8 @@ int ovl_want_write(struct dentry *dentry);
>  void ovl_drop_write(struct dentry *dentry);
>  struct dentry *ovl_workdir(struct dentry *dentry);
>  const struct cred *ovl_override_creds(struct super_block *sb);
> +ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
> +                           size_t size);
>  struct super_block *ovl_same_sb(struct super_block *sb);
>  int ovl_can_decode_fh(struct super_block *sb);
>  struct dentry *ovl_indexdir(struct super_block *sb);
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index f5678a3f8350..bed12aed902c 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -40,6 +40,20 @@ const struct cred *ovl_override_creds(struct super_block *sb)
>         return override_creds(ofs->creator_cred);
>  }
>
> +ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
> +                           size_t size)
> +{
> +       struct xattr_gs_args args = {};
> +
> +       args.dentry = dentry;
> +       args.inode = d_inode(dentry);
> +       args.name = name;
> +       args.buffer = buf;
> +       args.size = size;
> +       args.flags = XATTR_NOSECURITY;
> +       return __vfs_getxattr(&args);
> +}
> +

We do not understand each other.
I commented on this several times.
please put the wrapper helper ovl_do_getxattr() in overlayfs.h
next to the other ovl_do_ wrapper helpers and add pr_debug()
as all other wrappers have.

Thanks,
Amir.

Re: [PATCH v14 0/5] overlayfs override_creds=off & nested get xattr fix

[Amir Goldstein]

On Tue, Oct 22, 2019 at 11:45 PM Mark Salyzyn <salyzyn@android.com> wrote:
>
> Patch series:
>
> Mark Salyzyn (5):
>   Add flags option to get xattr method paired to __vfs_getxattr
>   overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh
>   overlayfs: handle XATTR_NOSECURITY flag for get xattr method
>   overlayfs: internal getxattr operations without sepolicy checking
>   overlayfs: override_creds=off option bypass creator_cred
>
> The first four patches address fundamental security issues that should
> be solved regardless of the override_creds=off feature.
>
> The fifth adds the feature depends on these other fixes.
>
> By default, all access to the upper, lower and work directories is the
> recorded mounter's MAC and DAC credentials.  The incoming accesses are
> checked against the caller's credentials.
>
> If the principles of least privilege are applied for sepolicy, the
> mounter's credentials might not overlap the credentials of the caller's
> when accessing the overlayfs filesystem.  For example, a file that a
> lower DAC privileged caller can execute, is MAC denied to the
> generally higher DAC privileged mounter, to prevent an attack vector.
>
> We add the option to turn off override_creds in the mount options; all
> subsequent operations after mount on the filesystem will be only the
> caller's credentials.  The module boolean parameter and mount option
> override_creds is also added as a presence check for this "feature",
> existence of /sys/module/overlay/parameters/overlay_creds
>
> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Stephen Smalley <sds@tycho.nsa.gov>
> Cc: linux-unionfs@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
>
> ---
> v14:
> - Rejoin, rebase and a few adjustments.
>
> v13:
> - Pull out first patch and try to get it in alone feedback, some
>   Acks, and then <crickets> because people forgot why we were doing i.

Mark,

I do not see the first patch on fsdevel
and I am confused from all the suggested APIs
I recall Christoph's comment on v8 for not using xattr_gs_args
and just adding flags to existing get() method.
I agree to that comment.

I remember asking - don't remember the answer -
do you have any testing for this feature?
I have a WIP branch to run unionmount-testsuite not as root,
which is a start, but I didn't get to finish the work.
Let me know if you want to take up this work.

Thanks,
Amir.

Re: [PATCH v14 0/5] overlayfs override_creds=off & nested get xattr fix

[Mark Salyzyn]

On 10/22/19 11:54 PM, Amir Goldstein wrote:
> On Tue, Oct 22, 2019 at 11:45 PM Mark Salyzyn <salyzyn@android.com> wrote:
>> Patch series:
>>
>> Mark Salyzyn (5):
>>    Add flags option to get xattr method paired to __vfs_getxattr
>>    overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh
>>    overlayfs: handle XATTR_NOSECURITY flag for get xattr method
>>    overlayfs: internal getxattr operations without sepolicy checking
>>    overlayfs: override_creds=off option bypass creator_cred
>>
>> The first four patches address fundamental security issues that should
>> be solved regardless of the override_creds=off feature.
>>
>> The fifth adds the feature depends on these other fixes.
>>
>> By default, all access to the upper, lower and work directories is the
>> recorded mounter's MAC and DAC credentials.  The incoming accesses are
>> checked against the caller's credentials.
>>
>> If the principles of least privilege are applied for sepolicy, the
>> mounter's credentials might not overlap the credentials of the caller's
>> when accessing the overlayfs filesystem.  For example, a file that a
>> lower DAC privileged caller can execute, is MAC denied to the
>> generally higher DAC privileged mounter, to prevent an attack vector.
>>
>> We add the option to turn off override_creds in the mount options; all
>> subsequent operations after mount on the filesystem will be only the
>> caller's credentials.  The module boolean parameter and mount option
>> override_creds is also added as a presence check for this "feature",
>> existence of /sys/module/overlay/parameters/overlay_creds
>>
>> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
>> Cc: Miklos Szeredi <miklos@szeredi.hu>
>> Cc: Jonathan Corbet <corbet@lwn.net>
>> Cc: Vivek Goyal <vgoyal@redhat.com>
>> Cc: Eric W. Biederman <ebiederm@xmission.com>
>> Cc: Amir Goldstein <amir73il@gmail.com>
>> Cc: Randy Dunlap <rdunlap@infradead.org>
>> Cc: Stephen Smalley <sds@tycho.nsa.gov>
>> Cc: linux-unionfs@vger.kernel.org
>> Cc: linux-doc@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>>
>> ---
>> v14:
>> - Rejoin, rebase and a few adjustments.
>>
>> v13:
>> - Pull out first patch and try to get it in alone feedback, some
>>    Acks, and then <crickets> because people forgot why we were doing i.
> Mark,
>
> I do not see the first patch on fsdevel
> and I am confused from all the suggested APIs
> I recall Christoph's comment on v8 for not using xattr_gs_args
> and just adding flags to existing get() method.
> I agree to that comment.

As already responded, third (?) patch version was like that, gregkh@ 
said it passed the limit for number of arguments, is looking a bit silly 
(my paraphrase), and that it should be passed as a structure. Two others 
agreed. We gained because both set and get use the same structure after 
this change (this allows a simplified read-modify-write cycle).

We will need a quorum on this, 3 (structure) to 2 (flag) now (but really 
basically between Greg and Christoph?). Coding style issue: Add a flag, 
or switch to a common xattr argument  structure?

> I remember asking - don't remember the answer -
> do you have any testing for this feature?
Yes, on an unnamed 4.19-based and mainline-based Android and virtual 
cuttlefish product ... which was critically unworkable without this 
patch series.
> I have a WIP branch to run unionmount-testsuite not as root,
> which is a start, but I didn't get to finish the work.
> Let me know if you want to take up this work.
Please refer it in private email to me, no guarantees, my cycles are so 
sparse right now that it took a month to respin this patch series to 
upstream. If I can make it test on Android with overlayfs activated, big 
gain.
>
> Thanks,
> Amir.

Re: [PATCH v14 1/5] Add flags option to get xattr method paired to __vfs_getxattr

[Amir Goldstein]

[excessive CC list reduced]

On Wed, Oct 23, 2019 at 11:07 AM Andreas Dilger via samba-technical
<samba-technical@lists.samba.org> wrote:
>
>
> On Oct 22, 2019, at 2:44 PM, Mark Salyzyn <salyzyn@android.com> wrote:
> >
> > Replace arguments for get and set xattr methods, and __vfs_getxattr
> > and __vfs_setaxtr functions with a reference to the following now
> > common argument structure:
> >
> > struct xattr_gs_args {
> >       struct dentry *dentry;
> >       struct inode *inode;
> >       const char *name;
> >       union {
> >               void *buffer;
> >               const void *value;
> >       };
> >       size_t size;
> >       int flags;
> > };
>

> > Mark,
> >
> > I do not see the first patch on fsdevel
> > and I am confused from all the suggested APIs
> > I recall Christoph's comment on v8 for not using xattr_gs_args
> > and just adding flags to existing get() method.
> > I agree to that comment.
>
> As already responded, third (?) patch version was like that,

The problem is that because of the waaay too long CC list, most revisions
of the patch and discussion were bounced from fsdevel, most emails
I did not get and cannot find in archives, so the discussion around
them is not productive.

Please resend patch to fsdevel discarding the auto added CC list
of all fs maintainers.

> gregkh@
> said it passed the limit for number of arguments, is looking a bit silly

Well, you just matched get() to set() args list, so this is not a strong
argument IMO.

> (my paraphrase), and that it should be passed as a structure. Two others
> agreed. We gained because both set and get use the same structure after
> this change (this allows a simplified read-modify-write cycle).

That sounds like a nice benefit if this was user API, but are there any
kernel users that intend to make use of that read-modify-write cycle?
I don't think so.

>
> We will need a quorum on this, 3 (structure) to 2 (flag) now (but really
> basically between Greg and Christoph?). Coding style issue: Add a flag,
> or switch to a common xattr argument  structure?
>

IIRC, Christoph was asking why the silly struct and not simply add flags
(as did I). He probably did not see Greg's comments due to the list bounce
issue. If I read your second hand description of Greg's reaction correctly,
it doesn't sound so strong opinionated as well.
Me, I can live with flags or struct - I don't care, but...

Be prepared that if you are going ahead with struct you are going to
suffer from bike shedding, which has already started and you will be
instructed (just now) to also fix all the relevant and missing Documentation.
If, on the other hand, you can get Greg and the rest to concede to adding
flags arg and match get() arg list to set() arg list, you will have a much
easier job and the patch line count, especially in fs code will be *much*
smaller - just saying.

Thanks,
Amir.

Re: [PATCH v14 2/5] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh

[Amir Goldstein]

+ ebiederm and nfsd folks

On Wed, Oct 23, 2019 at 11:08 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
>
>
> On Tue, Oct 22, 2019 at 10:46 PM Mark Salyzyn <salyzyn@android.com> wrote:
> >
> > Assumption never checked, should fail if the mounter creds are not
> > sufficient.
>
> A bit more explanation would be nice.  Like a pointer to the explanation given in the open_by_handle_at(2) code where this check was presumably taken from.
>

Well, it's not that simple (TM).
If you are considering unprivileged overlay mounts, then this should be
ns_capable() check, even though open_by_handle_at(2) does not
currently allow userspace nfsd to decode file handles.

Unlike open_by_handle_at(2), overlayfs (currently) never exposes file
data via decoded origin fh. AFAIK, it only exposes the origin st_ino
st_dev and some nlink related accounting.

I have been trying to understand from code if nfsd exports are allowed
from non privileged containers and couldn't figure it out (?).
If non privileged container is allowed to export nosubtreecheck export
then non privileged container root can already decode file handles...

Thanks,
Amir.

Re: [PATCH v14 2/5] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh

[J. Bruce Fields]

On Sun, Oct 27, 2019 at 09:24:52AM +0200, Amir Goldstein wrote:
> Well, it's not that simple (TM).
> If you are considering unprivileged overlay mounts, then this should be
> ns_capable() check, even though open_by_handle_at(2) does not
> currently allow userspace nfsd to decode file handles.
> 
> Unlike open_by_handle_at(2), overlayfs (currently) never exposes file
> data via decoded origin fh. AFAIK, it only exposes the origin st_ino
> st_dev and some nlink related accounting.
> 
> I have been trying to understand from code if nfsd exports are allowed
> from non privileged containers and couldn't figure it out (?).
> If non privileged container is allowed to export nosubtreecheck export
> then non privileged container root can already decode file handles...

I don't see any special checks in nfsctl_transaction_write() or
write_threads().  I guess it's just depending on the (0600) file
permissions.  I'm vague on how file permissions work in containers.

The issue with filehandles is that they allow you to bypass directory
lookup permissions.  Keeping a file private by denying permission to
look it up doesn't sound like a good idea to me, honestly, but it does
work on local posix filesystems, so we don't want to break that.

Filehandles are generally pretty easy to guess, and can't be revoked, so
we're more worried about using them (with open_by_handle_at()) than
reading them (with name_to_handle_at()), but we try to prevent the
latter as well.

--b.

Re: [PATCH v14 4/5] overlayfs: internal getxattr operations without sepolicy checking

[Mark Salyzyn]

On 10/22/19 11:39 PM, Amir Goldstein wrote:
> On Tue, Oct 22, 2019 at 11:46 PM Mark Salyzyn <salyzyn@android.com> wrote:
>> Check impure, opaque, origin & meta xattr with no sepolicy audit
>> (using __vfs_getxattr) since these operations are internal to
>> overlayfs operations and do not disclose any data.  This became
>> an issue for credential override off since sys_admin would have
>> been required by the caller; whereas would have been inherently
>> present for the creator since it performed the mount.
>>
>> This is a change in operations since we do not check in the new
>> ovl_do_vfs_getxattr function if the credential override is off or
>> not.  Reasoning is that the sepolicy check is unnecessary overhead,
>> especially since the check can be expensive.
>>
>> Because for override credentials off, this affects _everyone_ that
>> underneath performs private xattr calls without the appropriate
>> sepolicy permissions and sys_admin capability.  Providing blanket
>> support for sys_admin would be bad for all possible callers.
>>
>> For the override credentials on, this will affect only the mounter,
>> should it lack sepolicy permissions. Not considered a security
>> problem since mounting by definition has sys_admin capabilities,
>> but sepolicy contexts would still need to be crafted.
>>
> It sounds reasonable to me, but I am not a "security person".
>
>> It should be noted that there is precedence, __vfs_getxattr is used
>> in other filesystems for their own internal trusted xattr management.
>>
> Urgh! "other" filesystems meaning ecryptfs_getxattr()?
> That looks like a loop hole to read any trusted xattr without any
> security checks. Not sure its a good example...

Yes. But it also makes sense since ecryptfs_getxattr is performed inside 
a layer where the security check is done above by the filesystem that 
called it (AFAIK)? This is used by the filesystem, or the security 
layers to pull the actual sepolicy, rather than getting an EPERM and no 
data. The xattr 'data' is needed by the internal layers.

-- Mark

Re: [PATCH v14 1/5] Add flags option to get xattr method paired to __vfs_getxattr

[Mark Salyzyn]

On 10/23/19 9:57 PM, Amir Goldstein wrote:
> [excessive CC list reduced]
>
> On Wed, Oct 23, 2019 at 11:07 AM Andreas Dilger via samba-technical
> <samba-technical@lists.samba.org> wrote:
>>
>> On Oct 22, 2019, at 2:44 PM, Mark Salyzyn <salyzyn@android.com> wrote:
>>> Replace arguments for get and set xattr methods, and __vfs_getxattr
>>> and __vfs_setaxtr functions with a reference to the following now
>>> common argument structure:
>>>
>>> struct xattr_gs_args {
>>>        struct dentry *dentry;
>>>        struct inode *inode;
>>>        const char *name;
>>>        union {
>>>                void *buffer;
>>>                const void *value;
>>>        };
>>>        size_t size;
>>>        int flags;
>>> };
>>> Mark,
>>>
>>> I do not see the first patch on fsdevel
>>> and I am confused from all the suggested APIs
>>> I recall Christoph's comment on v8 for not using xattr_gs_args
>>> and just adding flags to existing get() method.
>>> I agree to that comment.
>> As already responded, third (?) patch version was like that,
> The problem is that because of the waaay too long CC list, most revisions
> of the patch and discussion were bounced from fsdevel, most emails
> I did not get and cannot find in archives, so the discussion around
> them is not productive.
>
> Please resend patch to fsdevel discarding the auto added CC list
> of all fs maintainers.

git send-email is not my friend :-(

>> gregkh@
>> said it passed the limit for number of arguments, is looking a bit silly
> Well, you just matched get() to set() args list, so this is not a strong
> argument IMO.
>
>> (my paraphrase), and that it should be passed as a structure. Two others
>> agreed. We gained because both set and get use the same structure after
>> this change (this allows a simplified read-modify-write cycle).
> That sounds like a nice benefit if this was user API, but are there any
> kernel users that intend to make use of that read-modify-write cycle?
> I don't think so.
(one user)
>
>> We will need a quorum on this, 3 (structure) to 2 (flag) now (but really
>> basically between Greg and Christoph?). Coding style issue: Add a flag,
>> or switch to a common xattr argument  structure?
>>
> IIRC, Christoph was asking why the silly struct and not simply add flags
> (as did I). He probably did not see Greg's comments due to the list bounce
> issue. If I read your second hand description of Greg's reaction correctly,
> it doesn't sound so strong opinionated as well.
> Me, I can live with flags or struct - I don't care, but...
>
> Be prepared that if you are going ahead with struct you are going to
> suffer from bike shedding, which has already started and you will be
> instructed (just now) to also fix all the relevant and missing Documentation.
> If, on the other hand, you can get Greg and the rest to concede to adding
> flags arg and match get() arg list to set() arg list, you will have a much
> easier job and the patch line count, especially in fs code will be *much*
> smaller - just saying.

Respining back to the v4 version of the series incorporating some of the 
fixes on the way.

Automated testing in kernel not yet handled and will be noted in the 
respin.

> Thanks,
> Amir.

Mark