[PATCH v12 0/5] overlayfs override_creds=off

[PATCH v12 0/5] overlayfs override_creds=off

[Mark Salyzyn]

Patch series:

overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh
Add flags option to get xattr method paired to __vfs_getxattr
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.
on them).

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

---
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.

[PATCH v12 1/5] 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 - 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,
-- 
2.22.0.770.g0f2c4a37fd-goog

[PATCH v12 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) ->
handler->get(dentry...XATTR_NOSECURITY) ->
__vfs_getxattr(realdentry) -> lower_handler->get(realdentry) 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
---
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     | 8 ++++++--
 fs/overlayfs/overlayfs.h | 2 +-
 fs/overlayfs/super.c     | 7 ++++---
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 7663aeb85fa3..ce66f4050557 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -363,7 +363,7 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
 }
 
 int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
-		  void *value, size_t size)
+		  void *value, size_t size, int flags)
 {
 	ssize_t res;
 	const struct cred *old_cred;
@@ -371,7 +371,11 @@ int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
 		ovl_i_dentry_upper(inode) ?: ovl_dentry_lower(dentry);
 
 	old_cred = ovl_override_creds(dentry->d_sb);
-	res = vfs_getxattr(realdentry, name, value, size);
+	if (flags & XATTR_NOSECURITY)
+		res = __vfs_getxattr(realdentry, d_inode(realdentry), name,
+				     value, size);
+	else
+		res = vfs_getxattr(realdentry, name, value, size);
 	revert_creds(old_cred);
 	return res;
 }
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 6934bcf030f0..ab3d031c422b 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -356,7 +356,7 @@ int ovl_permission(struct inode *inode, int mask);
 int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
 		  const void *value, size_t size, int flags);
 int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
-		  void *value, size_t size);
+		  void *value, size_t size, int flags);
 ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size);
 struct posix_acl *ovl_get_acl(struct inode *inode, int type);
 int ovl_update_time(struct inode *inode, struct timespec64 *ts, int flags);
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 57df03f3259f..6f041e1fceda 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -856,7 +856,7 @@ ovl_posix_acl_xattr_get(const struct xattr_handler *handler,
 			struct dentry *dentry, struct inode *inode,
 			const char *name, void *buffer, size_t size, int flags)
 {
-	return ovl_xattr_get(dentry, inode, handler->name, buffer, size);
+	return ovl_xattr_get(dentry, inode, handler->name, buffer, size, flags);
 }
 
 static int __maybe_unused
@@ -919,7 +919,8 @@ ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
 
 static int ovl_own_xattr_get(const struct xattr_handler *handler,
 			     struct dentry *dentry, struct inode *inode,
-			     const char *name, void *buffer, size_t size)
+			     const char *name, void *buffer, size_t size,
+			     int flags)
 {
 	return -EOPNOTSUPP;
 }
@@ -937,7 +938,7 @@ static int ovl_other_xattr_get(const struct xattr_handler *handler,
 			       const char *name, void *buffer, size_t size,
 			       int flags)
 {
-	return ovl_xattr_get(dentry, inode, name, buffer, size);
+	return ovl_xattr_get(dentry, inode, name, buffer, size, flags);
 }
 
 static int ovl_other_xattr_set(const struct xattr_handler *handler,
-- 
2.22.0.770.g0f2c4a37fd-goog

[PATCH v12 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
---
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      | 24 +++++++++++++++---------
 3 files changed, 24 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 ab3d031c422b..9d26d8758513 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..f80b95423043 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -40,6 +40,12 @@ 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)
+{
+	return __vfs_getxattr(dentry, d_inode(dentry), name, buf, size);
+}
+
 struct super_block *ovl_same_sb(struct super_block *sb)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
@@ -537,9 +543,9 @@ 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 +556,13 @@ 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 +843,13 @@ 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 +858,7 @@ 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 +884,7 @@ 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 +896,7 @@ 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;
 	}
-- 
2.22.0.770.g0f2c4a37fd-goog

[PATCH v12 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
---
v12:
- 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.

a
---
 Documentation/filesystems/overlayfs.txt | 23 +++++++++++++++++++++++
 fs/overlayfs/copy_up.c                  |  2 +-
 fs/overlayfs/dir.c                      | 11 ++++++-----
 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, 87 insertions(+), 33 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 1da2f1668f08..d48125076602 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..49b8ffc1294f 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -563,7 +563,8 @@ 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);
@@ -579,7 +580,7 @@ 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);
 	return err;
 }
 
@@ -655,7 +656,7 @@ 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 +852,7 @@ 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 +1222,7 @@ 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..627a303c95da 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)->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 ce66f4050557..bc46f45f65ba 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;
 }
 
@@ -351,7 +351,7 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
 		WARN_ON(flags != XATTR_REPLACE);
 		err = vfs_removexattr(realdentry, name);
 	}
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 
 	/* copy c/mtime */
 	ovl_copyattr(d_inode(realdentry), inode);
@@ -376,7 +376,7 @@ int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
 				     value, size);
 	else
 		res = vfs_getxattr(realdentry, name, value, size);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	return res;
 }
 
@@ -400,7 +400,7 @@ 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;
 
@@ -435,7 +435,7 @@ 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;
 }
@@ -473,7 +473,7 @@ 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 9d26d8758513..ad1a11e7ecbd 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 28a2d12a1029..2637c5aadf7f 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 6f041e1fceda..2c1278451f38 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;
@@ -362,6 +367,9 @@ 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;
 }
 
@@ -402,6 +410,8 @@ enum {
 	OPT_XINO_AUTO,
 	OPT_METACOPY_ON,
 	OPT_METACOPY_OFF,
+	OPT_OVERRIDE_CREDS_ON,
+	OPT_OVERRIDE_CREDS_OFF,
 	OPT_ERR,
 };
 
@@ -420,6 +430,8 @@ 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}
 };
 
@@ -478,6 +490,7 @@ 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;
@@ -558,6 +571,14 @@ 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;
@@ -1674,7 +1695,6 @@ 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 f80b95423043..4720a7a6fea3 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)
 {
@@ -797,7 +805,7 @@ 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)
@@ -815,7 +823,7 @@ 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);
-- 
2.22.0.770.g0f2c4a37fd-goog

Re: [PATCH v12 0/5] overlayfs override_creds=off

[Casey Schaufler]

On 7/30/2019 10:28 AM, Mark Salyzyn wrote:
> Patch series:

Please add linux-security-module@vger.kernel.org to the CC
for all changes affecting handling of security xattrs.

>
> overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh
> Add flags option to get xattr method paired to __vfs_getxattr
> 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.
> on them).
>
> 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
>
> ---
> 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.
>