[RESEND PATCH v18 0/4] overlayfs override_creds=off & nested get xattr fix

[RESEND PATCH v18 0/4] overlayfs override_creds=off & nested get xattr fix

[Mark Salyzyn]

Mark Salyzyn (3):
  Add flags option to get xattr method paired to __vfs_getxattr
  overlayfs: handle XATTR_NOSECURITY flag for get xattr method
  overlayfs: override_creds=off option bypass creator_cred

Mark Salyzyn + John Stultz (1):
  overlayfs: inode_owner_or_capable called during execv

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

The fourth 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: John Stultz <john.stultz@linaro.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-unionfs@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: kernel-team@android.com
Cc: selinux@vger.kernel.org

---

v18
- rebase + fix minor cut and paste error for inode argument in __vfs_getxattr

v17
- correct some zero-day build failures.
- fix up documentation

v16
- rebase and merge of two patches.
- add adjustment to deal with execv when overrides is off.

v15
- Revert back to v4 with fixes from on the way from v5-v14. The single
  structure argument passing to address the complaints about too many
  arguments was rejected by the community.
- Drop the udner discussion fix for an additional CAP_DAC_READ_SEARCH
  check. Can address that independently.
- ToDo: upstream test frame for thes security fixes (currently testing
  is all in Android).

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   |  2 +-
 Documentation/filesystems/overlayfs.rst | 26 +++++++++++++++++-
 fs/9p/acl.c                             |  3 ++-
 fs/9p/xattr.c                           |  3 ++-
 fs/afs/xattr.c                          | 10 +++----
 fs/btrfs/xattr.c                        |  3 ++-
 fs/ceph/xattr.c                         |  3 ++-
 fs/cifs/xattr.c                         |  2 +-
 fs/ecryptfs/inode.c                     |  6 +++--
 fs/ecryptfs/mmap.c                      |  2 +-
 fs/erofs/xattr.c                        |  3 ++-
 fs/ext2/xattr_security.c                |  2 +-
 fs/ext2/xattr_trusted.c                 |  2 +-
 fs/ext2/xattr_user.c                    |  2 +-
 fs/ext4/xattr_hurd.c                    |  2 +-
 fs/ext4/xattr_security.c                |  2 +-
 fs/ext4/xattr_trusted.c                 |  2 +-
 fs/ext4/xattr_user.c                    |  2 +-
 fs/f2fs/xattr.c                         |  4 +--
 fs/fuse/xattr.c                         |  4 +--
 fs/gfs2/xattr.c                         |  3 ++-
 fs/hfs/attr.c                           |  2 +-
 fs/hfsplus/xattr.c                      |  3 ++-
 fs/hfsplus/xattr_security.c             |  3 ++-
 fs/hfsplus/xattr_trusted.c              |  3 ++-
 fs/hfsplus/xattr_user.c                 |  3 ++-
 fs/jffs2/security.c                     |  3 ++-
 fs/jffs2/xattr_trusted.c                |  3 ++-
 fs/jffs2/xattr_user.c                   |  3 ++-
 fs/jfs/xattr.c                          |  5 ++--
 fs/kernfs/inode.c                       |  3 ++-
 fs/nfs/nfs4proc.c                       |  9 ++++---
 fs/ocfs2/xattr.c                        |  9 ++++---
 fs/orangefs/xattr.c                     |  3 ++-
 fs/overlayfs/copy_up.c                  |  2 +-
 fs/overlayfs/dir.c                      | 17 +++++++-----
 fs/overlayfs/file.c                     | 26 +++++++++---------
 fs/overlayfs/inode.c                    | 23 ++++++++--------
 fs/overlayfs/namei.c                    |  6 ++---
 fs/overlayfs/overlayfs.h                |  7 +++--
 fs/overlayfs/ovl_entry.h                |  1 +
 fs/overlayfs/readdir.c                  |  8 +++---
 fs/overlayfs/super.c                    | 34 ++++++++++++++++++-----
 fs/overlayfs/util.c                     | 13 +++++++--
 fs/posix_acl.c                          |  2 +-
 fs/reiserfs/xattr_security.c            |  3 ++-
 fs/reiserfs/xattr_trusted.c             |  3 ++-
 fs/reiserfs/xattr_user.c                |  3 ++-
 fs/squashfs/xattr.c                     |  2 +-
 fs/ubifs/xattr.c                        |  3 ++-
 fs/xattr.c                              | 36 ++++++++++++-------------
 fs/xfs/xfs_xattr.c                      |  3 ++-
 include/linux/xattr.h                   |  9 ++++---
 include/uapi/linux/xattr.h              |  7 +++--
 mm/shmem.c                              |  3 ++-
 net/socket.c                            |  3 ++-
 security/commoncap.c                    |  6 +++--
 security/integrity/evm/evm_main.c       |  3 ++-
 security/selinux/hooks.c                | 11 +++++---
 security/smack/smack_lsm.c              |  5 ++--
 60 files changed, 242 insertions(+), 137 deletions(-)

-- 
2.29.0.rc1.297.gfa9743e501-goog

[RESEND PATCH v18 2/4] 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.

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_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: linux-fsdevel@vger.kernel.org
Cc: linux-unionfs@vger.kernel.org
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-kernel@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: kernel-team@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: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-doc@vger.kernel.org
Cc: selinux@vger.kernel.org

v18 - correct inode argument to __vfs_getxattr

v17 - rebase and add inode argument to __vfs_getxattr

v16 - rebase and merge internal getxattr operations patch

v15 - revert to v13 because xattr_gs_args rejected.

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     | 5 +++--
 fs/overlayfs/overlayfs.h | 6 ++++--
 fs/overlayfs/super.c     | 4 ++--
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index b584dca845ba..2b14291beb86 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -378,7 +378,7 @@ 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;
@@ -386,7 +386,8 @@ 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);
+	res = __vfs_getxattr(realdentry, d_inode(realdentry), name,
+			     value, size, flags);
 	revert_creds(old_cred);
 	return res;
 }
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index f8880aa2ba0e..06db4cf87f55 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -184,7 +184,9 @@ static inline ssize_t ovl_do_getxattr(struct ovl_fs *ofs, struct dentry *dentry,
 				      size_t size)
 {
 	const char *name = ovl_xattr(ofs, ox);
-	return vfs_getxattr(dentry, name, value, size);
+	struct inode *ip = d_inode(dentry);
+
+	return __vfs_getxattr(dentry, ip, name, value, size, XATTR_NOSECURITY);
 }
 
 static inline int ovl_do_setxattr(struct ovl_fs *ofs, struct dentry *dentry,
@@ -439,7 +441,7 @@ 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 f41353ba1e68..d447958badc2 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -930,7 +930,7 @@ 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
@@ -1012,7 +1012,7 @@ 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,
-- 
2.29.0.rc1.297.gfa9743e501-goog

[RESEND PATCH v18 3/4] 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: linux-fsdevel@vger.kernel.org
Cc: linux-unionfs@vger.kernel.org
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-security-module@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
Cc: selinux@vger.kernel.org

v18
- rebase

v17
- move credential section for override_creds=off as a level 3 heading
  subsection of the permissions section.

v16
- Rebase, cover a few more new ovl_revert_creds callpoints.

v15
- Rebase

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 in progress
  in other commiter's hands.

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.rst | 26 ++++++++++++++++++++++++-
 fs/overlayfs/copy_up.c                  |  2 +-
 fs/overlayfs/dir.c                      | 17 +++++++++-------
 fs/overlayfs/file.c                     | 24 +++++++++++------------
 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                  |  8 ++++----
 fs/overlayfs/super.c                    | 22 ++++++++++++++++++++-
 fs/overlayfs/util.c                     | 13 +++++++++++--
 11 files changed, 98 insertions(+), 40 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 580ab9a0fe31..dac065f73e37 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -195,7 +195,7 @@ handle it in two different ways:
 
 1. return EXDEV error: this error is returned by rename(2) when trying to
    move a file or directory across filesystem boundaries.  Hence
-   applications are usually prepared to hande this error (mv(1) for example
+   applications are usually prepared to handle this error (mv(1) for example
    recursively copies the directory tree).  This is the default behavior.
 
 2. If the "redirect_dir" feature is enabled, then the directory will be
@@ -324,6 +324,30 @@ and
 The resulting access permissions should be the same.  The difference is in
 the time of copy (on-demand vs. up-front).
 
+### Non overlapping credentials
+
+As noted above, 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 the mounter, a
+use case available in older versions of the driver, the override_creds mount
+flag can be turned off.  For when the use pattern has caller with legitimate
+credentials where the mounter does not.  For example init may have been the
+mounter, but the caller would have execute or read MAC permissions where
+init would not.  override_creds off means all access, incoming, upper, lower
+or working, will be tested against the caller.
+
+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 re-testing 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.
 
 Multiple lower layers
 ---------------------
diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index 955ecd4030f0..b9d97e7efd5c 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -962,7 +962,7 @@ static 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 28a075b5f5b2..3c6f5f6648b8 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -565,7 +565,7 @@ 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;
 
@@ -592,14 +592,15 @@ 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))
@@ -608,7 +609,9 @@ 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;
 }
 
@@ -684,7 +687,7 @@ 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;
 }
@@ -903,7 +906,7 @@ 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);
@@ -1273,7 +1276,7 @@ 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 efccb7c1f9bc..b1357bb067d9 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -59,7 +59,7 @@ static struct file *ovl_open_realfile(const struct file *file,
 		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,
@@ -209,7 +209,7 @@ 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;
 	ovl_inode_unlock(inode);
@@ -323,7 +323,7 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			ovl_aio_cleanup_handler(aio_req);
 	}
 out:
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 	ovl_file_accessed(file);
 
 	fdput(real);
@@ -388,7 +388,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 			ovl_aio_cleanup_handler(aio_req);
 	}
 out:
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 	fdput(real);
 
 out_unlock:
@@ -411,7 +411,7 @@ static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,
 
 	old_cred = ovl_override_creds(file_inode(in)->i_sb);
 	ret = generic_file_splice_read(real.file, ppos, pipe, len, flags);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(in)->i_sb, old_cred);
 
 	ovl_file_accessed(in);
 	fdput(real);
@@ -432,7 +432,7 @@ ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,
 
 	old_cred = ovl_override_creds(file_inode(out)->i_sb);
 	ret = iter_file_splice_write(pipe, real.file, ppos, len, flags);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(out)->i_sb, old_cred);
 
 	ovl_file_accessed(out);
 	fdput(real);
@@ -456,7 +456,7 @@ 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);
@@ -480,7 +480,7 @@ 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 */
@@ -508,7 +508,7 @@ 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);
@@ -530,7 +530,7 @@ 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);
 
@@ -552,7 +552,7 @@ static long ovl_real_ioctl(struct file *file, unsigned int cmd,
 	ret = security_file_ioctl(real.file, cmd, arg);
 	if (!ret)
 		ret = vfs_ioctl(real.file, cmd, arg);
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	fdput(real);
 
@@ -741,7 +741,7 @@ 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 2b14291beb86..fb0ec01774e6 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -80,7 +80,7 @@ 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);
@@ -272,7 +272,7 @@ 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;
 }
@@ -306,7 +306,7 @@ 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;
 }
@@ -323,7 +323,7 @@ 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;
 }
 
@@ -366,7 +366,7 @@ 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);
@@ -388,7 +388,7 @@ int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
 	old_cred = ovl_override_creds(dentry->d_sb);
 	res = __vfs_getxattr(realdentry, d_inode(realdentry), name,
 			     value, size, flags);
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	return res;
 }
 
@@ -416,7 +416,7 @@ 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;
 
@@ -451,7 +451,7 @@ 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;
 }
@@ -485,7 +485,7 @@ static int ovl_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 
 	old_cred = ovl_override_creds(inode->i_sb);
 	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 a6162c4076db..0e6290844c57 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -1097,7 +1097,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	ovl_dentry_update_reval(dentry, upperdentry,
 			DCACHE_OP_REVALIDATE | DCACHE_OP_WEAK_REVALIDATE);
 
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	if (origin_path) {
 		dput(origin_path->dentry);
 		kfree(origin_path);
@@ -1124,7 +1124,7 @@ 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);
 }
 
@@ -1176,7 +1176,7 @@ 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 06db4cf87f55..da4a25ac9b01 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -253,6 +253,7 @@ 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);
 int ovl_can_decode_fh(struct super_block *sb);
 struct dentry *ovl_indexdir(struct super_block *sb);
 bool ovl_index_all(struct super_block *sb);
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 1b5a2094df8e..597f00982ade 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -18,6 +18,7 @@ struct ovl_config {
 	int xino;
 	bool metacopy;
 	bool ovl_volatile;
+	bool override_creds;
 };
 
 struct ovl_sb {
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 01620ebae1bd..38d5b855c4a4 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;
 }
@@ -796,7 +796,7 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
 	}
 	err = 0;
 out:
-	revert_creds(old_cred);
+	ovl_revert_creds(dentry->d_sb, old_cred);
 	return err;
 }
 
@@ -848,7 +848,7 @@ static struct file *ovl_dir_open_realfile(const struct file *file,
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	res = ovl_path_open(realpath, O_RDONLY | (file->f_flags & O_LARGEFILE));
-	revert_creds(old_cred);
+	ovl_revert_creds(file_inode(file)->i_sb, old_cred);
 
 	return res;
 }
@@ -986,7 +986,7 @@ 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 d447958badc2..28c5d291f836 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;
@@ -366,6 +371,9 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 			   ofs->config.metacopy ? "on" : "off");
 	if (ofs->config.ovl_volatile)
 		seq_puts(m, ",volatile");
+	if (ofs->config.override_creds != ovl_override_creds_def)
+		seq_show_option(m, "override_creds",
+				ofs->config.override_creds ? "on" : "off");
 	return 0;
 }
 
@@ -418,6 +426,8 @@ enum {
 	OPT_METACOPY_ON,
 	OPT_METACOPY_OFF,
 	OPT_VOLATILE,
+	OPT_OVERRIDE_CREDS_ON,
+	OPT_OVERRIDE_CREDS_OFF,
 	OPT_ERR,
 };
 
@@ -437,6 +447,8 @@ static const match_table_t ovl_tokens = {
 	{OPT_METACOPY_ON,		"metacopy=on"},
 	{OPT_METACOPY_OFF,		"metacopy=off"},
 	{OPT_VOLATILE,			"volatile"},
+	{OPT_OVERRIDE_CREDS_ON,		"override_creds=on"},
+	{OPT_OVERRIDE_CREDS_OFF,	"override_creds=off"},
 	{OPT_ERR,			NULL}
 };
 
@@ -496,6 +508,7 @@ 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;
@@ -585,6 +598,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 			config->ovl_volatile = true;
 			break;
 
+		case OPT_OVERRIDE_CREDS_ON:
+			config->override_creds = true;
+			break;
+
+		case OPT_OVERRIDE_CREDS_OFF:
+			config->override_creds = false;
+			break;
+
 		default:
 			pr_err("unrecognized mount option \"%s\" or missing value\n",
 					p);
@@ -2007,7 +2028,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	kfree(splitlower);
 
 	sb->s_root = root_dentry;
-
 	return 0;
 
 out_free_oe:
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 23f475627d07..8d1e7f0db7bc 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -37,9 +37,18 @@ 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);
+}
+
+
 /*
  * Check if underlying fs supports file handles and try to determine encoding
  * type, in order to deduce maximum inode number used by fs.
@@ -823,7 +832,7 @@ 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)
@@ -841,7 +850,7 @@ 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);
-- 
2.29.0.rc1.297.gfa9743e501-goog

Re: [RESEND PATCH v18 0/4] overlayfs override_creds=off & nested get xattr fix

[Eric Biggers]

On Wed, Oct 21, 2020 at 08:18:59AM -0700, Mark Salyzyn wrote:
> Mark Salyzyn (3):
>   Add flags option to get xattr method paired to __vfs_getxattr
>   overlayfs: handle XATTR_NOSECURITY flag for get xattr method
>   overlayfs: override_creds=off option bypass creator_cred
> 
> Mark Salyzyn + John Stultz (1):
>   overlayfs: inode_owner_or_capable called during execv
> 
> The first three patches address fundamental security issues that should
> be solved regardless of the override_creds=off feature.
> 
> The fourth adds the feature depends on these other fixes.

FYI, I didn't receive patch 4, and neither https://lkml.kernel.org/linux-fsdevel
nor https://lkml.kernel.org/linux-unionfs have it either.

- Eric

Re: [RESEND PATCH v18 0/4] overlayfs override_creds=off & nested get xattr fix

[Mark Salyzyn]

On 10/21/20 10:19 PM, Eric Biggers wrote:
> On Wed, Oct 21, 2020 at 08:18:59AM -0700, Mark Salyzyn wrote:
>> Mark Salyzyn (3):
>>    Add flags option to get xattr method paired to __vfs_getxattr
>>    overlayfs: handle XATTR_NOSECURITY flag for get xattr method
>>    overlayfs: override_creds=off option bypass creator_cred
>>
>> Mark Salyzyn + John Stultz (1):
>>    overlayfs: inode_owner_or_capable called during execv
>>
>> The first three patches address fundamental security issues that should
>> be solved regardless of the override_creds=off feature.
>>
>> The fourth adds the feature depends on these other fixes.
> FYI, I didn't receive patch 4, and neither https://lkml.kernel.org/linux-fsdevel
> nor https://lkml.kernel.org/linux-unionfs have it either.
>
> - Eric

Resent again, thanks.

Re: [RESEND PATCH v18 2/4] overlayfs: handle XATTR_NOSECURITY flag for get xattr method

[Miklos Szeredi]

On Wed, Oct 21, 2020 at 5:19 PM Mark Salyzyn <salyzyn@android.com> wrote:
>
> 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.
>
> 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_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.

This would be a problem when unprivileged mounting of overlay is
introduced.  I'd really like to avoid weakening the current security
model.

The big API churn in the 1/4 patch also seems excessive considering
that this seems to be mostly a cosmetic issue for android.  Am I
missing something?

Thanks,
Miklos

Re: [RESEND PATCH v18 2/4] overlayfs: handle XATTR_NOSECURITY flag for get xattr method

[Mark Salyzyn]

On 10/30/20 8:07 AM, Miklos Szeredi wrote:
> On Wed, Oct 21, 2020 at 5:19 PM Mark Salyzyn <salyzyn@android.com> wrote:
>> 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.
>>
>> 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_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.
> This would be a problem when unprivileged mounting of overlay is
> introduced.  I'd really like to avoid weakening the current security
> model.

The current security model does not deal with non-overlapping security 
contexts between init (which on android has MAC permissions only when 
necessary, only enough permissions to perform the mount and other 
mundane operations, missing exec and read permissions in key spots) and 
user calls.

We are only weakening (that is actually an incorrect statement, security 
is there, just not double security of both mounter and caller) the 
security around calls that retrieve the xattr for administrative and 
internal purposes. No data is exposed to the caller that it would not 
otherwise have permissions for.

This patch becomes necessary when matched with the PATCH v18 3/4 of the 
series which fixes the user space break introduced in ~4.6 that formerly 
used the callers credentials for all accesses in all places. Security is 
weakened already as-is in overlayfs with all the overriding of the 
credentials for internal accesses to overlayfs mechanics based on the 
mounter credentials. Using the mounter credentials as a wider security 
hole is the problem, at least with PATCH v18 3/4 of the series we go 
back optionally to only using the caller's credentials to perform the 
operations. Admittedly some of the internal operations like mknod are 
privileged, but at least in Android's use case we are not using them 
with callers without the necessary credentials.

Android does not give the mounter more credentials than the callers, 
there is very little overlap in the MAC security.

> The big API churn in the 1/4 patch also seems excessive considering
> that this seems to be mostly a cosmetic issue for android.  Am I
> missing something?

Breaks sepolicy, it no longer has access to the context data at the 
overlayfs security boundary.

unknown is a symptom of being denied based on the denial to xattr data 
from the underlying filesystem layer. Being denied the security context 
of the target is not a good thing within the sepolicy security layer.

>
> Thanks,
> Miklos

Re: [RESEND PATCH v18 2/4] overlayfs: handle XATTR_NOSECURITY flag for get xattr method

[Tyler Hicks]

On 2020-10-30 09:00:35, Mark Salyzyn wrote:
> On 10/30/20 8:07 AM, Miklos Szeredi wrote:
> > On Wed, Oct 21, 2020 at 5:19 PM Mark Salyzyn <salyzyn@android.com> wrote:
> > > 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.
> > > 
> > > 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_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.
> > This would be a problem when unprivileged mounting of overlay is
> > introduced.  I'd really like to avoid weakening the current security
> > model.
> 
> The current security model does not deal with non-overlapping security
> contexts between init (which on android has MAC permissions only when
> necessary, only enough permissions to perform the mount and other mundane
> operations, missing exec and read permissions in key spots) and user calls.
> 
> We are only weakening (that is actually an incorrect statement, security is
> there, just not double security of both mounter and caller) the security
> around calls that retrieve the xattr for administrative and internal
> purposes. No data is exposed to the caller that it would not otherwise have
> permissions for.

We've ran into the same issues that Mark is trying to solve with this
series. I came across Mark's series while searching around before I
wrote up a similar patch to Mark's patch #3.

We have a confined process that sets up Overlayfs mounts, then that process
starts a service confined by another security context, then that service
may execute binaries that run under a third security context. In this
case, I'm talking about SELinux security contexts but it could be
AppArmor or anything else that you use to separate out
privileges/permissions at fine-grained detail.

We don't want to grant all the privileges/permissions required by the
service (and its helper utilities) to the process that sets up the
Overlayfs mounts because we've been very careful in separating them
apart with security policy. However, we want to make use of Overlayfs
and adding a mount option to bypass the check on the mounter's cred
seems like a safe way of using Overlayfs without violating our principle
of least privilege.

Tyler

> 
> This patch becomes necessary when matched with the PATCH v18 3/4 of the
> series which fixes the user space break introduced in ~4.6 that formerly
> used the callers credentials for all accesses in all places. Security is
> weakened already as-is in overlayfs with all the overriding of the
> credentials for internal accesses to overlayfs mechanics based on the
> mounter credentials. Using the mounter credentials as a wider security hole
> is the problem, at least with PATCH v18 3/4 of the series we go back
> optionally to only using the caller's credentials to perform the operations.
> Admittedly some of the internal operations like mknod are privileged, but at
> least in Android's use case we are not using them with callers without the
> necessary credentials.
> 
> Android does not give the mounter more credentials than the callers, there
> is very little overlap in the MAC security.
> 
> > The big API churn in the 1/4 patch also seems excessive considering
> > that this seems to be mostly a cosmetic issue for android.  Am I
> > missing something?
> 
> Breaks sepolicy, it no longer has access to the context data at the
> overlayfs security boundary.
> 
> unknown is a symptom of being denied based on the denial to xattr data from
> the underlying filesystem layer. Being denied the security context of the
> target is not a good thing within the sepolicy security layer.
> 
> > 
> > Thanks,
> > Miklos
> 
> 

Re: [RESEND PATCH v18 2/4] overlayfs: handle XATTR_NOSECURITY flag for get xattr method

[Tyler Hicks]

On 2021-02-05 12:01:55, Tyler Hicks wrote:
> On 2020-10-30 09:00:35, Mark Salyzyn wrote:
> > On 10/30/20 8:07 AM, Miklos Szeredi wrote:
> > > On Wed, Oct 21, 2020 at 5:19 PM Mark Salyzyn <salyzyn@android.com> wrote:
> > > > 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.
> > > > 
> > > > 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_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.
> > > This would be a problem when unprivileged mounting of overlay is
> > > introduced.  I'd really like to avoid weakening the current security
> > > model.
> > 
> > The current security model does not deal with non-overlapping security
> > contexts between init (which on android has MAC permissions only when
> > necessary, only enough permissions to perform the mount and other mundane
> > operations, missing exec and read permissions in key spots) and user calls.
> > 
> > We are only weakening (that is actually an incorrect statement, security is
> > there, just not double security of both mounter and caller) the security
> > around calls that retrieve the xattr for administrative and internal
> > purposes. No data is exposed to the caller that it would not otherwise have
> > permissions for.
> 
> We've ran into the same issues that Mark is trying to solve with this
> series. I came across Mark's series while searching around before I
> wrote up a similar patch to Mark's patch #3.
> 
> We have a confined process that sets up Overlayfs mounts, then that process
> starts a service confined by another security context, then that service
> may execute binaries that run under a third security context. In this
> case, I'm talking about SELinux security contexts but it could be
> AppArmor or anything else that you use to separate out
> privileges/permissions at fine-grained detail.
> 
> We don't want to grant all the privileges/permissions required by the
> service (and its helper utilities) to the process that sets up the
> Overlayfs mounts because we've been very careful in separating them
> apart with security policy. However, we want to make use of Overlayfs
> and adding a mount option to bypass the check on the mounter's cred
> seems like a safe way of using Overlayfs without violating our principle
> of least privilege.

I just realized that I missed one noteworthy aspect of our use case. We
would be alright if this functionality to bypass the mounter's cred
checks (conditional, based on a mount option) was only allowed for
read-only overlays[1].

I'm not sure if that would meet the needs of Android. Can you comment on
that, Mark?

Miklos, would that restriction make this series any more acceptable to
you?

Thanks!

Tyler

[1] https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html#multiple-lower-layers

> 
> Tyler
> 
> > 
> > This patch becomes necessary when matched with the PATCH v18 3/4 of the
> > series which fixes the user space break introduced in ~4.6 that formerly
> > used the callers credentials for all accesses in all places. Security is
> > weakened already as-is in overlayfs with all the overriding of the
> > credentials for internal accesses to overlayfs mechanics based on the
> > mounter credentials. Using the mounter credentials as a wider security hole
> > is the problem, at least with PATCH v18 3/4 of the series we go back
> > optionally to only using the caller's credentials to perform the operations.
> > Admittedly some of the internal operations like mknod are privileged, but at
> > least in Android's use case we are not using them with callers without the
> > necessary credentials.
> > 
> > Android does not give the mounter more credentials than the callers, there
> > is very little overlap in the MAC security.
> > 
> > > The big API churn in the 1/4 patch also seems excessive considering
> > > that this seems to be mostly a cosmetic issue for android.  Am I
> > > missing something?
> > 
> > Breaks sepolicy, it no longer has access to the context data at the
> > overlayfs security boundary.
> > 
> > unknown is a symptom of being denied based on the denial to xattr data from
> > the underlying filesystem layer. Being denied the security context of the
> > target is not a good thing within the sepolicy security layer.
> > 
> > > 
> > > Thanks,
> > > Miklos
> > 
> >