[PATCH 1/1] selinux, smack: don't bypass permissions check in inode_setsecctx hook

[Scott Mayhew]

Marek Gresko reports that the root user on an NFS client is able to
change the security labels on files on an NFS filesystem that is
exported with root squashing enabled.

The end of the kerneldoc comment for __vfs_setxattr_noperm() states:

 *  This function requires the caller to lock the inode's i_mutex before it
 *  is executed. It also assumes that the caller will make the appropriate
 *  permission checks.

nfsd_setattr() does do permissions checking via fh_verify() and
nfsd_permission(), but those don't do all the same permissions checks
that are done by security_inode_setxattr() and its related LSM hooks do.

Since nfsd_setattr() is the only consumer of security_inode_setsecctx(),
simplest solution appears to be to replace the call to
__vfs_setxattr_noperm() with a call to __vfs_setxattr_locked().  This
fixes the above issue and has the added benefit of causing nfsd to
recall conflicting delegations on a file when a client tries to change
its security label.

Reported-by: Marek Gresko <marek.gresko at protonmail.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218809
Signed-off-by: Scott Mayhew <smayhew at redhat.com>
---
 security/selinux/hooks.c   | 4 ++--
 security/smack/smack_lsm.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index bfa61e005aac..400eca4ad0fb 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6660,8 +6660,8 @@ static int selinux_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen
  */
 static int selinux_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen)
 {
-	return __vfs_setxattr_noperm(&nop_mnt_idmap, dentry, XATTR_NAME_SELINUX,
-				     ctx, ctxlen, 0);
+	return __vfs_setxattr_locked(&nop_mnt_idmap, dentry, XATTR_NAME_SELINUX,
+				     ctx, ctxlen, 0, NULL);
 }
 
 static int selinux_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen)
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 4164699cd4f6..002a1b9ed83a 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -4880,8 +4880,8 @@ static int smack_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen)
 
 static int smack_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen)
 {
-	return __vfs_setxattr_noperm(&nop_mnt_idmap, dentry, XATTR_NAME_SMACK,
-				     ctx, ctxlen, 0);
+	return __vfs_setxattr_locked(&nop_mnt_idmap, dentry, XATTR_NAME_SMACK,
+				     ctx, ctxlen, 0, NULL);
 }
 
 static int smack_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen)
-- 
2.46.0