[PATCH v2] vfs: bypass may_create_in_sticky check on newly-created files if task has CAP_FOWNER

[Jeff Layton <jlayton@kernel.org>]

From: Christian Brauner <brauner@kernel.org>

NFS server is exporting a sticky directory (mode 01777) with root
squashing enabled. Client has protect_regular enabled and then tries to
open a file as root in that directory. File is created (with ownership
set to nobody:nobody) but the open syscall returns an error. The problem
is may_create_in_sticky which rejects the open even though the file has
already been created.

Add a new condition to may_create_in_sticky. If the file was just
created, then allow bypassing the ownership check if the task has
CAP_FOWNER. With this change, the initial open of a file by root works,
but later opens of the same file will fail.

Note that we can contrive a similar situation by exporting with
all_squash and opening the file as an unprivileged user. This patch does
not fix that case. I suspect that that configuration is likely to be
fundamentally incompatible with the protect_* sysctls enabled on the
clients.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829
Reported-by: Yongchen Yang <yoyang@redhat.com>
Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/namei.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Hi Christian,

I left you as author here since this is basically identical to the patch
you suggested. Let me know if that's an issue.

-- Jeff

diff --git a/fs/namei.c b/fs/namei.c
index 1f28d3f463c3..26b602d1152b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1221,7 +1221,8 @@ int may_linkat(struct user_namespace *mnt_userns, struct path *link)
  * Returns 0 if the open is allowed, -ve on error.
  */
 static int may_create_in_sticky(struct user_namespace *mnt_userns,
-				struct nameidata *nd, struct inode *const inode)
+				struct nameidata *nd, struct inode *const inode,
+				bool created)
 {
 	umode_t dir_mode = nd->dir_mode;
 	kuid_t dir_uid = nd->dir_uid;
@@ -1230,7 +1231,8 @@ static int may_create_in_sticky(struct user_namespace *mnt_userns,
 	    (!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
 	    likely(!(dir_mode & S_ISVTX)) ||
 	    uid_eq(i_uid_into_mnt(mnt_userns, inode), dir_uid) ||
-	    uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)))
+	    uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)) ||
+	    (created && inode_owner_or_capable(mnt_userns, inode)))
 		return 0;
 
 	if (likely(dir_mode & 0002) ||
@@ -3496,7 +3498,8 @@ static int do_open(struct nameidata *nd,
 		if (d_is_dir(nd->path.dentry))
 			return -EISDIR;
 		error = may_create_in_sticky(mnt_userns, nd,
-					     d_backing_inode(nd->path.dentry));
+					     d_backing_inode(nd->path.dentry),
+					     (file->f_mode & FMODE_CREATED));
 		if (unlikely(error))
 			return error;
 	}
-- 
2.37.1