[PATCH] fs/stat: reject invalid flags for empty paths

[Avi Weiss <thnkslprpt@gmail.com>]

vfs_fstatat() and statx() take the file descriptor fast path when
AT_EMPTY_PATH is set, even if unsupported flag bits are present.
This bypasses vfs_statx() flag validation and lets both syscalls
succeed with AT_RECURSIVE.

Restrict the fast paths to supported flags so invalid combinations
fall through to vfs_statx() and return -EINVAL.

Fixes: e896474fe485 ("getname_maybe_null() - the third variant of pathname copy-in")
Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
---
 fs/stat.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/stat.c b/fs/stat.c
index a2e63c6c45b7..b75a411b502f 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -371,7 +371,9 @@ int vfs_fstatat(int dfd, const char __user *filename,
 {
 	CLASS(filename_maybe_null, name)(filename, flags);
 
-	if (!name && dfd >= 0)
+	if (!name && dfd >= 0 &&
+	    !(flags & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
+			AT_EMPTY_PATH | AT_STATX_SYNC_TYPE)))
 		return vfs_fstat(dfd, stat);
 
 	return vfs_statx(dfd, name, flags | AT_NO_AUTOMOUNT,
@@ -812,7 +814,9 @@ SYSCALL_DEFINE5(statx,
 {
 	CLASS(filename_maybe_null, name)(filename, flags);
 
-	if (!name && dfd >= 0)
+	if (!name && dfd >= 0 &&
+	    !(flags & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
+			AT_EMPTY_PATH | AT_STATX_SYNC_TYPE)))
 		return do_statx_fd(dfd, flags & ~AT_NO_AUTOMOUNT, mask, buffer);
 
 	return do_statx(dfd, name, flags, mask, buffer);

base-commit: 36aad21d50d1af3f6954bb0c988257a908202477
-- 
2.43.0