mm/shmem: make shmem_file() check the file's inode

shmem_file() only checks if file->f_mapping is a shmem mapping. This
mapping can belong to another file, however, as with a Coda file backed
by a tmpfs container after mmap().

So it does not guarantee that the file inode is an actual shmem inode.

Seal handling and memfd preservation use this check before accessing
shmem inode state, so they can access an inode of the wrong type.

Let's make shmem_file() check the inode's file operations instead, to
identify shmem inodes reliably. This will identify regular shmem inodes,
including memfds and ordinary tmpfs files, independently of wrapper file
operations.

Use shmem_mapping() in collapse_file() instead to not change the behavior
of collapse_file(): it operates on the page cache and uses mapping->host
for shmem inode operations. Focusing on the file does not seem relevant.

Fixes: c49fcfcda8b5 ("mm/shmem: update file sealing comments and file checking")
Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
Cc: stable@vger.kernel.org
Reported-by: Chris Mason <mason@kernel.org>
Suggested-by: Pratyush Yadav <pratyush@kernel.org>
Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Link: https://patch.msgid.link/20261031-fixes_core-v3-3-d6437171ecc3@kernel.org
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

1 files changed, 6 insertions, 9 deletions

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index a7c7a96a7cbf..f89b4b151133 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -117,8 +117,14 @@ extern unsigned long shmem_get_unmapped_area(struct file *, unsigned long addr,
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 extern int shmem_lock(struct file *file, int lock, struct ucounts *ucounts);
 #ifdef CONFIG_SHMEM
+bool shmem_file(struct file *file);
 bool shmem_mapping(const struct address_space *mapping);
 #else
+static inline bool shmem_file(struct file *file)
+{
+	return false;
+}
+
 static inline bool shmem_mapping(const struct address_space *mapping)
 {
 	return false;
@@ -193,15 +199,6 @@ static inline struct page *shmem_read_mapping_page(
 					mapping_gfp_mask(mapping));
 }
 
-static inline bool shmem_file(struct file *file)
-{
-	if (!IS_ENABLED(CONFIG_SHMEM))
-		return false;
-	if (!file || !file->f_mapping)
-		return false;
-	return shmem_mapping(file->f_mapping);
-}
-
 /* Must be called with inode lock taken exclusive. */
 static inline void shmem_freeze(struct inode *inode, bool freeze)
 {