[PATCH 6.6.y] mm/shmem: make shmem_file() check the file's inode

[Sasha Levin <sashal@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

[ Upstream commit d3710111839d11339e414b1bb3fea3d05ab83f6e ]

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>
[ sashal: Adjusted context to preserve the existing inline shmem_mapping() helper and surrounding declarations. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/shmem_fs.h | 15 ++++++---------
 mm/khugepaged.c          |  2 +-
 mm/shmem.c               |  5 +++++
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 134c686c8676c..e4b24b4442bb9 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -93,12 +93,18 @@ 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);
 extern const struct address_space_operations shmem_aops;
 static inline bool shmem_mapping(struct address_space *mapping)
 {
 	return mapping->a_ops == &shmem_aops;
 }
 #else
+static inline bool shmem_file(struct file *file)
+{
+	return false;
+}
+
 static inline bool shmem_mapping(struct address_space *mapping)
 {
 	return false;
@@ -159,15 +165,6 @@ 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);
-}
-
 /*
  * If fallocate(FALLOC_FL_KEEP_SIZE) has been used, there may be pages
  * beyond i_size's notion of EOF, which fallocate has committed to reserving:
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c0a3d09cfd976..dafa740e9bb2d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1785,6 +1785,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 			 struct collapse_control *cc)
 {
 	struct address_space *mapping = file->f_mapping;
+	const bool is_shmem = shmem_mapping(mapping);
 	struct page *page;
 	struct page *tmp, *dst;
 	struct folio *folio, *new_folio;
@@ -1793,7 +1794,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
 	int nr_none = 0, result = SCAN_SUCCEED;
-	bool is_shmem = shmem_file(file);
 
 	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
 	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
diff --git a/mm/shmem.c b/mm/shmem.c
index 0f7c7ec48837a..74517e446f63f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -263,6 +263,11 @@ static const struct vm_operations_struct shmem_vm_ops;
 static const struct vm_operations_struct shmem_anon_vm_ops;
 static struct file_system_type shmem_fs_type;
 
+bool shmem_file(struct file *file)
+{
+	return file && file_inode(file)->i_fop == &shmem_file_operations;
+}
+
 bool vma_is_anon_shmem(struct vm_area_struct *vma)
 {
 	return vma->vm_ops == &shmem_anon_vm_ops;
-- 
2.53.0