[PATCH v2] mm/filemap: fix NULL pointer dereference in do_read_cache_folio()

[ssrane_b23@ee.vjti.ac.in]

From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>

When read_cache_folio() is called with a NULL filler function on a mapping
that doesn't implement read_folio, a NULL pointer dereference occurs in
filemap_read_folio().

The crash occurs when:
1. build_id_parse() is called on a VMA backed by a file from a filesystem
   that doesn't implement address_space_operations.read_folio (e.g., procfs,
   sysfs, or other virtual filesystems)
2. read_cache_folio() is called with filler=NULL
3. do_read_cache_folio() sets filler = mapping->a_ops->read_folio (still NULL)
4. filemap_read_folio() calls filler() causing a NULL pointer dereference

The fix is to add a NULL check after the fallback assignment and return
-EIO, which is handled gracefully by the callers.

Reported-by: syzbot+09b7d050e4806540153d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=09b7d050e4806540153d
Fixes: ad41251c290d ("lib/buildid: implement sleepable build_id_parse() API")
Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
---
Changes in v2:
- Add early read_folio check in __build_id_parse() for unsupported filesystems.
-Add defensive !filler check in do_read_cache_folio() to prevent NULL dereference.
---
 lib/buildid.c | 3 +++
 mm/filemap.c  | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/lib/buildid.c b/lib/buildid.c
index c4b0f376fb34..3136cc92010f 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -298,6 +298,9 @@ static int __build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
 	/* only works for page backed storage  */
 	if (!vma->vm_file)
 		return -EINVAL;
+	/* check if filesystem supports page cache operations */
+	if (!vma->vm_file->f_mapping->a_ops->read_folio)
+		return -EINVAL;
 
 	freader_init_from_file(&r, buf, sizeof(buf), vma->vm_file, may_fault);
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 13f0259d993c..f700fe931d61 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3980,6 +3980,8 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
 
 	if (!filler)
 		filler = mapping->a_ops->read_folio;
+	if (!filler)
+		return ERR_PTR(-EIO);
 repeat:
 	folio = filemap_get_folio(mapping, index);
 	if (IS_ERR(folio)) {
-- 
2.34.1