[PATCH v4] Fixes: null pointer dereference in pfnmap_lockdep_assert

[Manas via B4 Relay]

From: Manas <manas18244@xxxxxxxxxxx>

syzbot has pointed to a possible null pointer dereference in
pfnmap_lockdep_assert. vm_file member of vm_area_struct is being
dereferenced without any checks.

This fix assigns mapping only if vm_file is not NULL.

Reported-by: syzbot+093d096417e7038a689b@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
---
This bug[1] triggers a general protection fault in follow_pfnmap_start
function. An assertion pfnmap_lockdep_assert inside this function
dereferences vm_file member of vm_area_struct. And panic gets triggered
when vm_file is NULL.

This patch assigns mapping only if vm_file is not NULL.

[1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b

Signed-off-by: Manas <manas18244@xxxxxxxxxxx>
---
Changes in v4:
- v4: simplify ternary conditional
- Link to v3: https://lore.kernel.org/r/20241004-fix-null-deref-v3-1-f9459b1cc95f@xxxxxxxxxxx

Changes in v3:
- v3: use assigned var instead of accessing member again 
- Link to v2: https://lore.kernel.org/r/20241004-fix-null-deref-v2-1-23ad90999cd1@xxxxxxxxxxx

Changes in v2:
- v2: use ternary operator according to feedback
- Link to v1: https://lore.kernel.org/r/20241003-fix-null-deref-v1-1-0a45df9d016a@xxxxxxxxxxx
---
 mm/memory.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 2366578015ad..4e4d598496a8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6346,10 +6346,13 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
 static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
 {
 #ifdef CONFIG_LOCKDEP
-	struct address_space *mapping = vma->vm_file->f_mapping;
+	struct address_space *mapping = NULL;
+
+	if (vma->vm_file)
+		mapping = vma->vm_file->f_mapping;
 
 	if (mapping)
-		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
+		lockdep_assert(lockdep_is_held(&mapping->i_mmap_rwsem) ||
 			       lockdep_is_held(&vma->vm_mm->mmap_lock));
 	else
 		lockdep_assert(lockdep_is_held(&vma->vm_mm->mmap_lock));

---
base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
change-id: 20241003-fix-null-deref-6bfa0337efc3

Best regards,
-- 
Manas <manas18244@xxxxxxxxxxx>