Hi Peter, thanks for reviewing.
On 03.10.2024 12:41, Peter Xu wrote:
On Thu, Oct 03, 2024 at 09:31:06PM +0530, Manas via B4 Relay wrote:
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 returns if vm_file member in vm_area_struct is 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 returns from the assertion pfnmap_lockdep_assert if vm_file
is found to be NULL.
[1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
Signed-off-by: Manas <manas18244@xxxxxxxxxxx>
Thanks for the patch!
---
mm/memory.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/mm/memory.c b/mm/memory.c
index 2366578015ad..b152a95e543f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6346,6 +6346,9 @@ 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
+ if (!vma->vm_file)
+ return;
+
Hmm I guess I wasn't careful enough here as I was mostly only thinking
about file mappings, but I just notice we have other pfnmaps like the vvar
mappings.. the mapping var can also already be reused later when available.
Logically even if !vm_file we can still check against mmap write lock. So
would it be better to do this instead:
struct address_space *mapping = vma->vm_file && vma->vm_file->f_mapping;
This will lead to `-Wint-conversion` error in the assignment. We can either do a
cast like the following:
struct address_space *mapping = (struct address_space *)(vma->vm_file && vma->vm_file->f_mapping);
But I am not sure if it is the canonical way of doing it. It will also lead to
warning about pointer from integer casting.
Or will a conditional like this work here?
struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
if (mapping)
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));
?
struct address_space *mapping = vma->vm_file->f_mapping;
if (mapping)
---
base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
change-id: 20241003-fix-null-deref-6bfa0337efc3
Best regards,
--
Manas <manas18244@xxxxxxxxxxx>
--
Peter Xu
--
Manas
