Re: [PATCH] fs/buffer: fix a NULL pointer dereference in drop_buffers()

[Matthew Wilcox]

On Wed, Nov 09, 2022 at 05:50:18PM +0800, Liu Shixin wrote:
> syzbot found a null-ptr-deref by KASAN:
> 
>  BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
>  BUG: KASAN: null-ptr-deref in atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
>  BUG: KASAN: null-ptr-deref in buffer_busy fs/buffer.c:2856 [inline]
>  BUG: KASAN: null-ptr-deref in drop_buffers+0x61/0x2f0 fs/buffer.c:2868
>  Read of size 4 at addr 0000000000000060 by task syz-executor.5/24786
> 
>  CPU: 0 PID: 24786 Comm: syz-executor.5 Not tainted 6.0.0-syzkaller-09589-g55be6084c8e0 #0
>  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/22/2022
>  Call Trace:
>   <TASK>
>   __dump_stack lib/dump_stack.c:88 [inline]
>   dump_stack_lvl+0x1e3/0x2cb lib/dump_stack.c:106
>   print_report+0xf1/0x220 mm/kasan/report.c:436
>   kasan_report+0xfb/0x130 mm/kasan/report.c:495
>   kasan_check_range+0x2a7/0x2e0 mm/kasan/generic.c:189
>   instrument_atomic_read include/linux/instrumented.h:71 [inline]
>   atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
>   buffer_busy fs/buffer.c:2856 [inline]
>   drop_buffers+0x61/0x2f0 fs/buffer.c:2868
>   try_to_free_buffers+0x2b1/0x640 fs/buffer.c:2898
> [...]
> 
> We use folio_has_private() to decide whether call filemap_release_folio(),
> which may call try_to_free_buffers() then. folio_has_private() return true
> for both PG_private and PG_private_2. We should only call try_to_free_buffers()
> for case PG_private. So we should recheck PG_private in try_to_free_buffers().
> 
> Reported-by: syzbot+fbdb4ec578ebdcfb9ed2@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 266cf658efcf ("FS-Cache: Recruit a page flags for cache management")

but this can only happen for a filesystem which uses both bufferheads
and PG_private_2.  afaik there aren't any of those in the tree.  so
this bug can't actually happen.

if you have your own filesystem that does, you need to submit it.