lkml.org 
[lkml]   [2020]   [Jul]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] binder: Don't use mmput() from shrinker function.
On Thu 16-07-20 08:36:52, Tetsuo Handa wrote:
> syzbot is reporting that mmput() from shrinker function has a risk of
> deadlock [1]. Don't start synchronous teardown of mm when called from
> shrinker function.

Please add the actual lock dependency to the changelog.

Anyway is this deadlock real? Mayve I have missed some details but the
call graph points to these two paths.
uprobe_mmap do_shrink_slab
uprobes_mmap_hash #lock
install_breakpoint binder_shrink_scan
set_swbp binder_alloc_free_page
uprobe_write_opcode __mmput
update_ref_ctr uprobe_clear_state
mutex_lock(&delayed_uprobe_lock) mutex_lock(&delayed_uprobe_lock);
allocation -> reclaim

But in order for this to happen the shrinker would have to do the last
put on the mm. But mm cannot go away from under uprobe_mmap so those two
paths cannot race with each other.

Unless I am missing something this is a false positive. I do not mind
using mmput_async from the shrinker as a workaround but the changelog
should be explicit about the fact.

> [1] https://syzkaller.appspot.com/bug?id=bc9e7303f537c41b2b0cc2dfcea3fc42964c2d45
>
> Reported-by: syzbot <syzbot+1068f09c44d151250c33@syzkaller.appspotmail.com>
> Reported-by: syzbot <syzbot+e5344baa319c9a96edec@syzkaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
> drivers/android/binder_alloc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> index 42c672f1584e..cbe6aa77d50d 100644
> --- a/drivers/android/binder_alloc.c
> +++ b/drivers/android/binder_alloc.c
> @@ -947,7 +947,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
> trace_binder_unmap_user_end(alloc, index);
> }
> mmap_read_unlock(mm);
> - mmput(mm);
> + mmput_async(mm);
>
> trace_binder_unmap_kernel_start(alloc, index);
>
> --
> 2.18.4
>

--
Michal Hocko
SUSE Labs

\
 
 \ /
  Last update: 2020-07-16 10:36    [W:0.050 / U:0.824 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site