Re: [BUG] kernel BUG at fs/userfaultfd.c:385 after 04f5866e41fb

[Oleg Nesterov <oleg@redhat.com>]

On 08/15, Kefeng Wang wrote:
>
> On 2019/8/14 23:41, Oleg Nesterov wrote:
> >
> > Heh, I didn't notice you too mentioned userfaultfd_release() in your email.
> > can you try the patch below?
>
> Your patch below fixes the issue, could you send a formal patch ASAP and also it
> should be queued into stable, I have test lts4.4, it works too, thanks.

Thanks.

Yes, I _think_ we need something like this patch anyway, but it needs more
discussion. And it is not clear if it really fixes this issue or it hides
another bug.


> I built kernel with wrong gcc version, and the KASAN is not enabled, When KASAN enabled,
> there is an UAF,
>
> [   67.393442] ==================================================================
> [   67.395531] BUG: KASAN: use-after-free in handle_userfault+0x12f/0xc70
> [   67.397001] Read of size 8 at addr ffff8883c622c160 by task syz-executor.9/5225

OK, thanks this probably confirms that .ctx points to nowhere because it
was freed by userfaultfd_release() without clearing vm_flags/userfaultfd_ctx.

But,

> [   67.430243] RIP: 0010:copy_user_handle_tail+0x2/0x10
> [   67.431586] Code: c3 0f 1f 80 00 00 00 00 66 66 90 83 fa 40 0f 82 70 ff ff ff 89 d1 f3 a4 31 c0 66 66 90 c3 66 2e 0f 1f 84 00 00 00 00 00 89 d1 <f3> a4 89 c8 66 66 90 c3 66 0f 1f 44 00 00 66 66 90 83 fa 08 0f 82
> [   67.436978] RSP: 0018:ffff8883c4e8f908 EFLAGS: 00010246
> [   67.438743] RAX: 0000000000000001 RBX: 0000000020ffd000 RCX: 0000000000001000
> [   67.441101] RDX: 0000000000001000 RSI: 0000000020ffd000 RDI: ffff8883c0aa4000
> [   67.442865] RBP: 0000000000001000 R08: ffffed1078154a00 R09: 0000000000000000
> [   67.444534] R10: 0000000000000200 R11: ffffed10781549ff R12: ffff8883c0aa4000
> [   67.446216] R13: ffff8883c6096000 R14: ffff88837721f838 R15: ffff8883c6096000
> [   67.448388]  _copy_from_user+0xa1/0xd0
> [   67.449655]  mcopy_atomic+0xb3d/0x1380
> [   67.450991]  ? lock_downgrade+0x3a0/0x3a0
> [   67.452337]  ? mm_alloc_pmd+0x130/0x130
> [   67.453618]  ? __might_fault+0x7d/0xe0
> [   67.454980]  userfaultfd_ioctl+0x14a2/0x1c30

This must not be called after __fput(). So I think there is something else,
may by just an unbalanced userfaultfd_ctx_put(). I dunno, I know nothing
about usefaultfd.

It would be nice to understand what this reproducer does...

Oleg.