Re: [syzbot] KASAN: slab-out-of-bounds Read in copy_data

[Laurent Vivier]

On 27/10/2021 19:03, Laurent Vivier wrote:
> On 27/10/2021 18:25, Laurent Vivier wrote:
> > On 27/10/2021 17:28, Michael S. Tsirkin wrote:
> > > On Wed, Oct 27, 2021 at 03:36:19PM +0200, Dmitry Vyukov wrote:
> > > > On Wed, 27 Oct 2021 at 15:11, Laurent Vivier <lvivier@xxxxxxxxxx> wrote:
> > > > > On 26/10/2021 18:39, syzbot wrote:
> > > > > > Hello,
> > > > > >
> > > > > > syzbot found the following issue on:
> > > > > >
> > > > > > HEAD commit:    9ae1fbdeabd3 Add linux-next specific files for 20211025
> > > > > > git tree:       linux-next
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1331363cb00000
> > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=aeb17e42bc109064
> > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=b86736b5935e0d25b446
> > > > > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for 
> > > > > > Debian) 2.35.2
> > > > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=116ce954b00000
> > > > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=132fcf62b00000
> > > > > >
> > > > > > The issue was bisected to:
> > > > > >
> > > > > > commit 22849b5ea5952d853547cc5e0651f34a246b2a4f
> > > > > > Author: Leon Romanovsky <leonro@xxxxxxxxxx>
> > > > > > Date:   Thu Oct 21 14:16:14 2021 +0000
> > > > > >
> > > > > >       devlink: Remove not-executed trap policer notifications
> > > > > >
> > > > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=137d8bfcb00000
> > > > > > final oops:     https://syzkaller.appspot.com/x/report.txt?x=10fd8bfcb00000
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=177d8bfcb00000
> > > > > >
> > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > > Reported-by: syzbot+b86736b5935e0d25b446@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > > > Fixes: 22849b5ea595 ("devlink: Remove not-executed trap policer notifications")
> > > > > >
> > > > > > ==================================================================
> > > > > > BUG: KASAN: slab-out-of-bounds in memcpy include/linux/fortify-string.h:225 [inline]
> > > > > > BUG: KASAN: slab-out-of-bounds in copy_data+0xf3/0x2e0 
> > > > > > drivers/char/hw_random/virtio-rng.c:68
> > > > > > Read of size 64 at addr ffff88801a7a1580 by task syz-executor989/6542
> > > > >
> > > > > I'm not able to reproduce the problem with next-20211026 and the C reproducer.
> > > > >
> > > > > And reviewing the code in copy_data() I don't see any issue.
> > > > >
> > > > > Is it possible to know what it the VM configuration used to test it?
> > > >
> > > > Hi Laurent,
> > > >
> > > > syzbot used e2-standard-2 GCE VM when that happened.
> > > > You can see some info about these VMs under the "VM info" link on the dashboard.
> > >
> > > Could you pls confirm whether reverting
> > > caaf2874ba27b92bca6f0298bf88bad94067ec37 addresses this?
> >
> > I've restarted the syzbot on top of "hwrng: virtio - don't wait on cleanup" [1] and the 
> > problem has not been triggered.
> >
> > See https://syzkaller.appspot.com/bug?extid=b86736b5935e0d25b446
>
> The problem seems to be introduced by the last patch:
>
> "hwrng: virtio - always add a pending request"

I think I understand the problem.

As we check data_avail != 0 before waiting on the completion, we can have a data_idx != 0.

The following change fixes the problem for me:

--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -52,6 +52,8 @@ static void request_entropy(struct virtrng_info *vi)
        struct scatterlist sg;

        reinit_completion(&vi->have_data);
+       vi->data_avail = 0;
+       vi->data_idx = 0;

        sg_init_one(&sg, vi->data, sizeof(vi->data));


MST, do you update the patch or do you want I send a new version?

Thanks,
Laurent