[syzbot] kernel panic: corrupted stack end in openat

[Arnd Bergmann]

On Wed, Mar 17, 2021 at 8:52 AM Dmitry Vyukov <dvyukov at google.com> wrote:
> On Tue, Mar 16, 2021 at 5:28 PM Arnd Bergmann <arnd at arndb.de> wrote:
> > On Tue, Mar 16, 2021 at 5:13 PM Dmitry Vyukov <dvyukov at google.com> wrote:
> > > On Tue, Mar 16, 2021 at 5:03 PM Arnd Bergmann <arnd at arndb.de> wrote:
> > > > On Tue, Mar 16, 2021 at 4:51 PM Russell King - ARM Linux admin
> > > > <linux at armlinux.org.uk> wrote:
> > > > > On Tue, Mar 16, 2021 at 04:44:45PM +0100, Arnd Bergmann wrote:
> > > > > > On Tue, Mar 16, 2021 at 11:17 AM Dmitry Vyukov <dvyukov at google.com> wrote:
> > > > > > > The compiler is gcc version 10.2.1 20210110 (Debian 10.2.1-6)
> > > > > >
> > > > > > Ok, building with Ubuntu 10.2.1-1ubuntu1 20201207 locally, that's
> > > > > > the closest I have installed, and I think the Debian and Ubuntu versions
> > > > > > are generally quite close in case of gcc since they are maintained by
> > > > > > the same packagers.
> > > > >
> > > > > ... which shouldn't be a problem - that's just over 1/4 of the stack
> > > > > space. Could it be the syzbot's gcc is doing something weird and
> > > > > inflating the stack frames?
> > > >
> > > > It's possible, I think that's really unlikely given that it's just Debian's
> > > > gcc, which is as close to mainline as the version I was using.
> > > >
> > > > Uwe's DEBUG_STACKOVERFLOW patch from a while ago might
> > > > help if this was the problem though:
> > > > https://lore.kernel.org/linux-arm-kernel/20200108082913.29710-1-u.kleine-koenig@pengutronix.de/
> > > >
> > > > My best guess is something going wrong in the interrupt
> > > > that triggered the preempt_schedule() which ended up calling
> > > > task_stack_end_corrupted() in schedule_debug(), as you suggested
> > > > earlier.
> > >
> > > FWIW I see slightly larger frames with the config:
> > >
> > > 073ab64 <ima_calc_field_array_hash_tfm>:
> > > 8073ab64:       e1a0c00d        mov     ip, sp
> > > 8073ab68:       e92ddff0        push    {r4, r5, r6, r7, r8, r9, sl,
> > > fp, ip, lr, pc}
> > > 8073ab6c:       e24cb004        sub     fp, ip, #4
> > > 8073ab70:       e24ddfa7        sub     sp, sp, #668    ; 0x29c
> >
> > Yes, this is the one that the compiler complained about when warning
> > for stack over 600 bytes. It's not called in this call chain though.
> >
> > > page_alloc can also do reclaim, I had the impression that reclaim can
> > > be quite heavy-weight in all respects.
> >
> > Yes, that is another possibility. What writable file systems or swap
> > do you normally have mounted that it could be writing to, and on
> > what storage device?
>
> The root fs is ext4 on virtio-blk.
>
> There are also several dozens of shrinkers that can be called during reclaim:
> https://elixir.bootlin.com/linux/latest/C/ident/unregister_shrinker

Right, unfortunately I don't see a smoking gun there either, unless you are
also using NFS or devicemapper.

Implementing VMAP_STACK as you suggested earlier is probably the
best way to figure out if there is an actual overrun of the stack.
Alternatively, adding support for GCC_PLUGIN_STACKLEAK might
also help find out if we ever get close to the limit. This is probably
less work, but it might not actually help in this case.

        Arnd