[syzbot] kernel panic: corrupted stack end in openat

[Dmitry Vyukov]

On Wed, Mar 17, 2021 at 9:32 AM Arnd Bergmann <arnd at arndb.de> wrote:
> > > > > <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.

VMAP_STACK is quite intrusive as far as I understand. For KASAN I
considered a simpler option: have a debug config that allocates an
extra page after the stack and mprotect's it. It wastes a physical
page per task (fine for a debug config), but I would assume should be
radically simpler to implement. In the end somebody implemented proper
VMAP_STACK support for KASAN, but I still think it may be a reasonable
compromise between time investment and value.