Re: kvm crash in 5.14.1?

[Darrick J. Wong]

On Wed, Sep 29, 2021 at 03:21:09PM +0000, Sean Christopherson wrote:
> On Tue, Sep 28, 2021, Stephen wrote:
> > Hello,
> > 
> > I got this crash again on 5.14.7 in the early morning of the 27th.
> > Things hung up shortly after I'd gone to bed. Uptime was 1 day 9 hours 9
> > minutes.
> 
> ...
> 
> > BUG: kernel NULL pointer dereference, address: 0000000000000068
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > PGD 0 P4D 0
> > Oops: 0000 [#1] SMP NOPTI
> > CPU: 21 PID: 8494 Comm: CPU 7/KVM Tainted: G            E     5.14.7 #32
> > Hardware name: Gigabyte Technology Co., Ltd. X570 AORUS ELITE WIFI/X570
> > AORUS ELITE WIFI, BIOS F35 07/08/2021
> > RIP: 0010:internal_get_user_pages_fast+0x738/0xda0
> > Code: 84 24 a0 00 00 00 65 48 2b 04 25 28 00 00 00 0f 85 54 06 00 00 48
> > 81 c4 a8 00 00 00 44 89 e0 5b 5d 41 5c 41 5d 41 5e 41 5f c3 <48> 81 78
> > 68 a0 a3 >
> 
> I haven't reproduced the crash, but the code signature (CMP against an absolute
> address) is quite distinct, and is consistent across all three crashes.  I'm pretty
> sure the issue is that page_is_secretmem() doesn't check for a null page->mapping,
> e.g. if the page is truncated, which IIUC can happen in parallel since gup() doesn't
> hold the lock.
> 
> I think this should fix the problems?
> 
> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
> index 21c3771e6a56..988528b5da43 100644
> --- a/include/linux/secretmem.h
> +++ b/include/linux/secretmem.h
> @@ -23,7 +23,7 @@ static inline bool page_is_secretmem(struct page *page)
>         mapping = (struct address_space *)
>                 ((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS);
> 
> -       if (mapping != page->mapping)
> +       if (!mapping || mapping != page->mapping)

I'll roll this out on my vm host and try to re-run the mass fuzztest
overnight, though IT claims they're going to kill power to the whole
datacenter until Monday(!)...

--D

>                 return false;
> 
>         return mapping->a_ops == &secretmem_aops;