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Date:   Mon, 4 Oct 2021 09:54:32 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Stephen <stephenackerman16@gmail.com>, kvm@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: kvm crash in 5.14.1?
Message-ID: <20211004165432.GA24266@magnolia>
References: <2b5ca6d3-fa7b-5e2f-c353-f07dcff993c1@gmail.com>
 <16c7a433-6e58-4213-bc00-5f6196fe22f5@gmail.com>
 <YVSEZTCbFZ+HD/f0@google.com>
 <20210930175957.GA10573@magnolia>
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On Thu, Sep 30, 2021 at 10:59:57AM -0700, Darrick J. Wong wrote:
> 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(!)...

...which they did, 30 minutes after I sent this email. :(

I'll hopefully be able to report back to the list in a day or two.

--D

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