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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id p17sm197322pfo.9.2021.09.29.08.21.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Sep 2021 08:21:13 -0700 (PDT) Date: Wed, 29 Sep 2021 15:21:09 +0000 From: Sean Christopherson To: Stephen Cc: djwong@kernel.org, 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: References: <2b5ca6d3-fa7b-5e2f-c353-f07dcff993c1@gmail.com> <16c7a433-6e58-4213-bc00-5f6196fe22f5@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <16c7a433-6e58-4213-bc00-5f6196fe22f5@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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) return false; return mapping->a_ops == &secretmem_aops;