Re: kvm crash in 5.14.1?

[Sean Christopherson <seanjc@google.com>]

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;