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Re: [PATCH v2 4/5] x86/shadow: fix UB pointer arithmetic in sh_mfn_is_a_page_table()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 18 Mar 2025 15:36:45 +0100
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- Cc: Tim Deegan <tim@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 18 Mar 2025 14:36:58 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 18.03.2025 13:53, Andrew Cooper wrote:
> On 18/03/2025 9:19 am, Roger Pau Monne wrote:
>> UBSAN complains with:
>>
>> UBSAN: Undefined behaviour in arch/x86/mm/shadow/private.h:515:30
>> pointer operation overflowed ffff82e000000000 to ffff82dfffffffe0
>> [...]
>> Xen call trace:
>> [<ffff82d040303882>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0
>> [<ffff82d040304cc3>] F
>> lib/xxhash64.c#__ubsan_handle_pointer_overflow+0xcb/0x100
>> [<ffff82d040471c5d>] F
>> arch/x86/mm/shadow/guest_2.c#sh_page_fault__guest_2+0x1e350
>> [<ffff82d0403b216b>] F lib/xxhash64.c#svm_vmexit_handler+0xdf3/0x2450
>> [<ffff82d0402049c0>] F lib/xxhash64.c#svm_stgi_label+0x5/0x15
>
> Something is definitely wonky in this backtrace.
>
>>
>> Fix by moving the call to mfn_to_page() after the check of whether the
>> passed gmfn is valid. This avoid the call to mfn_to_page() with an
>> INVALID_MFN parameter.
>>
>> While there make the page local variable const, it's not modified by the
>> function.
>>
>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>
> Whatever is wonky in the backtrace isn't related to this patch, so
> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, but the backtrace
> does want fixing.
Right, but the fix may need to be in the tool chain. I'd be curious what
the symbol table looks like that this was created from. Roger, was this
linked with GNU ld or LLVM? Are the filename anomalies also visible in
the corresponding xen-syms.map?
Jan
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