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Re: [PATCH v2 4/5] x86/shadow: fix UB pointer arithmetic in sh_mfn_is_a_page_table()
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 18 Mar 2025 12:53:30 +0000
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Tim Deegan <tim@xxxxxxx>
- Delivery-date: Tue, 18 Mar 2025 12:53:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
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