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Tue, 18 Mar 2025 08:27:06 -0700 (PDT) Date: Tue, 18 Mar 2025 16:27:01 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Andrew Cooper Cc: xen-devel@lists.xenproject.org, Jan Beulich , Tim Deegan Subject: Re: [PATCH v2 4/5] x86/shadow: fix UB pointer arithmetic in sh_mfn_is_a_page_table() Message-ID: References: <20250318091904.52903-1-roger.pau@citrix.com> <20250318091904.52903-5-roger.pau@citrix.com> <1871a0ff-5766-4707-8791-c20279c12fd9@citrix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1871a0ff-5766-4707-8791-c20279c12fd9@citrix.com> On Tue, Mar 18, 2025 at 12:53:30PM +0000, 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: > > [] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0 > > [] F lib/xxhash64.c#__ubsan_handle_pointer_overflow+0xcb/0x100 > > [] F arch/x86/mm/shadow/guest_2.c#sh_page_fault__guest_2+0x1e350 > > [] F lib/xxhash64.c#svm_vmexit_handler+0xdf3/0x2450 > > [] F lib/xxhash64.c#svm_stgi_label+0x5/0x15 > > Something is definitely wonky in this backtrace. Oh, yes, it's a TODO I have pending when using LLVM LD. I sent a fix very long time ago, but it was quite ugly. > > > > 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 > > Whatever is wonky in the backtrace isn't related to this patch, so > Acked-by: Andrew Cooper , but the backtrace > does want fixing. I can get the proper backtrace using clang + GNU LD. Thanks, Roger.