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Tue, 05 Nov 2024 07:19:25 -0800 (PST) Received: from [10.125.226.166] ([185.25.67.249]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5cee6b0fc4dsm1427225a12.84.2024.11.05.07.19.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 05 Nov 2024 07:19:24 -0800 (PST) Message-ID: Date: Tue, 5 Nov 2024 15:19:23 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 5/8] x86/traps: Cleanup and robustify decode_bug() To: Peter Zijlstra , x86@kernel.org Cc: linux-kernel@vger.kernel.org, alyssa.milburn@intel.com, scott.d.constable@intel.com, joao@overdrivepizza.com, jpoimboe@kernel.org, alexei.starovoitov@gmail.com, ebiggers@kernel.org, samitolvanen@google.com, kees@kernel.org References: <20241105113901.348320374@infradead.org> <20241105114522.285032152@infradead.org> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 05/11/2024 11:39 am, Peter Zijlstra wrote: > --- a/arch/x86/kernel/traps.c > +++ b/arch/x86/kernel/traps.c > @@ -110,24 +117,37 @@ __always_inline int decode_bug(unsigned > return BUG_NONE; > > v = *(u8 *)(addr++); > - if (v == SECOND_BYTE_OPCODE_UD2) > + if (v == SECOND_BYTE_OPCODE_UD2) { > + *len = addr - start; > return BUG_UD2; > + } > > - if (!IS_ENABLED(CONFIG_UBSAN_TRAP) || v != SECOND_BYTE_OPCODE_UD1) > + if (v != SECOND_BYTE_OPCODE_UD1) > return BUG_NONE; > > - /* Retrieve the immediate (type value) for the UBSAN UD1 */ > - v = *(u8 *)(addr++); > - if (X86_MODRM_RM(v) == 4) > - addr++; > - > *imm = 0; > - if (X86_MODRM_MOD(v) == 1) > - *imm = *(u8 *)addr; > - else if (X86_MODRM_MOD(v) == 2) > - *imm = *(u32 *)addr; > - else > - WARN_ONCE(1, "Unexpected MODRM_MOD: %u\n", X86_MODRM_MOD(v)); > + v = *(u8 *)(addr++); /* ModRM */ > + > + /* Decode immediate, if present */ > + if (X86_MODRM_MOD(v) != 3) { > + if (X86_MODRM_RM(v) == 4) > + addr++; /* Skip SIB byte */ > + > + if (X86_MODRM_MOD(v) == 1) { > + *imm = *(s8 *)addr; > + addr += 1; > + > + } else if (X86_MODRM_MOD(v) == 2) { > + *imm = *(s32 *)addr; > + addr += 4; > + } > + } > + > + /* record instruction length */ > + *len = addr - start; `ud1 0(%rip),%eax` has something to say about this length calculation[1]. You need the Mod = 0, RM = 5 case wired into addr += 4 without filling in imm. ~Andrew [1] or maybe you've got something rude to say about those of us who encode instructions like that...[2] [2] It's perhaps fortunate that decode_bug() doesn't know what a REX prefix is.