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([2001:b07:6468:f312:9af8:e5f5:7516:fa89]) by smtp.googlemail.com with ESMTPSA id by28-20020a0564021b1c00b0043acddee068sm881292edb.83.2022.07.14.04.36.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 Jul 2022 04:36:25 -0700 (PDT) Message-ID: <976510d2-c7ad-2108-27e0-4c3b82c210f1@redhat.com> Date: Thu, 14 Jul 2022 13:36:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.10.0 Subject: Re: [PATCH] x86/kvm: fix FASTOP_SIZE when return thunks are enabled Content-Language: en-US To: Peter Zijlstra , Thadeu Lima de Souza Cascardo Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org, stable@vger.kernel.org, Greg Kroah-Hartman , Naresh Kamboju , Borislav Petkov , Josh Poimboeuf , Linux Kernel Functional Testing References: <20220713171241.184026-1-cascardo@canonical.com> From: Paolo Bonzini In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/14/22 11:52, Peter Zijlstra wrote: > On Wed, Jul 13, 2022 at 02:12:41PM -0300, Thadeu Lima de Souza Cascardo wrote: >> The return thunk call makes the fastop functions larger, just like IBT >> does. Consider a 16-byte FASTOP_SIZE when CONFIG_RETHUNK is enabled. >> >> Otherwise, functions will be incorrectly aligned and when computing their >> position for differently sized operators, they will executed in the middle >> or end of a function, which may as well be an int3, leading to a crash >> like: > > Bah.. I did the SETcc stuff, but then forgot about the FASTOP :/ > > af2e140f3420 ("x86/kvm: Fix SETcc emulation for return thunks") > >> Fixes: aa3d480315ba ("x86: Use return-thunk in asm code") >> Signed-off-by: Thadeu Lima de Souza Cascardo >> Cc: Peter Zijlstra (Intel) >> Cc: Borislav Petkov >> Cc: Josh Poimboeuf >> Cc: Paolo Bonzini >> Reported-by: Linux Kernel Functional Testing >> --- >> arch/x86/kvm/emulate.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c >> index db96bf7d1122..d779eea1052e 100644 >> --- a/arch/x86/kvm/emulate.c >> +++ b/arch/x86/kvm/emulate.c >> @@ -190,7 +190,7 @@ >> #define X16(x...) X8(x), X8(x) >> >> #define NR_FASTOP (ilog2(sizeof(ulong)) + 1) >> -#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT)) >> +#define FASTOP_SIZE (8 * (1 + (HAS_KERNEL_IBT | IS_ENABLED(CONFIG_RETHUNK)))) > > Would it make sense to do something like this instead? Yes, definitely. Applied with a small tweak to make FASTOP_LENGTH more similar to SETCC_LENGTH: diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index db96bf7d1122..0a15b0fec6d9 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -189,8 +189,12 @@ #define X8(x...) X4(x), X4(x) #define X16(x...) X8(x), X8(x) -#define NR_FASTOP (ilog2(sizeof(ulong)) + 1) -#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT)) +#define NR_FASTOP (ilog2(sizeof(ulong)) + 1) +#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \ + IS_ENABLED(CONFIG_SLS)) +#define FASTOP_LENGTH (ENDBR_INSN_SIZE + 7 + RET_LENGTH) +#define FASTOP_SIZE (8 << ((FASTOP_LENGTH > 8) & 1) << ((FASTOP_LENGTH > 16) & 1)) +static_assert(FASTOP_LENGTH <= FASTOP_SIZE); struct opcode { u64 flags; @@ -442,8 +446,6 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop); * RET | JMP __x86_return_thunk [1,5 bytes; CONFIG_RETHUNK] * INT3 [1 byte; CONFIG_SLS] */ -#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \ - IS_ENABLED(CONFIG_SLS)) #define SETCC_LENGTH (ENDBR_INSN_SIZE + 3 + RET_LENGTH) #define SETCC_ALIGN (4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1)) static_assert(SETCC_LENGTH <= SETCC_ALIGN); Paolo