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Tue, 01 Oct 2024 04:20:05 -0700 (PDT) Received: from [10.125.226.166] ([185.25.67.249]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a93c2776d92sm688817266b.29.2024.10.01.04.20.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 01 Oct 2024 04:20:04 -0700 (PDT) Message-ID: Date: Tue, 1 Oct 2024 12:20:02 +0100 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 13/14] x86: BHI stubs To: Peter Zijlstra , Josh Poimboeuf Cc: x86@kernel.org, linux-kernel@vger.kernel.org, alyssa.milburn@intel.com, scott.d.constable@intel.com, joao@overdrivepizza.com, jose.marchesi@oracle.com, hjl.tools@gmail.com, ndesaulniers@google.com, samitolvanen@google.com, nathan@kernel.org, ojeda@kernel.org, kees@kernel.org, alexei.starovoitov@gmail.com References: <20240927194856.096003183@infradead.org> <20240927194925.707462984@infradead.org> <20240930213030.ixbsyzziy6frh62f@treble> <54d392d3-32b3-4832-89e1-d2ada1af22a8@citrix.com> <20240930223848.ulipiky3uw52ej56@treble> <20241001110310.GM5594@noisy.programming.kicks-ass.net> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 01/10/2024 12:03 pm, Peter Zijlstra wrote: > On Mon, Sep 30, 2024 at 03:38:48PM -0700, Josh Poimboeuf wrote: >> On Mon, Sep 30, 2024 at 11:23:38PM +0100, Andrew Cooper wrote: >>> On 30/09/2024 10:30 pm, Josh Poimboeuf wrote: >>>> On Fri, Sep 27, 2024 at 09:49:09PM +0200, Peter Zijlstra wrote: >>>>> +SYM_INNER_LABEL(__bhi_args_0, SYM_L_LOCAL) >>>>> + UNWIND_HINT_FUNC >>>>> + cmovne %r10, %rdi >>>> IIUC, this works because if the "jz" in the CFI preamble mispredicts to >>>> the __bhi_args_* code, "cmovne" will zero out the speculative value of >>>> rdi. >>>> >>>> Why use %r10 instead of a literal $0? Also how do you know %r10 is 0? >>> There's no encoding for CMOVcc which takes an $imm. >> Ah. >> >>> %r10 is guaranteed zero after the FineIBT prologue >> If the "jz" in the FineIBT prologue mispredicts, isn't %r10 non-zero by >> definition? > Since I just wrote the comment... > > * FineIBT-BHI: > * > * __cfi_foo: > * endbr64 > * subl 0x12345678, %r10d > * jz foo-1 > * ud2 > * foo-1: > * call __bhi_args_XXX > * foo+4: > * ... code here ... > * ret > * > * direct caller: > * call foo+4 > * > * indirect caller: > * lea foo(%rip), %r11 > * ... > * movl $0x12345678, %r10d > * subl $16, %r11 A compiler would normally spell this:     lea foo-16(%rip), %r11 > * nop4 > * call *%r11 > > And lets take a random bhi function: > > + .align 16 > +SYM_INNER_LABEL(__bhi_args_0_1, SYM_L_LOCAL) > + UNWIND_HINT_FUNC > + cmovne %r10, %rdi > + cmovne %r10, %rsi > + ANNOTATE_UNRET_SAFE > + ret > + int3 > > So the case you worry about is SUBL does *not* result in 0, but we > speculate JZ true and end up in CALL, and do CMOVne. > > Since we speculated Z, we must then also not do the CMOV, so the value > of R10 is irrelevant, it will not be used. The thing however is that > CMOV will unconditionally put a store dependency on the target register > (RDI, RSI in the above sequence) and as such any further speculative > code trying to use those registers will stall. How does that help? The write dependency doesn't stop a dependent load from executing in the shadow of a mispredicted branch. This is why SLH forces the pointer to 0 on the bad speculation path. ~Andrew