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Peter Anvin" , Peter Zijlstra , Josh Poimboeuf , Andy Lutomirski , Jonathan Corbet , Sean Christopherson , Paolo Bonzini , tony.luck@intel.com, ak@linux.intel.com, tim.c.chen@linux.intel.com Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kvm@vger.kernel.org, Alyssa Milburn , Daniel Sneddon , antonio.gomez.iglesias@linux.intel.com, Pawan Gupta , Alyssa Milburn Subject: [PATCH 1/6] x86/bugs: Add asm helpers for executing VERW Message-ID: <20231020-delay-verw-v1-1-cff54096326d@linux.intel.com> X-Mailer: b4 0.12.3 References: <20231020-delay-verw-v1-0-cff54096326d@linux.intel.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20231020-delay-verw-v1-0-cff54096326d@linux.intel.com> X-Spam-Status: No, score=-0.8 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on agentk.vger.email Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (agentk.vger.email [0.0.0.0]); Fri, 20 Oct 2023 13:45:33 -0700 (PDT) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1780308855887674140 X-GMAIL-MSGID: 1780308855887674140 MDS mitigation requires clearing the CPU buffers before returning to user. This needs to be done late in the exit-to-user path. Current location of VERW leaves a possibility of kernel data ending up in CPU buffers for memory accesses done after VERW such as: 1. Kernel data accessed by an NMI between VERW and return-to-user can remain in CPU buffers ( since NMI returning to kernel does not execute VERW to clear CPU buffers. 2. Alyssa reported that after VERW is executed, CONFIG_GCC_PLUGIN_STACKLEAK=y scrubs the stack used by a system call. Memory accesses during stack scrubbing can move kernel stack contents into CPU buffers. 3. When caller saved registers are restored after a return from function executing VERW, the kernel stack accesses can remain in CPU buffers(since they occur after VERW). To fix this VERW needs to be moved very late in exit-to-user path. In preparation for moving VERW to entry/exit asm code, create macros that can be used in asm. Also make them depend on a new feature flag X86_FEATURE_USER_CLEAR_CPU_BUF. Reported-by: Alyssa Milburn Signed-off-by: Pawan Gupta --- arch/x86/include/asm/cpufeatures.h | 2 +- arch/x86/include/asm/nospec-branch.h | 25 +++++++++++++++++++++++++ 2 files changed, 26 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h index 58cb9495e40f..3f018dfedd5f 100644 --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -308,10 +308,10 @@ #define X86_FEATURE_SMBA (11*32+21) /* "" Slow Memory Bandwidth Allocation */ #define X86_FEATURE_BMEC (11*32+22) /* "" Bandwidth Monitoring Event Configuration */ #define X86_FEATURE_USER_SHSTK (11*32+23) /* Shadow stack support for user mode applications */ - #define X86_FEATURE_SRSO (11*32+24) /* "" AMD BTB untrain RETs */ #define X86_FEATURE_SRSO_ALIAS (11*32+25) /* "" AMD BTB untrain RETs through aliasing */ #define X86_FEATURE_IBPB_ON_VMEXIT (11*32+26) /* "" Issue an IBPB only on VMEXIT */ +#define X86_FEATURE_USER_CLEAR_CPU_BUF (11*32+27) /* "" Clear CPU buffers before returning to user */ /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */ #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */ diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index c55cc243592e..e1b623a27e1b 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -111,6 +111,24 @@ #define RESET_CALL_DEPTH_FROM_CALL #endif +/* + * Macro to execute VERW instruction to mitigate transient data sampling + * attacks such as MDS. On affected systems a microcode update overloaded VERW + * instruction to also clear the CPU buffers. + * + * Note: Only the memory operand variant of VERW clears the CPU buffers. To + * handle the case when VERW is executed after user registers are restored, use + * RIP to point the memory operand to a part NOPL instruction that contains + * __KERNEL_DS. + */ +#define __EXEC_VERW(m) verw _ASM_RIP(m) + +#define EXEC_VERW \ + __EXEC_VERW(551f); \ + /* nopl __KERNEL_DS(%rax) */ \ + .byte 0x0f, 0x1f, 0x80, 0x00, 0x00; \ +551: .word __KERNEL_DS; \ + /* * Fill the CPU return stack buffer. * @@ -329,6 +347,13 @@ #endif .endm +/* Clear CPU buffers before returning to user */ +.macro USER_CLEAR_CPU_BUFFERS + ALTERNATIVE "jmp .Lskip_verw_\@;", "", X86_FEATURE_USER_CLEAR_CPU_BUF + EXEC_VERW +.Lskip_verw_\@: +.endm + #else /* __ASSEMBLY__ */ #define ANNOTATE_RETPOLINE_SAFE \