From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3090C1C6AB; Mon, 4 Mar 2024 21:52:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709589166; cv=none; b=GDCwQgTMcpJL75Fh9s/W5xHftZ76QiN8lGGVyuS4G1JXH2uThY4W0QtCz19TaI1qJeWZZMxHsAoT5KATK6wZjgZtmVNmTol/U+Po8TnMn8xrKJIQi32G/ZlvhLeDtaA2HPkmo7XJtmackwRtMXPfGDb8o146SxE2pEx3tFUWCIU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709589166; c=relaxed/simple; bh=zZF/ybR2GrhySrqfjYe6e0a0DyFzEstJtiOjPXxU45w=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=KxdNvxKkw1JrSy8XIEUnXHXtzTL+cQaoBdl2zq1fNbk2qsRild8Egrn1KUWeydddzsQQTYJ7XmwVsAM7z24UOxycAFjI6yQUbcmQhoQwbv966TcMTt3LLkFwqXgw3wLIR9YtCLFtTiLVzhFEdfcEP0qO9QmdBk1lPTn2Q1xVALg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=s6CUqRxP; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="s6CUqRxP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BA408C433F1; Mon, 4 Mar 2024 21:52:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1709589166; bh=zZF/ybR2GrhySrqfjYe6e0a0DyFzEstJtiOjPXxU45w=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=s6CUqRxPjVM+KN7o8xlgf6osIlyWVSsFc1LcOreaCC/xxqvUjMXy5laqhPLffWzwB hDFoA3Axu23AD1CaX1gSBHOCjxqCQ7y4qfnbAwZwmBDIh36GvmMzVGkmKeUVwVjViG KuAJm2AQ7tpjG/nXXCRGOTfEB95GgJGhOK1vKvGY= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Alyssa Milburn , Andrew Cooper , Peter Zijlstra , Pawan Gupta , Dave Hansen Subject: [PATCH 6.1 206/215] x86/bugs: Add asm helpers for executing VERW Date: Mon, 4 Mar 2024 21:24:29 +0000 Message-ID: <20240304211603.525758714@linuxfoundation.org> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20240304211556.993132804@linuxfoundation.org> References: <20240304211556.993132804@linuxfoundation.org> User-Agent: quilt/0.67 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.1-stable review patch. If anyone has any objections, please let me know. ------------------ From: Pawan Gupta commit baf8361e54550a48a7087b603313ad013cc13386 upstream. 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 VERW patching depend on a new feature flag X86_FEATURE_CLEAR_CPU_BUF. [pawan: - Runtime patch jmp instead of verw in macro CLEAR_CPU_BUFFERS due to lack of relative addressing support for relocations in kernels < v6.5. - Add UNWIND_HINT_EMPTY to avoid warning: arch/x86/entry/entry.o: warning: objtool: mds_verw_sel+0x0: unreachable instruction] Reported-by: Alyssa Milburn Suggested-by: Andrew Cooper Suggested-by: Peter Zijlstra Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Link: https://lore.kernel.org/all/20240213-delay-verw-v8-1-a6216d83edb7%40linux.intel.com Signed-off-by: Greg Kroah-Hartman --- arch/x86/entry/entry.S | 23 +++++++++++++++++++++++ arch/x86/include/asm/cpufeatures.h | 2 +- arch/x86/include/asm/nospec-branch.h | 15 +++++++++++++++ 3 files changed, 39 insertions(+), 1 deletion(-) --- a/arch/x86/entry/entry.S +++ b/arch/x86/entry/entry.S @@ -6,6 +6,9 @@ #include #include #include +#include +#include +#include .pushsection .noinstr.text, "ax" @@ -20,3 +23,23 @@ SYM_FUNC_END(entry_ibpb) EXPORT_SYMBOL_GPL(entry_ibpb); .popsection + +/* + * Define the VERW operand that is disguised as entry code so that + * it can be referenced with KPTI enabled. This ensure VERW can be + * used late in exit-to-user path after page tables are switched. + */ +.pushsection .entry.text, "ax" + +.align L1_CACHE_BYTES, 0xcc +SYM_CODE_START_NOALIGN(mds_verw_sel) + UNWIND_HINT_EMPTY + ANNOTATE_NOENDBR + .word __KERNEL_DS +.align L1_CACHE_BYTES, 0xcc +SYM_CODE_END(mds_verw_sel); +/* For KVM */ +EXPORT_SYMBOL_GPL(mds_verw_sel); + +.popsection + --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -304,7 +304,7 @@ #define X86_FEATURE_UNRET (11*32+15) /* "" AMD BTB untrain return */ #define X86_FEATURE_USE_IBPB_FW (11*32+16) /* "" Use IBPB during runtime firmware calls */ #define X86_FEATURE_RSB_VMEXIT_LITE (11*32+17) /* "" Fill RSB on VM exit when EIBRS is enabled */ - +#define X86_FEATURE_CLEAR_CPU_BUF (11*32+18) /* "" Clear CPU buffers using VERW */ #define X86_FEATURE_MSR_TSX_CTRL (11*32+20) /* "" MSR IA32_TSX_CTRL (Intel) implemented */ --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -194,6 +194,19 @@ #endif .endm +/* + * Macro to execute VERW instruction that mitigate transient data sampling + * attacks such as MDS. On affected systems a microcode update overloaded VERW + * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF. + * + * Note: Only the memory operand variant of VERW clears the CPU buffers. + */ +.macro CLEAR_CPU_BUFFERS + ALTERNATIVE "jmp .Lskip_verw_\@", "", X86_FEATURE_CLEAR_CPU_BUF + verw _ASM_RIP(mds_verw_sel) +.Lskip_verw_\@: +.endm + #else /* __ASSEMBLY__ */ #define ANNOTATE_RETPOLINE_SAFE \ @@ -375,6 +388,8 @@ DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear); +extern u16 mds_verw_sel; + #include /**