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From: Sasha Levin <sashal@kernel.org>
To: linux-kernel@vger.kernel.org, tglx@linutronix.de, bp@alien8.de,
	luto@kernel.org
Cc: hpa@zytor.com, dave.hansen@intel.com, tony.luck@intel.com,
	ak@linux.intel.com, ravi.v.shankar@intel.com,
	chang.seok.bae@intel.com, Sasha Levin <sashal@kernel.org>
Subject: [PATCH v10 14/18] x86/speculation/swapgs: Check FSGSBASE in enabling SWAPGS mitigation
Date: Thu, 23 Apr 2020 19:22:03 -0400	[thread overview]
Message-ID: <20200423232207.5797-15-sashal@kernel.org> (raw)
In-Reply-To: <20200423232207.5797-1-sashal@kernel.org>

From: Tony Luck <tony.luck@intel.com>

Before enabling FSGSBASE the kernel could safely assume that the content
of GS base was a user address. Thus any speculative access as the result
of a mispredicted branch controlling the execution of SWAPGS would be to
a user address. So systems with speculation-proof SMAP did not need to
add additional LFENCE instructions to mitigate.

With FSGSBASE enabled a hostile user can set GS base to a kernel address.
So they can make the kernel speculatively access data they wish to leak
via a side channel. This means that SMAP provides no protection.

Add FSGSBASE as an additional condition to enable the fence-based SWAPGS
mitigation.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kernel/cpu/bugs.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index ed54b3b21c396..487603ea51cd1 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -450,14 +450,12 @@ static void __init spectre_v1_select_mitigation(void)
 		 * If FSGSBASE is enabled, the user can put a kernel address in
 		 * GS, in which case SMAP provides no protection.
 		 *
-		 * [ NOTE: Don't check for X86_FEATURE_FSGSBASE until the
-		 *	   FSGSBASE enablement patches have been merged. ]
-		 *
 		 * If FSGSBASE is disabled, the user can only put a user space
 		 * address in GS.  That makes an attack harder, but still
 		 * possible if there's no SMAP protection.
 		 */
-		if (!smap_works_speculatively()) {
+		if (boot_cpu_has(X86_FEATURE_FSGSBASE) ||
+		    !smap_works_speculatively()) {
 			/*
 			 * Mitigation can be provided from SWAPGS itself or
 			 * PTI as the CR3 write in the Meltdown mitigation
-- 
2.20.1


  parent reply	other threads:[~2020-04-23 23:22 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-23 23:21 [PATCH v10 00/18] Enable FSGSBASE instructions Sasha Levin
2020-04-23 23:21 ` [PATCH v10 01/18] x86/ptrace: Prevent ptrace from clearing the FS/GS selector Sasha Levin
2020-04-25 22:46   ` Andy Lutomirski
2020-04-23 23:21 ` [PATCH v10 02/18] selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write Sasha Levin
2020-04-23 23:21 ` [PATCH v10 03/18] x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Sasha Levin
2020-04-23 23:21 ` [PATCH v10 04/18] x86/entry/64: Clean up paranoid exit Sasha Levin
2020-04-23 23:21 ` [PATCH v10 05/18] x86/entry/64: Switch CR3 before SWAPGS in paranoid entry Sasha Levin
2020-04-23 23:21 ` [PATCH v10 06/18] x86/entry/64: Introduce the FIND_PERCPU_BASE macro Sasha Levin
2020-04-23 23:21 ` [PATCH v10 07/18] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit Sasha Levin
2020-04-23 23:21 ` [PATCH v10 08/18] x86/entry/64: Document GSBASE handling in the paranoid path Sasha Levin
2020-04-23 23:21 ` [PATCH v10 09/18] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Sasha Levin
2020-04-23 23:21 ` [PATCH v10 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions Sasha Levin
2020-04-23 23:22 ` [PATCH v10 11/18] x86/fsgsbase/64: Use FSGSBASE in switch_to() if available Sasha Levin
2020-04-23 23:22 ` [PATCH v10 12/18] x86/fsgsbase/64: move save_fsgs to header file Sasha Levin
2020-04-23 23:22 ` [PATCH v10 13/18] x86/fsgsbase/64: Use FSGSBASE instructions on thread copy and ptrace Sasha Levin
2020-04-23 23:22 ` Sasha Levin [this message]
2020-04-23 23:22 ` [PATCH v10 15/18] selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE Sasha Levin
2020-04-23 23:22 ` [PATCH v10 16/18] x86/fsgsbase/64: Enable FSGSBASE on 64bit by default and add a chicken bit Sasha Levin
2020-04-23 23:22 ` [PATCH v10 17/18] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Sasha Levin
2020-04-23 23:22 ` [PATCH v10 18/18] Documentation/x86/64: Add documentation for GS/FS addressing mode Sasha Levin
2020-05-10  8:09 ` [PATCH v10 00/18] Enable FSGSBASE instructions Vegard Nossum
2020-05-10  8:29   ` Vegard Nossum
2020-05-10 10:15     ` Thomas Gleixner
2020-05-10 14:17       ` Sasha Levin
2020-05-11  0:48     ` Andi Kleen
2020-05-11  0:50       ` Andi Kleen
2020-05-11  5:03         ` Sasha Levin

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