[PATCH v12 14/18] x86/speculation/swapgs: Check FSGSBASE in enabling SWAPGS mitigation

[Sasha Levin]

From: Tony Luck <tony.luck@xxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 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