The following commit has been merged into the x86/fsgsbase branch of tip: Commit-ID: 978e1342c3c4d7b20808fd5875d9ac0d57db22ee Gitweb: https://git.kernel.org/tip/978e1342c3c4d7b20808fd5875d9ac0d57db22ee Author: Tony Luck <tony.l...@intel.com> AuthorDate: Thu, 28 May 2020 16:13:54 -04:00 Committer: Thomas Gleixner <t...@linutronix.de> CommitterDate: Thu, 18 Jun 2020 15:47:02 +02:00
x86/speculation/swapgs: Check FSGSBASE in enabling SWAPGS mitigation 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.l...@intel.com> Signed-off-by: Chang S. Bae <chang.seok....@intel.com> Signed-off-by: Sasha Levin <sas...@kernel.org> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Link: https://lkml.kernel.org/r/20200528201402.1708239-9-sas...@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 0b71970..5ea5fbd 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -543,14 +543,12 @@ 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
