From 04dcbdb8057827b043b3c71aa397c4c63e67d086 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Mon, 18 Feb 2019 23:42:51 +0100 Subject: x86/speculation/mds: Clear CPU buffers on exit to user Add a static key which controls the invocation of the CPU buffer clear mechanism on exit to user space and add the call into prepare_exit_to_usermode() and do_nmi() right before actually returning. Add documentation which kernel to user space transition this covers and explain why some corner cases are not mitigated. Signed-off-by: Thomas Gleixner Reviewed-by: Greg Kroah-Hartman Reviewed-by: Borislav Petkov Reviewed-by: Frederic Weisbecker Reviewed-by: Jon Masters Tested-by: Jon Masters --- arch/x86/include/asm/nospec-branch.h | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'arch/x86/include/asm/nospec-branch.h') diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index 67cb9b2082b1..65b747286d96 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -318,6 +318,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp); DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb); DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb); +DECLARE_STATIC_KEY_FALSE(mds_user_clear); + #include /** @@ -343,6 +345,17 @@ static inline void mds_clear_cpu_buffers(void) asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc"); } +/** + * mds_user_clear_cpu_buffers - Mitigation for MDS vulnerability + * + * Clear CPU buffers if the corresponding static key is enabled + */ +static inline void mds_user_clear_cpu_buffers(void) +{ + if (static_branch_likely(&mds_user_clear)) + mds_clear_cpu_buffers(); +} + #endif /* __ASSEMBLY__ */ /* -- cgit v1.2.3-59-g8ed1b