From 36971fccf81038a0770128056b23be273ab867da Mon Sep 17 00:00:00 2001 From: Lai Jiangshan Date: Fri, 26 Nov 2021 18:11:21 +0800 Subject: [PATCH] x86/entry: Add a fence for kernel entry SWAPGS in paranoid_entry() Commit a030cf48d2a3d ("x86/speculation: Prepare entry code for Spectre v1 swapgs mitigations") added FENCE_SWAPGS_{KERNEL|USER}_ENTRY for conditional SWAPGS. In paranoid_entry(), it uses only FENCE_SWAPGS_KERNEL_ENTRY for both branches. This is because the fence is required for both cases since the CR3 write is conditional even when PTI is enabled. But 8599aa317efe2 ("x86/entry/64: Switch CR3 before SWAPGS in paranoid entry") changed the order of SWAPGS and the CR3 write. And it missed the needed FENCE_SWAPGS_KERNEL_ENTRY for the user gsbase case. Add it back by changing the branches so that FENCE_SWAPGS_KERNEL_ENTRY can cover both branches. [ bp: Massage, fix typos, remove obsolete comment while at it. ] Fixes: 8599aa317efe2 ("x86/entry/64: Switch CR3 before SWAPGS in paranoid entry") Signed-off-by: Lai Jiangshan Signed-off-by: Borislav Petkov Link: https://lkml.kernel.org/r/20211126101209.8613-2-jiangshanlai@gmail.com --- arch/x86/entry/entry_64.S | 16 +++++----------- 1 file changed, 5 insertions(+), 11 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index e38a4cf795d96..f1a8b5b2af964 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -890,6 +890,7 @@ SYM_CODE_START_LOCAL(paranoid_entry) .Lparanoid_entry_checkgs: /* EBX = 1 -> kernel GSBASE active, no restore required */ movl $1, %ebx + /* * The kernel-enforced convention is a negative GSBASE indicates * a kernel value. No SWAPGS needed on entry and exit. @@ -897,21 +898,14 @@ SYM_CODE_START_LOCAL(paranoid_entry) movl $MSR_GS_BASE, %ecx rdmsr testl %edx, %edx - jns .Lparanoid_entry_swapgs - ret + js .Lparanoid_kernel_gsbase -.Lparanoid_entry_swapgs: + /* EBX = 0 -> SWAPGS required on exit */ + xorl %ebx, %ebx swapgs +.Lparanoid_kernel_gsbase: - /* - * The above SAVE_AND_SWITCH_TO_KERNEL_CR3 macro doesn't do an - * unconditional CR3 write, even in the PTI case. So do an lfence - * to prevent GS speculation, regardless of whether PTI is enabled. - */ FENCE_SWAPGS_KERNEL_ENTRY - - /* EBX = 0 -> SWAPGS required on exit */ - xorl %ebx, %ebx ret SYM_CODE_END(paranoid_entry) -- 2.39.5