x86/entry: Add a fence for kernel entry SWAPGS in paranoid_entry()
Commit
18ec54fdd6d18 ("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
96b2371413e8f ("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: 96b2371413e8f ("x86/entry/64: Switch CR3 before SWAPGS in paranoid entry")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211126101209.8613-2-jiangshanlai@gmail.com
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e38a4cf..f1a8b5b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -890,6 +890,7 @@
.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 @@
movl $MSR_GS_BASE, %ecx
rdmsr
testl %edx, %edx
- jns .Lparanoid_entry_swapgs
- ret
-
-.Lparanoid_entry_swapgs:
- swapgs
-
- /*
- * 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
+ js .Lparanoid_kernel_gsbase
/* EBX = 0 -> SWAPGS required on exit */
xorl %ebx, %ebx
+ swapgs
+.Lparanoid_kernel_gsbase:
+
+ FENCE_SWAPGS_KERNEL_ENTRY
ret
SYM_CODE_END(paranoid_entry)