[PATCH V6 01/49] x86/entry: Add fence for kernel entry swapgs in paranoid_entry()

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

Commit 18ec54fdd6d18 ("x86/speculation: Prepare entry code for Spectre
v1 swapgs mitigations") adds FENCE_SWAPGS_{KERNEL|USER}_ENTRY
for conditional swapgs.  And in paranoid_entry(), it uses only
FENCE_SWAPGS_KERNEL_ENTRY for both branches.  It is because the fence
is required for both cases since the CR3 write is conditinal even PTI
is enabled.

But commit 96b2371413e8f ("x86/entry/64: Switch CR3 before SWAPGS in
paranoid entry") switches the code order and changes the branches.
And it misses the needed FENCE_SWAPGS_KERNEL_ENTRY for user gsbase case.

Add it back by changing the branches so that FENCE_SWAPGS_KERNEL_ENTRY
can cover both branches.

Fixes: Commit 96b2371413e8f ("x86/entry/64: Switch CR3 before SWAPGS in paranoid entry")
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: Sasha Levin <sashal@xxxxxxxxxx>
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e38a4cf795d9..8582709576bf 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -897,11 +897,12 @@ 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
@@ -909,9 +910,6 @@ SYM_CODE_START_LOCAL(paranoid_entry)
 	 * 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)
 
-- 
2.19.1.6.gb485710b