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Subject[PATCH V6 02/49] x86/entry: Use the correct fence macro after swapgs in kernel CR3
Date
From: Lai Jiangshan <laijs@linux.alibaba.com>

The commit c75890700455 ("x86/entry/64: Remove unneeded kernel CR3
switching") removes a CR3 write in the faulting path of load_gs_index.

But the path's FENCE_SWAPGS_USER_ENTRY has no fence operation if PTI
is enabled. Rahter, it depends on the CR3 write of SWITCH_TO_KERNEL_CR3.
So the path should use FENCE_SWAPGS_KERNEL_ENTRY if SWITCH_TO_KERNEL_CR3
is removed.

Fixes: c75890700455 ("x86/entry/64: Remove unneeded kernel CR3 switching")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
arch/x86/entry/entry_64.S | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 8582709576bf..4967edded48d 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -991,11 +991,6 @@ SYM_CODE_START_LOCAL(error_entry)
pushq %r12
ret

-.Lerror_entry_done_lfence:
- FENCE_SWAPGS_KERNEL_ENTRY
-.Lerror_entry_done:
- ret
-
/*
* There are two places in the kernel that can potentially fault with
* usergs. Handle them here. B stepping K8s sometimes report a
@@ -1018,8 +1013,15 @@ SYM_CODE_START_LOCAL(error_entry)
* .Lgs_change's error handler with kernel gsbase.
*/
SWAPGS
- FENCE_SWAPGS_USER_ENTRY
- jmp .Lerror_entry_done
+
+ /*
+ * The above code has no serializing instruction. So do an lfence
+ * to prevent GS speculation, regardless of whether it is kernel
+ * gsbase or user gsbase.
+ */
+.Lerror_entry_done_lfence:
+ FENCE_SWAPGS_KERNEL_ENTRY
+ ret

.Lbstep_iret:
/* Fix truncated RIP */
--
2.19.1.6.gb485710b
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