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x86/entry: Use the correct fence macro after swapgs in kernel CR3

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The commit

  c75890700455 ("x86/entry/64: Remove unneeded kernel CR3 switching")

removed 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, see spectre_v1_select_mitigation().

Rather, it depended on the serializing CR3 write of SWITCH_TO_KERNEL_CR3
and since it got removed, add a FENCE_SWAPGS_KERNEL_ENTRY call to make
sure speculation is blocked.

 [ bp: Massage commit message and comment. ]

Fixes: c75890700455 ("x86/entry/64: Remove unneeded kernel CR3 switching")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211126101209.8613-3-jiangshanlai@gmail.com
This commit is contained in:
Lai Jiangshan 2021-11-26 18:11:22 +08:00 committed by Borislav Petkov
parent c07e45553d
commit 1367afaa2e
1 changed files with 8 additions and 7 deletions
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15
arch/x86/entry/entry_64.S
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@ -987,11 +987,6 @@ 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
@ -1014,8 +1009,14 @@ SYM_CODE_START_LOCAL(error_entry)
* .Lgs_change's error handler with kernel gsbase.
*/
SWAPGS
FENCE_SWAPGS_USER_ENTRY
jmp .Lerror_entry_done
/*
* Issue an LFENCE to prevent GS speculation, regardless of whether it is a
* kernel or user gsbase.
*/
.Lerror_entry_done_lfence:
FENCE_SWAPGS_KERNEL_ENTRY
ret
.Lbstep_iret:
/* Fix truncated RIP */