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x86/entry: Add a fence for kernel entry SWAPGS in paranoid_entry()

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  18ec54fdd6 ("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

  96b2371413 ("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: 96b2371413 ("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
This commit is contained in:
Lai Jiangshan 2021-11-26 18:11:21 +08:00 committed by Borislav Petkov
parent 1d5379d047
commit c07e45553d
1 changed files with 6 additions and 12 deletions
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18
arch/x86/entry/entry_64.S
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@ -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
.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)