Commit 1367afaa authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Borislav Petkov
Browse files

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



The commit

  c7589070 ("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: c7589070 ("x86/entry/64: Remove unneeded kernel CR3 switching")
Signed-off-by: default avatarLai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211126101209.8613-3-jiangshanlai@gmail.com
parent c07e4555

arch/x86/entry/entry_64.S

+8 −7
Original line number Diff line number Diff line
@@ -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 */