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

Commit

  e05deb47eed1d ("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

  f86cbc276c054 ("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: f86cbc276c054 ("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

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e38a4cf795d962bbb8312dc6241333b8939a6b8b..f1a8b5b2af964f5b99ce67050a0f03a79b2ff8a2 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -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
+       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
-        * 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
-
-       /* EBX = 0 -> SWAPGS required on exit */
-       xorl    %ebx, %ebx
        ret
 SYM_CODE_END(paranoid_entry)