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    Subject[PATCH V5 02/50] 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 14ffe12807ba..6189a0dc83ab 100644
    --- a/arch/x86/entry/entry_64.S
    +++ b/arch/x86/entry/entry_64.S
    @@ -993,11 +993,6 @@ 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
    @@ -1020,8 +1015,15 @@ 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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