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    SubjectRe: [PATCH 09/24] x86/traps: Add fence_swapgs_{user,kernel}_entry()
    On Wed, Sep 01, 2021 at 01:50:10AM +0800, Lai Jiangshan wrote:

    > +/*
    > + * Mitigate Spectre v1 for conditional swapgs code paths.
    > + *
    > + * fence_swapgs_user_entry is used in the user entry swapgs code path, to
    > + * prevent a speculative swapgs when coming from kernel space.
    > + *
    > + * fence_swapgs_kernel_entry is used in the kernel entry non-swapgs code path,
    > + * to prevent the swapgs from getting speculatively skipped when coming from
    > + * user space.
    > + */
    > +static __always_inline void fence_swapgs_user_entry(void)
    > +{
    > + alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_USER);
    > +}
    > +
    > +static __always_inline void fence_swapgs_kernel_entry(void)
    > +{
    > + alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_KERNEL);
    > +}

    I think slightly larger primitives might make more sense; that is
    include the swapgs in these function and drop the fence_ prefix.

    Something a bit like...

    --- a/arch/x86/entry/traps.c
    +++ b/arch/x86/entry/traps.c
    @@ -853,20 +853,22 @@ static __always_inline void ist_restore_
    /*
    * Mitigate Spectre v1 for conditional swapgs code paths.
    *
    - * fence_swapgs_user_entry is used in the user entry swapgs code path, to
    - * prevent a speculative swapgs when coming from kernel space.
    + * swapgs_user_entry is used in the user entry swapgs code path, to prevent a
    + * speculative swapgs when coming from kernel space.
    *
    - * fence_swapgs_kernel_entry is used in the kernel entry non-swapgs code path,
    - * to prevent the swapgs from getting speculatively skipped when coming from
    - * user space.
    + * swapgs_kernel_entry is used in the kernel entry non-swapgs code path, to
    + * prevent the swapgs from getting speculatively skipped when coming from user
    + * space.
    */
    -static __always_inline void fence_swapgs_user_entry(void)
    +static __always_inline void swapgs_user_entry(void)
    {
    + native_swapgs();
    alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_USER);
    }

    -static __always_inline void fence_swapgs_kernel_entry(void)
    +static __always_inline void swapgs_kernel_entry(void)
    {
    + native_swapgs();
    alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_KERNEL);
    }

    @@ -896,8 +898,7 @@ struct pt_regs *do_error_entry(struct pt
    * We entered from user mode.
    * Switch to kernel gsbase and CR3.
    */
    - native_swapgs();
    - fence_swapgs_user_entry();
    + swapgs_user_entry();
    switch_to_kernel_cr3();

    /* Put pt_regs onto the task stack. */
    @@ -917,8 +918,7 @@ struct pt_regs *do_error_entry(struct pt
    * We came from an IRET to user mode, so we have user
    * gsbase and CR3. Switch to kernel gsbase and CR3:
    */
    - native_swapgs();
    - fence_swapgs_user_entry();
    + swapgs_user_entry();
    switch_to_kernel_cr3();

    /*
    @@ -936,8 +936,7 @@ struct pt_regs *do_error_entry(struct pt
    * handler with kernel gsbase.
    */
    if (eregs->ip == (unsigned long)asm_load_gs_index_gs_change) {
    - native_swapgs();
    - fence_swapgs_user_entry();
    + swapgs_user_entry();
    } else {
    fence_swapgs_kernel_entry();
    }
    @@ -1017,14 +1016,12 @@ static __always_inline unsigned long ist
    if ((long)gsbase < 0)
    return 1;

    - native_swapgs();
    -
    /*
    * The above ist_switch_to_kernel_cr3() 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();
    + swapgs_kernel_entry();

    /* SWAPGS required on exit */
    return 0;
    @@ -1089,7 +1086,7 @@ void paranoid_exit(struct ist_regs *ist)
    }

    if (!ist->gsbase)
    - native_swapgs();
    + swapgs_user_entry();
    }
    #endif

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