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    SubjectRe: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support
    On Fri, Aug 20, 2021 at 11:11:52AM -0700, Yu-cheng Yu wrote:
    > diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
    > new file mode 100644
    > index 000000000000..6432baf4de1f
    > --- /dev/null
    > +++ b/arch/x86/include/asm/cet.h
    > @@ -0,0 +1,30 @@
    > +/* SPDX-License-Identifier: GPL-2.0 */
    > +#ifndef _ASM_X86_CET_H
    > +#define _ASM_X86_CET_H
    > +
    > +#ifndef __ASSEMBLY__
    > +#include <linux/types.h>
    > +
    > +struct task_struct;
    > +
    > +/*
    > + * Per-thread CET status
    > + */

    That comment is superfluous and wrong now. The struct name says exactly
    what that thing is.

    > +static unsigned long alloc_shstk(unsigned long size)
    > +{
    > + int flags = MAP_ANONYMOUS | MAP_PRIVATE;
    > + struct mm_struct *mm = current->mm;
    > + unsigned long addr, populate;

    s/populate/unused/

    > +
    > + mmap_write_lock(mm);
    > + addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHADOW_STACK, 0,
    > + &populate, NULL);
    > + mmap_write_unlock(mm);
    > +
    > + return addr;
    > +}
    > +
    > +int shstk_setup(void)
    > +{
    > + struct thread_shstk *shstk = &current->thread.shstk;
    > + unsigned long addr, size;
    > + int err;
    > +
    > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    > + return 0;
    > +
    > + size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
    > + addr = alloc_shstk(size);
    > + if (IS_ERR_VALUE(addr))
    > + return PTR_ERR((void *)addr);
    > +
    > + start_update_msrs();

    You're setting CET_U with the MSR writes below. Why do you need to do
    XRSTOR here? To zero out PL[012]_SSP?

    If so, you can WRMSR those too - no need for a full XRSTOR...

    > + err = wrmsrl_safe(MSR_IA32_PL3_SSP, addr + size);
    > + if (!err)
    > + wrmsrl_safe(MSR_IA32_U_CET, CET_SHSTK_EN);
    > + end_update_msrs();
    > +
    > + if (!err) {
    > + shstk->base = addr;
    > + shstk->size = size;
    > + }
    > +
    > + return err;
    > +}

    --
    Regards/Gruss,
    Boris.

    https://people.kernel.org/tglx/notes-about-netiquette

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