Re: [PATCH v3 04/37] x86/cpufeatures: Enable CET CR4 bit for shadow stack

Borislav Petkov
2022-11-07T18:05:11Z

On Fri, Nov 04, 2022 at 03:35:31PM -0700, Rick Edgecombe wrote:
> static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> {
> - u64 msr = CET_ENDBR_EN;
> + bool kernel_ibt = HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT);
> + bool user_shstk;
> + u64 msr = 0;
>
> - if (!HAS_KERNEL_IBT ||
> - !cpu_feature_enabled(X86_FEATURE_IBT))
> + /*
> + * Enable user shadow stack only if the Linux defined user shadow stack
> + * cap was not cleared by command line.
> + */
> + user_shstk = cpu_feature_enabled(X86_FEATURE_SHSTK) &&
> + IS_ENABLED(CONFIG_X86_USER_SHADOW_STACK) &&
> + !test_bit(X86_FEATURE_USER_SHSTK, (unsigned long *)cpu_caps_cleared);

Huh, why poke at cpu_caps_cleared?

Look below:

> + if (!kernel_ibt && !user_shstk)
> return;
>
> + if (user_shstk)
> + set_cpu_cap(c, X86_FEATURE_USER_SHSTK);
> +
> + if (kernel_ibt)
> + msr = CET_ENDBR_EN;
> +
> wrmsrl(MSR_IA32_S_CET, msr);
> cr4_set_bits(X86_CR4_CET);
>
> - if (!ibt_selftest()) {
> + if (kernel_ibt && !ibt_selftest()) {
> pr_err("IBT selftest: Failed!\n");
> setup_clear_cpu_cap(X86_FEATURE_IBT);
> return;
> }
> }
> +#else /* CONFIG_X86_CET */
> +static inline void setup_cet(struct cpuinfo_x86 *c) {}
> +#endif
>
> __noendbr void cet_disable(void)
> {
> - if (cpu_feature_enabled(X86_FEATURE_IBT))
> - wrmsrl(MSR_IA32_S_CET, 0);
> + if (!(cpu_feature_enabled(X86_FEATURE_IBT) ||
> + cpu_feature_enabled(X86_FEATURE_SHSTK)))
> + return;
> +
> + wrmsrl(MSR_IA32_S_CET, 0);
> + wrmsrl(MSR_IA32_U_CET, 0);

Here you need to do

setup_clear_cpu_cap(X86_FEATURE_IBT);
setup_clear_cpu_cap(X86_FEATURE_SHSTK);

and then the cpu_feature_enabled() test above alone should suffice.

But, before you do that, I'd like to ask you to update your patchset
ontop of tip/master because the conflicts are getting non-trivial. This
one doesn't even want to apply with a large fuzz:

$ patch -p1 --dry-run -F20 -i /tmp/new
checking file arch/x86/kernel/cpu/common.c
Hunk #1 FAILED at 596.
1 out of 1 hunk FAILED

Thx.

--
Regards/Gruss,
Boris.