Re: [PATCH v7 34/41] x86/shstk: Support WRSS for userspace

[Borislav Petkov <bp@alien8.de>]

On Mon, Feb 27, 2023 at 02:29:50PM -0800, Rick Edgecombe wrote:
> For the current shadow stack implementation, shadow stacks contents can't
> easily be provisioned with arbitrary data. This property helps apps
> protect themselves better, but also restricts any potential apps that may
> want to do exotic things at the expense of a little security.
> 
> The x86 shadow stack feature introduces a new instruction, WRSS, which
> can be enabled to write directly to shadow stack permissioned memory from

s/permissioned //

By now it is clear that shadow stack memory is a special thing anyway.

> userspace. Allow it to get enabled via the prctl interface.
> 
> Only enable the userspace WRSS instruction, which allows writes to
> userspace shadow stacks from userspace. Do not allow it to be enabled
> independently of shadow stack, as HW does not support using WRSS when
> shadow stack is disabled.
> 
> From a fault handler perspective, WRSS will behave very similar to WRUSS,
> which is treated like a user access from a #PF err code perspective.

...

> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 65ec1965cd28..2d3b35c957ad 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -310,6 +310,17 @@ void msrs_free(struct msr *msrs);
>  int msr_set_bit(u32 msr, u8 bit);
>  int msr_clear_bit(u32 msr, u8 bit);
>  
> +/* Helper that can never get accidentally un-inlined. */
> +#define set_clr_bits_msrl(msr, set, clear)	do {	\

Uff, pls kill this thing.

Our MSR interfaces universe is already insane and arch/x86/lib/msr.c
already has similar attempts to what you're doing here in addition to
all the other gunk in msr.h.

I highly doubt this can't be done the usual way, lemme see...

> +	u64 __val, __new_val, __msr = msr;		\
> +							\
> +	rdmsrl(__msr, __val);				\
> +	__new_val = (__val & ~(clear)) | (set);		\
> +							\
> +	if (__new_val != __val)				\
> +		wrmsrl(__msr, __new_val);		\
> +} while (0)
> +
>  #ifdef CONFIG_SMP
>  int rdmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h);
>  int wrmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 l, u32 h);
> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index 7dfd9dc00509..e31495668056 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -28,5 +28,6 @@
>  
>  /* ARCH_SHSTK_ features bits */
>  #define ARCH_SHSTK_SHSTK		(1ULL <<  0)
> +#define ARCH_SHSTK_WRSS			(1ULL <<  1)
>  
>  #endif /* _ASM_X86_PRCTL_H */
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 0a3decab70ee..009cb3fa0ae5 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -363,6 +363,36 @@ void shstk_free(struct task_struct *tsk)
>  	unmap_shadow_stack(shstk->base, shstk->size);
>  }
>  
> +static int wrss_control(bool enable)
> +{
> +	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
> +		return -EOPNOTSUPP;
> +
> +	/*
> +	 * Only enable wrss if shadow stack is enabled. If shadow stack is not

"WRSS". Insns in all caps pls.

> +	 * enabled, wrss will already be disabled, so don't bother clearing it

Ditto.

> +	 * when disabling.
> +	 */
> +	if (!features_enabled(ARCH_SHSTK_SHSTK))
> +		return -EPERM;
> +
> +	/* Already enabled/disabled? */
> +	if (features_enabled(ARCH_SHSTK_WRSS) == enable)
> +		return 0;
> +
> +	fpregs_lock_and_load();
> +	if (enable) {
> +		set_clr_bits_msrl(MSR_IA32_U_CET, CET_WRSS_EN, 0);
> +		features_set(ARCH_SHSTK_WRSS);
> +	} else {
> +		set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_WRSS_EN);
> +		features_clr(ARCH_SHSTK_WRSS);
> +	}
> +	fpregs_unlock();

Yes, doing it the "usual" way is more readable because it is a common
code pattern which one encounters all around arch/x86/.

Diff ontop:

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 009cb3fa0ae5..914feff26b23 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -365,6 +365,8 @@ void shstk_free(struct task_struct *tsk)
 
 static int wrss_control(bool enable)
 {
+	u64 msrval;
+
 	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
 		return -EOPNOTSUPP;
 
@@ -381,13 +383,22 @@ static int wrss_control(bool enable)
 		return 0;
 
 	fpregs_lock_and_load();
+	rdmsrl(MSR_IA32_U_CET, msrval);
+
 	if (enable) {
-		set_clr_bits_msrl(MSR_IA32_U_CET, CET_WRSS_EN, 0);
 		features_set(ARCH_SHSTK_WRSS);
+		msrval |= CET_WRSS_EN;
 	} else {
-		set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_WRSS_EN);
 		features_clr(ARCH_SHSTK_WRSS);
+		if (!(msrval & CET_WRSS_EN))
+			goto unlock;
+
+		msrval &= ~CET_WRSS_EN;
 	}
+
+	wrmsrl(MSR_IA32_U_CET, msrval);
+
+unlock:
 	fpregs_unlock();
 
 	return 0;

-- 
Regards/Gruss,
    Boris.

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