Re: [tip: x86/urgent] x86/entry, selftests: Further improve user entry sanity checks

[peterz]

On Sat, Jul 04, 2020 at 05:49:10PM -0000, tip-bot2 for Andy Lutomirski wrote:

> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index f392a8b..e83b3f1 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -49,6 +49,23 @@
>  static void check_user_regs(struct pt_regs *regs)
>  {
>  	if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) {
> +		/*
> +		 * Make sure that the entry code gave us a sensible EFLAGS
> +		 * register.  Native because we want to check the actual CPU
> +		 * state, not the interrupt state as imagined by Xen.
> +		 */
> +		unsigned long flags = native_save_fl();
> +		WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
> +				      X86_EFLAGS_NT));

This triggers with AC|TF on my !SMAP enabled machine.

something like so then?

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index a8f9315b9eae..76410964585f 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -18,8 +18,15 @@ static __always_inline void arch_check_user_regs(struct pt_regs *regs)
 		 * state, not the interrupt state as imagined by Xen.
 		 */
 		unsigned long flags = native_save_fl();
-		WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
-				      X86_EFLAGS_NT));
+		unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
+
+		/*
+		 * For !SMAP hardware we patch out CLAC on entry.
+		 */
+		if (boot_cpu_has(X86_FEATURE_SMAP))
+			mask |= X86_EFLAGS_AC;
+
+		WARN_ON_ONCE(flags & mask);
 
 		/* We think we came from user mode. Make sure pt_regs agrees. */
 		WARN_ON_ONCE(!user_mode(regs));