x86/entry: Fix AC assertion

The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
improve user entry sanity checks") unconditionally triggers on a IVB
machine because it does not support SMAP.

For !SMAP hardware the CLAC/STAC instructions are patched out and thus if
userspace sets AC, it is still have set after entry.

Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200902133200.666781610@infradead.org

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index a8f9315..6fe54b2 100644 (file)
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -18,8 +18,16 @@ 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) ||
+                   (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
+                       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));