linux-kernel - [PATCH] x86/entry: Fix AC assertion

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Message-ID: <20200824101428.GS1362448@hirez.programming.kicks-ass.net>
Date:   Mon, 24 Aug 2020 12:14:28 +0200
From:   peterz@...radead.org
To:     x86@...nel.org
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] x86/entry: Fix AC assertion


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

For !SMAP hardware we patch out CLAC/STAC instructions and thus if
userspace sets AC, we'll still have it set after entry.

Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/include/asm/entry-common.h |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- 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_u
 		 * 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));