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

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Message-ID: <159922536478.20229.11152935257298904584.tip-bot2@tip-bot2>
Date:   Fri, 04 Sep 2020 13:16:04 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org,
        x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: [tip: x86/urgent] x86/entry: Fix AC assertion

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     662a0221893a3d58aa72719671844264306f6e4b
Gitweb:        https://git.kernel.org/tip/662a0221893a3d58aa72719671844264306f6e4b
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Wed, 02 Sep 2020 15:25:50 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Fri, 04 Sep 2020 15:09:29 +02:00

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@...radead.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Daniel Thompson <daniel.thompson@...aro.org>
Acked-by: Andy Lutomirski <luto@...nel.org>
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/r/20200902133200.666781610@infradead.org

---
 arch/x86/include/asm/entry-common.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index a8f9315..6fe54b2 100644
--- 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));