linux-kernel - [PATCH] x86/AMD: Fix ASM constraints in amd_clear

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Date:   Wed, 9 Aug 2023 21:23:56 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     LKML <linux-kernel@...r.kernel.org>
CC:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>
Subject: [PATCH] x86/AMD: Fix ASM constraints in amd_clear_divider()

DIV writes its results into %eax and %edx, meaning that they need to be output
constraints too.  It happens to be benign in this case as the registers don't
change value, but the compiler should still know.

Fixes: 77245f1c3c64 ("x86/CPU/AMD: Do not leak quotient data after a division by 0")
Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com>
---
CC: Borislav Petkov <bp@...en8.de>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Ingo Molnar <mingo@...hat.com>
CC: Dave Hansen <dave.hansen@...ux.intel.com>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
---
 arch/x86/kernel/cpu/amd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index b55d8f82b621..8585a4be1912 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1325,6 +1325,8 @@ bool cpu_has_ibpb_brtype_microcode(void)
  */
 void noinstr amd_clear_divider(void)
 {
+	unsigned int a = 0, d = 0;
+
 	asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
-		     :: "a" (0), "d" (0), "r" (1));
+		     : "+a" (a), "+d" (d) : "r" (1));
 }

base-commit: cacc6e22932f373a91d7be55a9b992dc77f4c59b
-- 
2.30.2