linux-kernel - [PATCH] x86/entry: fix entry_INT80

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Date:   Tue, 16 Aug 2022 09:11:37 +0200
From:   Juergen Gross <jgross@...e.com>
To:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Juergen Gross <jgross@...e.com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, stable@...r.kernel.org
Subject: [PATCH] x86/entry: fix entry_INT80_compat for Xen PV guests

Commit c89191ce67ef ("x86/entry: Convert SWAPGS to swapgs and remove
the definition of SWAPGS") missed one use case of SWAPGS in
entry_INT80_compat. Removing of the SWAPGS macro led to asm just
using "swapgs", as it is accepting instructions in capital letters,
too.

This in turn leads to splats in Xen PV guests like:

[   36.145223] general protection fault, maybe for address 0x2d: 0000 [#1] PREEMPT SMP NOPTI
[   36.145794] CPU: 2 PID: 1847 Comm: ld-linux.so.2 Not tainted 5.19.1-1-default #1 openSUSE Tumbleweed f3b44bfb672cdb9f235aff53b57724eba8b9411b
[   36.146608] Hardware name: HP ProLiant ML350p Gen8, BIOS P72 11/14/2013
[   36.148126] RIP: e030:entry_INT80_compat+0x3/0xa3

Fix that by open coding this single instance of the SWAPGS macro.

Cc: <stable@...r.kernel.org> # 5.19
Fixes: c89191ce67ef ("x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS")
Signed-off-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/entry/entry_64_compat.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 682338e7e2a3..4dd19819053a 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -311,7 +311,7 @@ SYM_CODE_START(entry_INT80_compat)
 	 * Interrupts are off on entry.
 	 */
 	ASM_CLAC			/* Do this early to minimize exposure */
-	SWAPGS
+	ALTERNATIVE "swapgs", "", X86_FEATURE_XENPV
 
 	/*
 	 * User tracing code (ptrace or signal handlers) might assume that
-- 
2.35.3