linux-kernel - [PATCH] x86/xen: fix xen_hypercall

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Message-ID: <20250205091056.17796-1-jgross@suse.com>
Date: Wed,  5 Feb 2025 10:10:56 +0100
From: Juergen Gross <jgross@...e.com>
To: linux-kernel@...r.kernel.org,
	x86@...nel.org
Cc: Juergen Gross <jgross@...e.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	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>,
	xen-devel@...ts.xenproject.org
Subject: [PATCH] x86/xen: fix xen_hypercall_hvm() to not clobber %rbx

xen_hypercall_hvm(), which is used when running as a Xen PVH guest at
most only once during early boot, is clobbering %rbx. Depending on
whether the caller relies on %rbx to be preserved across the call or
not, this clobbering might result in an early crash of the system.

This can be avoided by not modifying %rbx in xen_hypercall_hvm().

Fixes: b4845bb63838 ("x86/xen: add central hypercall functions")
Signed-off-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/xen/xen-head.S | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 9252652afe59..4378b817ed32 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -117,8 +117,7 @@ SYM_FUNC_START(xen_hypercall_hvm)
 	pop %ebx
 	pop %eax
 #else
-	lea xen_hypercall_amd(%rip), %rbx
-	cmp %rax, %rbx
+	cmp xen_hypercall_amd(%rip), %rax
 #ifdef CONFIG_FRAME_POINTER
 	pop %rax	/* Dummy pop. */
 #endif
-- 
2.43.0