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Re: [PATCH] x86/xen: fix xen_hypercall_hvm() to not clobber %rbx
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx
- From: Jürgen Groß <jgross@xxxxxxxx>
- Date: Wed, 5 Feb 2025 11:04:34 +0100
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- Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 05 Feb 2025 10:04:44 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05.02.25 10:38, Andrew Cooper wrote:
On 05/02/2025 9:17 am, Jürgen Groß wrote:
On 05.02.25 10:16, Andrew Cooper wrote:
On 05/02/2025 9:10 am, Juergen Gross wrote:
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@xxxxxxxx>
---
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)
The 32bit case, out of context up here, also clobbers %ebx.
~Andrew
pop %ebx
It does not, as this part of the context is showing.
Hmm, so it is, and worse, it can't be changed to match the 64bit side.
That's nasty.
But while I'm here looking at the code, what's up with
#ifdef CONFIG_FRAME_POINTER
pushq $0 /* Dummy push for stack alignment. */
#endif
?
That's covered by FRAME_{START,END} normally, and Linux's preferred
stack alignment is 8 not 16.
I've added this due to a review comment by Jan. As he is more into ABI
matters, I believed him.
Google is telling me you are right, so I'll remove those extra hunks in
V2 of the patch adding FRAME_END.
Juergen
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