From: Ard Biesheuvel <ardb@kernel.org>

Calling C code via a different mapping than it was linked at is
problematic, because the compiler assumes that RIP-relative and absolute
symbol references are interchangeable. GCC in particular may use
RIP-relative per-CPU variable references even when not using -fpic.

So call xen_prepare_pvh() via its kernel virtual mapping on x86_64, so
that those RIP-relative references produce the correct values. This
matches the pre-existing behavior for i386, which also invokes
xen_prepare_pvh() via the kernel virtual mapping before invoking
startup_32 with paging disabled again.

Fixes: 7243b93345f7 ("xen/pvh: Bootstrap PVH guest")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/platform/pvh/head.S | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index f7235ef87bc3..a308b79a887c 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -101,7 +101,11 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
 	xor %edx, %edx
 	wrmsr
 
-	call xen_prepare_pvh
+	/* Call xen_prepare_pvh() via the kernel virtual mapping */
+	leaq xen_prepare_pvh(%rip), %rax
+	addq $__START_KERNEL_map, %rax
+	ANNOTATE_RETPOLINE_SAFE
+	call *%rax
 
 	/* startup_64 expects boot_params in %rsi. */
 	mov $_pa(pvh_bootparams), %rsi
-- 
2.46.0.792.g87dc391469-goog

On 2024-09-25 11:01, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Calling C code via a different mapping than it was linked at is
> problematic, because the compiler assumes that RIP-relative and absolute
> symbol references are interchangeable. GCC in particular may use
> RIP-relative per-CPU variable references even when not using -fpic.
> 
> So call xen_prepare_pvh() via its kernel virtual mapping on x86_64, so
> that those RIP-relative references produce the correct values. This
> matches the pre-existing behavior for i386, which also invokes
> xen_prepare_pvh() via the kernel virtual mapping before invoking
> startup_32 with paging disabled again.
> 
> Fixes: 7243b93345f7 ("xen/pvh: Bootstrap PVH guest")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Tested-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>

I found that before this change xen_prepare_pvh() would call through 
some pv_ops function pointers into the kernel virtual mapping.

Regards,
Jason