[PATCH v3 1/5] x86/pvh: Call C code via the kernel virtual mapping

Ard Biesheuvel
2024-10-09T16:09:07Z

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")
Tested-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/platform/pvh/head.S | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index 64fca49cd88f..ce4fd8d33da4 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -172,7 +172,14 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
movq %rbp, %rbx
subq $_pa(pvh_start_xen), %rbx
movq %rbx, phys_base(%rip)
- call xen_prepare_pvh
+
+ /* Call xen_prepare_pvh() via the kernel virtual mapping */
+ leaq xen_prepare_pvh(%rip), %rax
+ subq phys_base(%rip), %rax
+ addq $__START_KERNEL_map, %rax
+ ANNOTATE_RETPOLINE_SAFE
+ call *%rax
+
/*
* Clear phys_base. __startup_64 will *add* to its value,
* so reset to 0.