[RFC PATCH 2/2] x86/sev: Disable jump tables in SEV startup code

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

When retpolines and IBT are both disabled, the compiler is free to use
jump tables to optimize switch instructions. However, these are emitted
by Clang as absolute references into .rodata, e.g.,

        jmp    *-0x7dfffe90(,%r9,8)
                        R_X86_64_32S    .rodata+0x170

Given that this code will execute before that address in .rodata has even
been mapped, it is guaranteed to crash a SEV-SNP guest in a way that is
difficult to diagnose.

So disable jump tables when building this code. It would be better if we
could attach this annotation to the __head macro but this appears to be
impossible.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/coco/sev/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/coco/sev/Makefile b/arch/x86/coco/sev/Makefile
index 08de37559307..dcb06dc8b5ae 100644
--- a/arch/x86/coco/sev/Makefile
+++ b/arch/x86/coco/sev/Makefile
@@ -2,6 +2,10 @@
 
 obj-y += core.o
 
+# jump tables are emitted using absolute references in non-PIC code
+# so they cannot be used in the early SEV startup code
+CFLAGS_core.o += -fno-jump-tables
+
 ifdef CONFIG_FUNCTION_TRACER
 CFLAGS_REMOVE_core.o = -pg
 endif
-- 
2.48.1.262.g85cc9f2d1e-goog