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x86/sev: Disable jump tables in SEV startup code

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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:

        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>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250127114334.1045857-6-ardb+git@google.com
This commit is contained in:
Ard Biesheuvel 2025-01-27 12:43:37 +01:00 • committed by Ingo Molnar
parent 9c5968db9e
commit 1105ab42a8
1 changed files with 4 additions and 0 deletions
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4
arch/x86/coco/sev/Makefile
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@ -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