[PATCH] x86/boot: Disable jump tables in PIC code

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

From: Ard Biesheuvel <ardb@kernel.org>

objtool already struggles to identify jump tables correctly in non-PIC
code, where the idiom is something like

  jmpq  *table(,%idx,8)

and the table is a list of absolute addresses of jump targets.

When using -fPIC, both the table reference as well as the jump targets
are emitted in a RIP-relative manner, resulting in something like

  leaq    table(%rip), %tbl
  movslq  (%tbl,%idx,4), %offset
  addq    %offset, %tbl
  jmpq    *%tbl

and the table is a list of offsets of the jump targets relative to the
start of the entire table.

Considering that this sequence of instructions can be interleaved with
other instructions that have nothing to do with the jump table in
question, it is extremely difficult to infer the control flow by
deriving the jump targets from the indirect jump, the location of the
table and the relative offsets it contains.

So let's not bother and disable jump tables for code built with -fPIC
under arch/x86/boot/startup.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/startup/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index e5f77e65c083..4062582144f6 100644
--- a/arch/x86/boot/startup/Makefile
+++ b/arch/x86/boot/startup/Makefile
@@ -5,6 +5,7 @@ KBUILD_CFLAGS		+= -D__DISABLE_EXPORTS -mcmodel=small -fPIC \
 			   -Os -DDISABLE_BRANCH_PROFILING \
 			   $(DISABLE_STACKLEAK_PLUGIN) \
 			   -fno-stack-protector -D__NO_FORTIFY \
+			   -fno-jump-tables \
 			   -include $(srctree)/include/linux/hidden.h
 
 # disable ftrace hooks and LTO

base-commit: ff4c0560ab020d34baf0aa6434f66333d25ae524
-- 
2.49.0.805.g082f7c87e0-goog