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x86/retpoline: Create a retpoline thunk array 

Stick all the retpolines in a single symbol and have the individual
thunks as inner labels, this should guarantee thunk order and layout.

Previously there were 16 (or rather 15 without rsp) separate symbols and
a toolchain might reasonably expect it could displace them however it
liked, with disregard for their relative position.

However, now they're part of a larger symbol. Any change to their
relative position would disrupt this larger _array symbol and thus not
be sound.

This is the same reasoning used for data symbols. On their own there
is no guarantee about their relative position wrt to one aonther, but
we're still able to do arrays because an array as a whole is a single
larger symbol.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.169659320@infradead.org
This commit is contained in:
Peter Zijlstra 2021-10-26 14:01:41 +02:00
parent 6fda8a3886
commit 1a6f74429c
2 changed files with 16 additions and 6 deletions
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8
arch/x86/include/asm/nospec-branch.h
View File

@ -12,6 +12,8 @@
#include <asm/msr-index.h>
#include <asm/unwind_hints.h>
#define RETPOLINE_THUNK_SIZE 32
/*
* Fill the CPU return stack buffer.
*
@ -120,11 +122,15 @@
#ifdef CONFIG_RETPOLINE
typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
#define GEN(reg) \
extern asmlinkage void __x86_indirect_thunk_ ## reg (void);
extern retpoline_thunk_t __x86_indirect_thunk_ ## reg;
#include <asm/GEN-for-each-reg.h>
#undef GEN
extern retpoline_thunk_t __x86_indirect_thunk_array[];
#ifdef CONFIG_X86_64
/*

14
arch/x86/lib/retpoline.S
View File

@ -28,16 +28,14 @@
.macro THUNK reg
.align 32
SYM_FUNC_START(__x86_indirect_thunk_\reg)
.align RETPOLINE_THUNK_SIZE
SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL)
UNWIND_HINT_EMPTY
ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
__stringify(RETPOLINE \reg), X86_FEATURE_RETPOLINE, \
__stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), X86_FEATURE_RETPOLINE_AMD
SYM_FUNC_END(__x86_indirect_thunk_\reg)
.endm
/*
@ -55,10 +53,16 @@ SYM_FUNC_END(__x86_indirect_thunk_\reg)
#define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)
#define EXPORT_THUNK(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
.align RETPOLINE_THUNK_SIZE
SYM_CODE_START(__x86_indirect_thunk_array)
#define GEN(reg) THUNK reg
#include <asm/GEN-for-each-reg.h>
#undef GEN
.align RETPOLINE_THUNK_SIZE
SYM_CODE_END(__x86_indirect_thunk_array)
#define GEN(reg) EXPORT_THUNK(reg)
#include <asm/GEN-for-each-reg.h>
#undef GEN