[PATCH 5.10 034/148] objtool/x86: Rewrite retpoline thunk calls

Greg Kroah-Hartman
2022-07-23T09:59:30Z

From: Peter Zijlstra <peterz@infradead.org>

commit 9bc0bb50727c8ac69fbb33fb937431cf3518ff37 upstream.

When the compiler emits: "CALL __x86_indirect_thunk_\reg" for an
indirect call, have objtool rewrite it to:

ALTERNATIVE "call __x86_indirect_thunk_\reg",
"call *%reg", ALT_NOT(X86_FEATURE_RETPOLINE)

Additionally, in order to not emit endless identical
.altinst_replacement chunks, use a global symbol for them, see
__x86_indirect_alt_*.

This also avoids objtool from having to do code generation.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lkml.kernel.org/r/20210326151300.320177914@infradead.org
[bwh: Backported to 5.10: include "arch_elf.h" instead of "arch/elf.h"]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/asm-prototypes.h | 12 ++-
arch/x86/lib/retpoline.S | 41 +++++++++++
tools/objtool/arch/x86/decode.c | 117 ++++++++++++++++++++++++++++++++++
3 files changed, 167 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/asm-prototypes.h
+++ b/arch/x86/include/asm/asm-prototypes.h
@@ -19,11 +19,19 @@ extern void cmpxchg8b_emu(void);

#ifdef CONFIG_RETPOLINE

-#define DECL_INDIRECT_THUNK(reg) \
+#undef GEN
+#define GEN(reg) \
extern asmlinkage void __x86_indirect_thunk_ ## reg (void);
+#include <asm/GEN-for-each-reg.h>
+
+#undef GEN
+#define GEN(reg) \
+ extern asmlinkage void __x86_indirect_alt_call_ ## reg (void);
+#include <asm/GEN-for-each-reg.h>

#undef GEN
-#define GEN(reg) DECL_INDIRECT_THUNK(reg)
+#define GEN(reg) \
+ extern asmlinkage void __x86_indirect_alt_jmp_ ## reg (void);
#include <asm/GEN-for-each-reg.h>

#endif /* CONFIG_RETPOLINE */
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -10,6 +10,8 @@
#include <asm/unwind_hints.h>
#include <asm/frame.h>

+ .section .text.__x86.indirect_thunk
+
.macro RETPOLINE reg
ANNOTATE_INTRA_FUNCTION_CALL
call .Ldo_rop_\@
@@ -25,9 +27,9 @@
.endm

.macro THUNK reg
- .section .text.__x86.indirect_thunk

.align 32
+
SYM_FUNC_START(__x86_indirect_thunk_\reg)

ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
@@ -39,6 +41,32 @@ SYM_FUNC_END(__x86_indirect_thunk_\reg)
.endm

/*
+ * This generates .altinstr_replacement symbols for use by objtool. They,
+ * however, must not actually live in .altinstr_replacement since that will be
+ * discarded after init, but module alternatives will also reference these
+ * symbols.
+ *
+ * Their names matches the "__x86_indirect_" prefix to mark them as retpolines.
+ */
+.macro ALT_THUNK reg
+
+ .align 1
+
+SYM_FUNC_START_NOALIGN(__x86_indirect_alt_call_\reg)
+ ANNOTATE_RETPOLINE_SAFE
+1: call *%\reg
+2: .skip 5-(2b-1b), 0x90
+SYM_FUNC_END(__x86_indirect_alt_call_\reg)
+
+SYM_FUNC_START_NOALIGN(__x86_indirect_alt_jmp_\reg)
+ ANNOTATE_RETPOLINE_SAFE
+1: jmp *%\reg
+2: .skip 5-(2b-1b), 0x90
+SYM_FUNC_END(__x86_indirect_alt_jmp_\reg)
+
+.endm
+
+/*
* Despite being an assembler file we can't just use .irp here
* because __KSYM_DEPS__ only uses the C preprocessor and would
* only see one instance of "__x86_indirect_thunk_\reg" rather
@@ -61,3 +89,14 @@ SYM_FUNC_END(__x86_indirect_thunk_\reg)
#define GEN(reg) EXPORT_THUNK(reg)
#include <asm/GEN-for-each-reg.h>

+#undef GEN
+#define GEN(reg) ALT_THUNK reg
+#include <asm/GEN-for-each-reg.h>
+
+#undef GEN
+#define GEN(reg) __EXPORT_THUNK(__x86_indirect_alt_call_ ## reg)
+#include <asm/GEN-for-each-reg.h>
+
+#undef GEN
+#define GEN(reg) __EXPORT_THUNK(__x86_indirect_alt_jmp_ ## reg)
+#include <asm/GEN-for-each-reg.h>
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -16,6 +16,7 @@
#include "../../arch.h"
#include "../../warn.h"
#include <asm/orc_types.h>
+#include "arch_elf.h"

static unsigned char op_to_cfi_reg[][2] = {
{CFI_AX, CFI_R8},
@@ -585,6 +586,122 @@ const char *arch_nop_insn(int len)
return nops[len-1];
}

+/* asm/alternative.h ? */
+
+#define ALTINSTR_FLAG_INV (1 << 15)
+#define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV)
+
+struct alt_instr {
+ s32 instr_offset; /* original instruction */
+ s32 repl_offset; /* offset to replacement instruction */
+ u16 cpuid; /* cpuid bit set for replacement */
+ u8 instrlen; /* length of original instruction */
+ u8 replacementlen; /* length of new instruction */
+} __packed;
+
+static int elf_add_alternative(struct elf *elf,
+ struct instruction *orig, struct symbol *sym,
+ int cpuid, u8 orig_len, u8 repl_len)
+{
+ const int size = sizeof(struct alt_instr);
+ struct alt_instr *alt;
+ struct section *sec;
+ Elf_Scn *s;
+
+ sec = find_section_by_name(elf, ".altinstructions");
+ if (!sec) {
+ sec = elf_create_section(elf, ".altinstructions",
+ SHF_WRITE, size, 0);
+
+ if (!sec) {
+ WARN_ELF("elf_create_section");
+ return -1;
+ }
+ }
+
+ s = elf_getscn(elf->elf, sec->idx);
+ if (!s) {
+ WARN_ELF("elf_getscn");
+ return -1;
+ }
+
+ sec->data = elf_newdata(s);
+ if (!sec->data) {
+ WARN_ELF("elf_newdata");
+ return -1;
+ }
+
+ sec->data->d_size = size;
+ sec->data->d_align = 1;
+
+ alt = sec->data->d_buf = malloc(size);
+ if (!sec->data->d_buf) {
+ perror("malloc");
+ return -1;
+ }
+ memset(sec->data->d_buf, 0, size);
+
+ if (elf_add_reloc_to_insn(elf, sec, sec->sh.sh_size,
+ R_X86_64_PC32, orig->sec, orig->offset)) {
+ WARN("elf_create_reloc: alt_instr::instr_offset");
+ return -1;
+ }
+
+ if (elf_add_reloc(elf, sec, sec->sh.sh_size + 4,
+ R_X86_64_PC32, sym, 0)) {
+ WARN("elf_create_reloc: alt_instr::repl_offset");
+ return -1;
+ }
+
+ alt->cpuid = cpuid;
+ alt->instrlen = orig_len;
+ alt->replacementlen = repl_len;
+
+ sec->sh.sh_size += size;
+ sec->changed = true;
+
+ return 0;
+}
+
+#define X86_FEATURE_RETPOLINE ( 7*32+12)
+
+int arch_rewrite_retpolines(struct objtool_file *file)
+{
+ struct instruction *insn;
+ struct reloc *reloc;
+ struct symbol *sym;
+ char name[32] = "";
+
+ list_for_each_entry(insn, &file->retpoline_call_list, call_node) {
+
+ if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
+ continue;
+
+ reloc = insn->reloc;
+
+ sprintf(name, "__x86_indirect_alt_%s_%s",
+ insn->type == INSN_JUMP_DYNAMIC ? "jmp" : "call",
+ reloc->sym->name + 21);
+
+ sym = find_symbol_by_name(file->elf, name);
+ if (!sym) {
+ sym = elf_create_undef_symbol(file->elf, name);
+ if (!sym) {
+ WARN("elf_create_undef_symbol");
+ return -1;
+ }
+ }
+
+ if (elf_add_alternative(file->elf, insn, sym,
+ ALT_NOT(X86_FEATURE_RETPOLINE), 5, 5)) {
+ WARN("elf_add_alternative");
+ return -1;
+ }
+ }
+
+ return 0;
+}
+
int arch_decode_hint_reg(struct instruction *insn, u8 sp_reg)
{
struct cfi_reg *cfa = &insn->cfi.cfa;