[PATCH v4 07/16] objtool: Convert fixed location stack protector accesses

Brian Gerst
2024-03-22T16:52:56Z

Older versions of GCC fixed the location of the stack protector canary
at %gs:40. Use objtool to convert these accesses to normal percpu
accesses to __stack_chk_guard.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
---
arch/x86/Kconfig | 4 ++
scripts/Makefile.lib | 2 +
tools/objtool/arch/x86/decode.c | 46 +++++++++++++
tools/objtool/arch/x86/special.c | 91 +++++++++++++++++++++++++
tools/objtool/builtin-check.c | 9 ++-
tools/objtool/check.c | 12 ++++
tools/objtool/elf.c | 34 +++++++--
tools/objtool/include/objtool/arch.h | 3 +
tools/objtool/include/objtool/builtin.h | 2 +
tools/objtool/include/objtool/elf.h | 6 ++
10 files changed, 204 insertions(+), 5 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 88d72227e3cb..121cfb9ffc0e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -417,6 +417,10 @@ config CC_HAS_SANE_STACKPROTECTOR
We have to make sure stack protector is unconditionally disabled if
the compiler does not allow control of the segment and symbol.

+config STACKPROTECTOR_OBJTOOL
+ bool
+ default n
+
menu "Processor type and features"

config SMP
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 1bd59b8db05f..6bc4c69a9e50 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -258,6 +258,8 @@ objtool := $(objtree)/tools/objtool/objtool
objtool-args-$(CONFIG_HAVE_JUMP_LABEL_HACK) += --hacks=jump_label
objtool-args-$(CONFIG_HAVE_NOINSTR_HACK) += --hacks=noinstr
objtool-args-$(CONFIG_MITIGATION_CALL_DEPTH_TRACKING) += --hacks=skylake
+objtool-args-$(CONFIG_STACKPROTECTOR_OBJTOOL) += --hacks=stackprotector
+objtool-args-$(CONFIG_SMP) += --smp
objtool-args-$(CONFIG_X86_KERNEL_IBT) += --ibt
objtool-args-$(CONFIG_FINEIBT) += --cfi
objtool-args-$(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL) += --mcount
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 3a1d80a7878d..583a16b8bf47 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -144,6 +144,18 @@ static bool has_notrack_prefix(struct insn *insn)
return false;
}

+static bool has_gs_prefix(struct insn *insn)
+{
+ int i;
+
+ for (i = 0; i < insn->prefixes.nbytes; i++) {
+ if (insn->prefixes.bytes[i] == 0x65)
+ return true;
+ }
+
+ return false;
+}
+
int arch_decode_instruction(struct objtool_file *file, const struct section *sec,
unsigned long offset, unsigned int maxlen,
struct instruction *insn)
@@ -408,10 +420,44 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec

break;

+ case 0x2b:
+ case 0x3b:
+ case 0x39:
+ if (!rex_w)
+ break;
+
+ /* sub %gs:0x28, reg */
+ /* cmp %gs:0x28, reg */
+ /* cmp reg, %gs:0x28 */
+ if (has_gs_prefix(&ins) &&
+ modrm_mod == 0 &&
+ modrm_rm == 4 &&
+ sib_index == 4 &&
+ sib_base == 5 &&
+ ins.displacement.value == 0x28)
+ {
+ insn->type = INSN_STACKPROTECTOR;
+ break;
+ }
+
+ break;
+
case 0x8b:
if (!rex_w)
break;

+ /* mov %gs:0x28, reg */
+ if (has_gs_prefix(&ins) &&
+ modrm_mod == 0 &&
+ modrm_rm == 4 &&
+ sib_index == 4 &&
+ sib_base == 5 &&
+ ins.displacement.value == 0x28)
+ {
+ insn->type = INSN_STACKPROTECTOR;
+ break;
+ }
+
if (rm_is_mem(CFI_BP)) {

/* mov disp(%rbp), reg */
diff --git a/tools/objtool/arch/x86/special.c b/tools/objtool/arch/x86/special.c
index 4134d27c696b..020b6040c487 100644
--- a/tools/objtool/arch/x86/special.c
+++ b/tools/objtool/arch/x86/special.c
@@ -3,6 +3,9 @@

#include <objtool/special.h>
#include <objtool/builtin.h>
+#include <objtool/warn.h>
+#include <objtool/check.h>
+#include <objtool/elf.h>

#define X86_FEATURE_POPCNT (4 * 32 + 23)
#define X86_FEATURE_SMAP (9 * 32 + 20)
@@ -137,3 +140,91 @@ struct reloc *arch_find_switch_table(struct objtool_file *file,

return rodata_reloc;
}
+
+/*
+ * Convert op %gs:0x28, reg -> op __stack_chk_guard(%rip), reg
+ * op is MOV, SUB, or CMP.
+ *
+ * This can be removed when the minimum supported GCC version is raised
+ * to 8.1 or later.
+ */
+int arch_hack_stackprotector(struct objtool_file *file)
+{
+ struct section *sec;
+ struct symbol *__stack_chk_guard;
+ struct instruction *insn;
+
+ int i;
+
+ __stack_chk_guard = find_symbol_by_name(file->elf, "__stack_chk_guard");
+
+ for_each_sec(file, sec) {
+ int count = 0;
+ int idx;
+ struct section *rsec = sec->rsec;
+
+ sec_for_each_insn(file, sec, insn) {
+ if (insn->type == INSN_STACKPROTECTOR)
+ count++;
+ }
+
+ if (!count)
+ continue;
+
+ if (!__stack_chk_guard)
+ __stack_chk_guard = elf_create_undef_symbol(file->elf, "__stack_chk_guard");
+
+ if (!rsec) {
+ idx = 0;
+ rsec = sec->rsec = elf_create_rela_section(file->elf, sec, count);
+ } else {
+ idx = sec_num_entries(rsec);
+ if (elf_extend_rela_section(file->elf, rsec, count))
+ return -1;
+ }
+
+ sec_for_each_insn(file, sec, insn) {
+ unsigned char *data = sec->data->d_buf + insn->offset;
+
+ if (insn->type != INSN_STACKPROTECTOR)
+ continue;
+
+ if (insn->len != 9)
+ goto invalid;
+
+ /* Convert GS prefix to DS if !SMP */
+ if (data[0] != 0x65)
+ goto invalid;
+ if (!opts.smp)
+ data[0] = 0x3e;
+
+ /* Set Mod=00, R/M=101. Preserve Reg */
+ data[3] = (data[3] & 0x38) | 5;
+
+ /* Displacement 0 */
+ data[4] = 0;
+ data[5] = 0;
+ data[6] = 0;
+ data[7] = 0;
+
+ /* Pad with NOP */
+ data[8] = 0x90;
+
+ if (!elf_init_reloc_data_sym(file->elf, sec, insn->offset + 4, idx++, __stack_chk_guard, -4))
+ return -1;
+
+ continue;
+
+invalid:
+ fprintf(stderr, "Invalid stackprotector instruction at %s+0x%lx: ", sec->name, insn->offset);
+ for (i = 0; i < insn->len; i++)
+ fprintf(stderr, "%02x ", data[i]);
+ fprintf(stderr, "\n");
+ return -1;
+ }
+
+ mark_sec_changed(file->elf, sec, true);
+ }
+
+ return 0;
+}
diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index 5e21cfb7661d..0ab2efb45c0e 100644
--- a/tools/objtool/builtin-check.c
+++ b/tools/objtool/builtin-check.c
@@ -62,12 +62,17 @@ static int parse_hacks(const struct option *opt, const char *str, int unset)
found = true;
}

+ if (!str || strstr(str, "stackprotector")) {
+ opts.hack_stackprotector = true;
+ found = true;
+ }
+
return found ? 0 : -1;
}

static const struct option check_options[] = {
OPT_GROUP("Actions:"),
- OPT_CALLBACK_OPTARG('h', "hacks", NULL, NULL, "jump_label,noinstr,skylake", "patch toolchain bugs/limitations", parse_hacks),
+ OPT_CALLBACK_OPTARG('h', "hacks", NULL, NULL, "jump_label,noinstr,skylake,stackprotector", "patch toolchain bugs/limitations", parse_hacks),
OPT_BOOLEAN('i', "ibt", &opts.ibt, "validate and annotate IBT"),
OPT_BOOLEAN('m', "mcount", &opts.mcount, "annotate mcount/fentry calls for ftrace"),
OPT_BOOLEAN('n', "noinstr", &opts.noinstr, "validate noinstr rules"),
@@ -94,6 +99,7 @@ static const struct option check_options[] = {
OPT_BOOLEAN(0, "sec-address", &opts.sec_address, "print section addresses in warnings"),
OPT_BOOLEAN(0, "stats", &opts.stats, "print statistics"),
OPT_BOOLEAN('v', "verbose", &opts.verbose, "verbose warnings"),
+ OPT_BOOLEAN(0, "smp", &opts.smp, "building an SMP kernel"),

OPT_END(),
};
@@ -133,6 +139,7 @@ static bool opts_valid(void)
{
if (opts.hack_jump_label ||
opts.hack_noinstr ||
+ opts.hack_stackprotector ||
opts.ibt ||
opts.mcount ||
opts.noinstr ||
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0a2c161fc04d..0056dd99ff7f 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1315,6 +1315,11 @@ __weak bool arch_is_embedded_insn(struct symbol *sym)
return false;
}

+__weak int arch_hack_stackprotector(struct objtool_file *file)
+{
+ return 0;
+}
+
static struct reloc *insn_reloc(struct objtool_file *file, struct instruction *insn)
{
struct reloc *reloc;
@@ -4824,6 +4829,13 @@ int check(struct objtool_file *file)
warnings += ret;
}

+ if (opts.hack_stackprotector) {
+ ret = arch_hack_stackprotector(file);
+ if (ret < 0)
+ goto out;
+ warnings += ret;
+ }
+
free_insns(file);

if (opts.verbose)
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index cfb970727c8a..2af99b2a054c 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -846,6 +846,32 @@ elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size)
return sym;
}

+struct symbol *
+elf_create_undef_symbol(struct elf *elf, const char *sym_name)
+{
+ struct symbol *sym = calloc(1, sizeof(*sym));
+ char *name = strdup(sym_name);
+
+ if (!sym || !name) {
+ perror("malloc");
+ return NULL;
+ }
+
+ sym->name = name;
+ sym->sec = find_section_by_index(elf, 0);
+
+ sym->sym.st_name = elf_add_string(elf, NULL, name);
+ sym->sym.st_info = GELF_ST_INFO(STB_GLOBAL, STT_NOTYPE);
+ sym->sym.st_value = 0;
+ sym->sym.st_size = 0;
+
+ sym = __elf_create_symbol(elf, sym);
+ if (sym)
+ elf_add_symbol(elf, sym);
+
+ return sym;
+}
+
static struct reloc *elf_init_reloc(struct elf *elf, struct section *rsec,
unsigned int reloc_idx,
unsigned long offset, struct symbol *sym,
@@ -924,7 +950,7 @@ struct reloc *elf_init_reloc_data_sym(struct elf *elf, struct section *sec,
struct symbol *sym,
s64 addend)
{
- if (sym->sec && (sec->sh.sh_flags & SHF_EXECINSTR)) {
+ if (sym->sec && (sym->sec->sh.sh_flags & SHF_EXECINSTR)) {
WARN("bad call to %s() for text symbol %s",
__func__, sym->name);
return NULL;
@@ -1196,9 +1222,9 @@ struct section *elf_create_section(struct elf *elf, const char *name,
return sec;
}

-static struct section *elf_create_rela_section(struct elf *elf,
- struct section *sec,
- unsigned int reloc_nr)
+struct section *elf_create_rela_section(struct elf *elf,
+ struct section *sec,
+ unsigned int reloc_nr)
{
struct section *rsec;
struct reloc_block *block;
diff --git a/tools/objtool/include/objtool/arch.h b/tools/objtool/include/objtool/arch.h
index 0b303eba660e..c60fec88b3af 100644
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -28,6 +28,7 @@ enum insn_type {
INSN_CLD,
INSN_TRAP,
INSN_ENDBR,
+ INSN_STACKPROTECTOR,
INSN_OTHER,
};

@@ -96,4 +97,6 @@ int arch_rewrite_retpolines(struct objtool_file *file);

bool arch_pc_relative_reloc(struct reloc *reloc);

+int arch_hack_stackprotector(struct objtool_file *file);
+
#endif /* _ARCH_H */
diff --git a/tools/objtool/include/objtool/builtin.h b/tools/objtool/include/objtool/builtin.h
index fcca6662c8b4..5085d3135e6b 100644
--- a/tools/objtool/include/objtool/builtin.h
+++ b/tools/objtool/include/objtool/builtin.h
@@ -13,6 +13,7 @@ struct opts {
bool hack_jump_label;
bool hack_noinstr;
bool hack_skylake;
+ bool hack_stackprotector;
bool ibt;
bool mcount;
bool noinstr;
@@ -38,6 +39,7 @@ struct opts {
bool sec_address;
bool stats;
bool verbose;
+ bool smp;
};

extern struct opts opts;
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 7851467f6878..b5eec9e4a65d 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -120,6 +120,10 @@ struct elf *elf_open_read(const char *name, int flags);
struct section *elf_create_section(struct elf *elf, const char *name,
size_t entsize, unsigned int nr);

+struct section *elf_create_rela_section(struct elf *elf,
+ struct section *sec,
+ unsigned int reloc_nr);
+
int elf_extend_rela_section(struct elf *elf,
struct section *rsec,
int add_relocs);
@@ -130,6 +134,8 @@ struct section *elf_create_section_pair(struct elf *elf, const char *name,

struct symbol *elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size);

+struct symbol *elf_create_undef_symbol(struct elf *elf, const char *sym_name);
+
struct reloc *elf_init_reloc_text_sym(struct elf *elf, struct section *sec,
unsigned long offset,
unsigned int reloc_idx,
--
2.44.0