Add CONFIG_CFI_CLANG error handling and allow the config to be selected
on x86_64.
Signed-off-by: Sami Tolvanen <[email protected]>
---
arch/x86/Kconfig | 1 +
arch/x86/include/asm/linkage.h | 7 ++++++
arch/x86/kernel/traps.c | 39 +++++++++++++++++++++++++++++++++-
3 files changed, 46 insertions(+), 1 deletion(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b0142e01002e..01db5c5c4dde 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -108,6 +108,7 @@ config X86
select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64
select ARCH_SUPPORTS_NUMA_BALANCING if X86_64
select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP if NR_CPUS <= 4096
+ select ARCH_SUPPORTS_CFI_CLANG if X86_64
select ARCH_SUPPORTS_LTO_CLANG
select ARCH_SUPPORTS_LTO_CLANG_THIN
select ARCH_USE_BUILTIN_BSWAP
diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
index 85865f1645bd..d20acf5ebae3 100644
--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -25,6 +25,13 @@
#define RET ret
#endif
+#ifdef CONFIG_CFI_CLANG
+#define __CFI_TYPE(name) \
+ .fill 10, 1, 0x90 ASM_NL \
+ .4byte __kcfi_typeid_##name ASM_NL \
+ .fill 2, 1, 0xcc
+#endif
+
#else /* __ASSEMBLY__ */
#ifdef CONFIG_SLS
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 1563fb995005..b9e46e6ed83b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -40,6 +40,7 @@
#include <linux/hardirq.h>
#include <linux/atomic.h>
#include <linux/ioasid.h>
+#include <linux/cfi.h>
#include <asm/stacktrace.h>
#include <asm/processor.h>
@@ -295,6 +296,41 @@ static inline void handle_invalid_op(struct pt_regs *regs)
ILL_ILLOPN, error_get_trap_addr(regs));
}
+#ifdef CONFIG_CFI_CLANG
+void *arch_get_cfi_target(unsigned long addr, struct pt_regs *regs)
+{
+ char buffer[MAX_INSN_SIZE];
+ int offset;
+ struct insn insn;
+ unsigned long *target;
+
+ /*
+ * The expected CFI check instruction sequence:
+ * cmpl <id>, -6(%reg) ; 7 bytes
+ * je .Ltmp1 ; 2 bytes
+ * ud2 ; <- addr
+ * .Ltmp1:
+ *
+ * Therefore, the target address is in a register that we can
+ * decode from the cmpl instruction.
+ */
+ if (copy_from_kernel_nofault(buffer, (void *)addr - 9, MAX_INSN_SIZE))
+ return NULL;
+ if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64))
+ return NULL;
+ if (insn.opcode.value != 0x81)
+ return NULL;
+
+ offset = insn_get_modrm_rm_off(&insn, regs);
+ if (offset < 0)
+ return NULL;
+
+ target = (void *)regs + offset;
+
+ return (void *)*target;
+}
+#endif
+
static noinstr bool handle_bug(struct pt_regs *regs)
{
bool handled = false;
@@ -312,7 +348,8 @@ static noinstr bool handle_bug(struct pt_regs *regs)
*/
if (regs->flags & X86_EFLAGS_IF)
raw_local_irq_enable();
- if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+ if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
+ report_cfi(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
regs->ip += LEN_UD2;
handled = true;
}
--
2.36.0.464.gb9c8b46e94-goog
On Fri, Apr 29, 2022 at 01:36:44PM -0700, Sami Tolvanen wrote:
> Add CONFIG_CFI_CLANG error handling and allow the config to be selected
> on x86_64.
Might be useful to have an example output of all ths somewhere, because
unless I go build my own clang again, I can't tell from these patches
what actual codegen looks like.
Going from the below, I seem to be able to reverse engineer some of it:
.long \signature
int3
int3
my_func:
ENDBR
...
ret
And then the callsites look like (clang *always* uses r11, right?):
cmpl \signature, -6(%r11)
je 1f
ud2
1:
call __x86_indirect_thunk_r11
> Signed-off-by: Sami Tolvanen <[email protected]>
> ---
> arch/x86/Kconfig | 1 +
> arch/x86/include/asm/linkage.h | 7 ++++++
> arch/x86/kernel/traps.c | 39 +++++++++++++++++++++++++++++++++-
> 3 files changed, 46 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b0142e01002e..01db5c5c4dde 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -108,6 +108,7 @@ config X86
> select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64
> select ARCH_SUPPORTS_NUMA_BALANCING if X86_64
> select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP if NR_CPUS <= 4096
> + select ARCH_SUPPORTS_CFI_CLANG if X86_64
> select ARCH_SUPPORTS_LTO_CLANG
> select ARCH_SUPPORTS_LTO_CLANG_THIN
> select ARCH_USE_BUILTIN_BSWAP
> diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
> index 85865f1645bd..d20acf5ebae3 100644
> --- a/arch/x86/include/asm/linkage.h
> +++ b/arch/x86/include/asm/linkage.h
> @@ -25,6 +25,13 @@
> #define RET ret
> #endif
>
> +#ifdef CONFIG_CFI_CLANG
> +#define __CFI_TYPE(name) \
> + .fill 10, 1, 0x90 ASM_NL \
> + .4byte __kcfi_typeid_##name ASM_NL \
> + .fill 2, 1, 0xcc
> +#endif
> +
> #else /* __ASSEMBLY__ */
>
> #ifdef CONFIG_SLS
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 1563fb995005..b9e46e6ed83b 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -40,6 +40,7 @@
> #include <linux/hardirq.h>
> #include <linux/atomic.h>
> #include <linux/ioasid.h>
> +#include <linux/cfi.h>
>
> #include <asm/stacktrace.h>
> #include <asm/processor.h>
> @@ -295,6 +296,41 @@ static inline void handle_invalid_op(struct pt_regs *regs)
> ILL_ILLOPN, error_get_trap_addr(regs));
> }
>
> +#ifdef CONFIG_CFI_CLANG
> +void *arch_get_cfi_target(unsigned long addr, struct pt_regs *regs)
> +{
> + char buffer[MAX_INSN_SIZE];
> + int offset;
> + struct insn insn;
> + unsigned long *target;
Reverse xmas please..
> +
> + /*
> + * The expected CFI check instruction sequence:
> + * ? cmpl? ? <id>, -6(%reg) ; 7 bytes
> + * je .Ltmp1 ; 2 bytes
> + * ud2 ; <- addr
> + * .Ltmp1:
> + *
> + * Therefore, the target address is in a register that we can
> + * decode from the cmpl instruction.
> + */
> + if (copy_from_kernel_nofault(buffer, (void *)addr - 9, MAX_INSN_SIZE))
> + return NULL;
> + if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64))
> + return NULL;
insn_decode_kernel()
> + if (insn.opcode.value != 0x81)
> + return NULL;
That's not sufficient to uniquely identify cmp, you also need to look at
the modrm to find r==7 I think.
> +
> + offset = insn_get_modrm_rm_off(&insn, regs);
> + if (offset < 0)
> + return NULL;
> +
> + target = (void *)regs + offset;
> +
> + return (void *)*target;
> +}
> +#endif
> +
> static noinstr bool handle_bug(struct pt_regs *regs)
> {
> bool handled = false;
> @@ -312,7 +348,8 @@ static noinstr bool handle_bug(struct pt_regs *regs)
> */
> if (regs->flags & X86_EFLAGS_IF)
> raw_local_irq_enable();
> - if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
> + if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
> + report_cfi(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
This way you'll first get a BUG splat and then tack on the CFI thing.
Seems a bit daft to have two splats.
> regs->ip += LEN_UD2;
> handled = true;
> }
> --
> 2.36.0.464.gb9c8b46e94-goog
>
On Sat, Apr 30, 2022 at 2:24 AM Peter Zijlstra <[email protected]> wrote:
>
> On Fri, Apr 29, 2022 at 01:36:44PM -0700, Sami Tolvanen wrote:
> > - if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
> > + if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
> > + report_cfi(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
>
> This way you'll first get a BUG splat and then tack on the CFI thing.
The CFI ud2 isn't in the bug table, which means find_bug returns
BUG_TRAP_TYPE_NONE and report_bug bails out before printing out
anything.
Sami