diff mbox series

[v4,1/4] x86/ibt: factor out cfi and fineibt offset

Message ID 20250303132837.498938-2-dongml2@chinatelecom.cn (mailing list archive)
State New
Headers show
Series per-function storage support | expand

Commit Message

Menglong Dong March 3, 2025, 1:28 p.m. UTC 
For now, the layout of cfi and fineibt is hard coded, and the padding is
fixed on 16 bytes.

Factor out FINEIBT_INSN_OFFSET and CFI_INSN_OFFSET. CFI_INSN_OFFSET is
the offset of cfi, which is the same as FUNCTION_ALIGNMENT when
CALL_PADDING is enabled. And FINEIBT_INSN_OFFSET is the offset where we
put the fineibt preamble on, which is 16 for now.

When the FUNCTION_ALIGNMENT is bigger than 16, we place the fineibt
preamble on the last 16 bytes of the padding for better performance, which
means the fineibt preamble don't use the space that cfi uses.

The FINEIBT_INSN_OFFSET is not used in fineibt_caller_start and
fineibt_paranoid_start, as it is always "0x10". Note that we need to
update the offset in fineibt_caller_start and fineibt_paranoid_start if
FINEIBT_INSN_OFFSET changes.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
---
v4:
- rebase to the newest tip/x86/core, the fineibt has some updating
---
 arch/x86/include/asm/cfi.h    | 13 +++++++++----
 arch/x86/kernel/alternative.c | 18 +++++++++++-------
 arch/x86/net/bpf_jit_comp.c   | 22 +++++++++++-----------
 3 files changed, 31 insertions(+), 22 deletions(-)

Comments

Peter Zijlstra March 3, 2025, 4:54 p.m. UTC | #1 
On Mon, Mar 03, 2025 at 09:28:34PM +0800, Menglong Dong wrote:
> For now, the layout of cfi and fineibt is hard coded, and the padding is
> fixed on 16 bytes.
> 
> Factor out FINEIBT_INSN_OFFSET and CFI_INSN_OFFSET. CFI_INSN_OFFSET is
> the offset of cfi, which is the same as FUNCTION_ALIGNMENT when
> CALL_PADDING is enabled. And FINEIBT_INSN_OFFSET is the offset where we
> put the fineibt preamble on, which is 16 for now.
> 
> When the FUNCTION_ALIGNMENT is bigger than 16, we place the fineibt
> preamble on the last 16 bytes of the padding for better performance, which
> means the fineibt preamble don't use the space that cfi uses.
> 
> The FINEIBT_INSN_OFFSET is not used in fineibt_caller_start and
> fineibt_paranoid_start, as it is always "0x10". Note that we need to
> update the offset in fineibt_caller_start and fineibt_paranoid_start if
> FINEIBT_INSN_OFFSET changes.
> 
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>

I'm confused as to what exactly you mean.

Preamble will have __cfi symbol and some number of NOPs right before
actual symbol like:

__cfi_foo:
  mov $0x12345678, %reg
  nop
  nop
  nop
  ...
foo:

FineIBT must be at foo-16, has nothing to do with performance. This 16
can also be spelled: fineibt_preamble_size.

The total size of the preamble is FUNCTION_PADDING_BYTES + CFI_CLANG*5.

If you increase FUNCTION_PADDING_BYTES by another 5, which is what you
want I think, then we'll have total preamble of 21 bytes; 5 bytes kCFI,
16 bytes nop.

Then kCFI expects hash to be at -20, while FineIBT must be at -16.

This then means there is no unambiguous hole for you to stick your
meta-data thing (whatever that is).

There are two options: make meta data location depend on cfi_mode, or
have __apply_fineibt() rewrite kCFI to also be at -16, so that you can
have -21 for your 5 bytes.

I think I prefer latter.

In any case, I don't think we need *_INSN_OFFSET. At most we need
PREAMBLE_SIZE.

Hmm?
diff mbox series

Patch

diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h
index 2f6a01f098b5..04525f2f6bf2 100644
--- a/arch/x86/include/asm/cfi.h
+++ b/arch/x86/include/asm/cfi.h
@@ -108,6 +108,14 @@  extern bhi_thunk __bhi_args_end[];
 
 struct pt_regs;
 
+#ifdef CONFIG_CALL_PADDING
+#define FINEIBT_INSN_OFFSET	16
+#define CFI_INSN_OFFSET		CONFIG_FUNCTION_ALIGNMENT
+#else
+#define FINEIBT_INSN_OFFSET	0
+#define CFI_INSN_OFFSET		5
+#endif
+
 #ifdef CONFIG_CFI_CLANG
 enum bug_trap_type handle_cfi_failure(struct pt_regs *regs);
 #define __bpfcall
@@ -118,11 +126,8 @@  static inline int cfi_get_offset(void)
 {
 	switch (cfi_mode) {
 	case CFI_FINEIBT:
-		return 16;
 	case CFI_KCFI:
-		if (IS_ENABLED(CONFIG_CALL_PADDING))
-			return 16;
-		return 5;
+		return CFI_INSN_OFFSET;
 	default:
 		return 0;
 	}
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 32e4b801db99..0088d2313f33 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -917,7 +917,7 @@  void __init_or_module noinline apply_seal_endbr(s32 *start, s32 *end)
 
 		poison_endbr(addr);
 		if (IS_ENABLED(CONFIG_FINEIBT))
-			poison_cfi(addr - 16);
+			poison_cfi(addr);
 	}
 }
 
@@ -980,12 +980,13 @@  u32 cfi_get_func_hash(void *func)
 {
 	u32 hash;
 
-	func -= cfi_get_offset();
 	switch (cfi_mode) {
 	case CFI_FINEIBT:
+		func -= FINEIBT_INSN_OFFSET;
 		func += 7;
 		break;
 	case CFI_KCFI:
+		func -= CFI_INSN_OFFSET;
 		func += 1;
 		break;
 	default:
@@ -1372,7 +1373,7 @@  static int cfi_rewrite_preamble(s32 *start, s32 *end)
 		 * have determined there are no indirect calls to it and we
 		 * don't need no CFI either.
 		 */
-		if (!is_endbr(addr + 16))
+		if (!is_endbr(addr + CFI_INSN_OFFSET))
 			continue;
 
 		hash = decode_preamble_hash(addr, &arity);
@@ -1380,6 +1381,7 @@  static int cfi_rewrite_preamble(s32 *start, s32 *end)
 			 addr, addr, 5, addr))
 			return -EINVAL;
 
+		addr += (CFI_INSN_OFFSET - FINEIBT_INSN_OFFSET);
 		text_poke_early(addr, fineibt_preamble_start, fineibt_preamble_size);
 		WARN_ON(*(u32 *)(addr + fineibt_preamble_hash) != 0x12345678);
 		text_poke_early(addr + fineibt_preamble_hash, &hash, 4);
@@ -1402,10 +1404,10 @@  static void cfi_rewrite_endbr(s32 *start, s32 *end)
 	for (s = start; s < end; s++) {
 		void *addr = (void *)s + *s;
 
-		if (!exact_endbr(addr + 16))
+		if (!exact_endbr(addr + CFI_INSN_OFFSET))
 			continue;
 
-		poison_endbr(addr + 16);
+		poison_endbr(addr + CFI_INSN_OFFSET);
 	}
 }
 
@@ -1543,12 +1545,12 @@  static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
 		return;
 
 	case CFI_FINEIBT:
-		/* place the FineIBT preamble at func()-16 */
+		/* place the FineIBT preamble at func()-FINEIBT_INSN_OFFSET */
 		ret = cfi_rewrite_preamble(start_cfi, end_cfi);
 		if (ret)
 			goto err;
 
-		/* rewrite the callers to target func()-16 */
+		/* rewrite the callers to target func()-FINEIBT_INSN_OFFSET */
 		ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
 		if (ret)
 			goto err;
@@ -1588,6 +1590,7 @@  static void poison_cfi(void *addr)
 	 */
 	switch (cfi_mode) {
 	case CFI_FINEIBT:
+		addr -= FINEIBT_INSN_OFFSET;
 		/*
 		 * FineIBT prefix should start with an ENDBR.
 		 */
@@ -1607,6 +1610,7 @@  static void poison_cfi(void *addr)
 		break;
 
 	case CFI_KCFI:
+		addr -= CFI_INSN_OFFSET;
 		/*
 		 * kCFI prefix should start with a valid hash.
 		 */
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 72776dcb75aa..ee86a5df5ffb 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -415,6 +415,12 @@  static int emit_call(u8 **prog, void *func, void *ip);
 static void emit_fineibt(u8 **pprog, u8 *ip, u32 hash, int arity)
 {
 	u8 *prog = *pprog;
+#ifdef CONFIG_CALL_PADDING
+	int i;
+
+	for (i = 0; i < CFI_INSN_OFFSET - 16; i++)
+		EMIT1(0x90);
+#endif
 
 	EMIT_ENDBR();
 	EMIT3_off32(0x41, 0x81, 0xea, hash);		/* subl $hash, %r10d	*/
@@ -432,20 +438,14 @@  static void emit_fineibt(u8 **pprog, u8 *ip, u32 hash, int arity)
 static void emit_kcfi(u8 **pprog, u32 hash)
 {
 	u8 *prog = *pprog;
+#ifdef CONFIG_CALL_PADDING
+	int i;
+#endif
 
 	EMIT1_off32(0xb8, hash);			/* movl $hash, %eax	*/
 #ifdef CONFIG_CALL_PADDING
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
+	for (i = 0; i < CFI_INSN_OFFSET - 5; i++)
+		EMIT1(0x90);
 #endif
 	EMIT_ENDBR();