diff mbox series

[bpf-next,v3,2/3] bpf: use check_add_overflow() to check for addition overflows

Message ID 20240712080127.136608-3-shung-hsi.yu@suse.com (mailing list archive)
State Accepted
Commit 28a4411076b254c67842348e3b25c2fb41a94cad
Delegated to: BPF
Headers show
Series Use overflow.h helpers to check for overflows | expand

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Commit Message

Shung-Hsi Yu July 12, 2024, 8:01 a.m. UTC 
signed_add*_overflows() was added back when there was no overflow-check
helper. With the introduction of such helpers in commit f0907827a8a91
("compiler.h: enable builtin overflow checkers and add fallback code"), we
can drop signed_add*_overflows() in kernel/bpf/verifier.c and use the
generic check_add_overflow() instead.

This will make future refactoring easier, and takes advantage of
compiler-emitted hardware instructions that efficiently implement these
checks.

After the change GCC 13.3.0 generates cleaner assembly on x86_64:

	err = adjust_scalar_min_max_vals(env, insn, dst_reg, *src_reg);
   13625:	mov    0x28(%rbx),%r9  /*  r9 = src_reg->smin_value */
   13629:	mov    0x30(%rbx),%rcx /* rcx = src_reg->smax_value */
   ...
	if (check_add_overflow(*dst_smin, src_reg->smin_value, dst_smin) ||
   141c1:	mov    %r9,%rax
   141c4:	add    0x28(%r12),%rax
   141c9:	mov    %rax,0x28(%r12)
   141ce:	jo     146e4 <adjust_reg_min_max_vals+0x1294>
	    check_add_overflow(*dst_smax, src_reg->smax_value, dst_smax)) {
   141d4:	add    0x30(%r12),%rcx
   141d9:	mov    %rcx,0x30(%r12)
	if (check_add_overflow(*dst_smin, src_reg->smin_value, dst_smin) ||
   141de:	jo     146e4 <adjust_reg_min_max_vals+0x1294>
   ...
		*dst_smin = S64_MIN;
   146e4:	movabs $0x8000000000000000,%rax
   146ee:	mov    %rax,0x28(%r12)
		*dst_smax = S64_MAX;
   146f3:	sub    $0x1,%rax
   146f7:	mov    %rax,0x30(%r12)

Before the change it gives:

	s64 smin_val = src_reg->smin_value;
     675:	mov    0x28(%rsi),%r8
	s64 smax_val = src_reg->smax_value;
	u64 umin_val = src_reg->umin_value;
	u64 umax_val = src_reg->umax_value;
     679:	mov    %rdi,%rax /* rax = dst_reg */
	if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
     67c:	mov    0x28(%rdi),%rdi /* rdi = dst_reg->smin_value */
	u64 umin_val = src_reg->umin_value;
     680:	mov    0x38(%rsi),%rdx
	u64 umax_val = src_reg->umax_value;
     684:	mov    0x40(%rsi),%rcx
	s64 res = (s64)((u64)a + (u64)b);
     688:	lea    (%r8,%rdi,1),%r9 /* r9 = dst_reg->smin_value + src_reg->smin_value */
	return res < a;
     68c:	cmp    %r9,%rdi
     68f:	setg   %r10b /* r10b = (dst_reg->smin_value + src_reg->smin_value) > dst_reg->smin_value */
	if (b < 0)
     693:	test   %r8,%r8
     696:	js     72b <scalar_min_max_add+0xbb>
	    signed_add_overflows(dst_reg->smax_value, smax_val)) {
		dst_reg->smin_value = S64_MIN;
		dst_reg->smax_value = S64_MAX;
     69c:	movabs $0x7fffffffffffffff,%rdi
	s64 smax_val = src_reg->smax_value;
     6a6:	mov    0x30(%rsi),%r8
		dst_reg->smin_value = S64_MIN;
     6aa:	00 00 00 	movabs $0x8000000000000000,%rsi
	if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
     6b4:	test   %r10b,%r10b /* (dst_reg->smin_value + src_reg->smin_value) > dst_reg->smin_value ? goto 6cb */
     6b7:	jne    6cb <scalar_min_max_add+0x5b>
	    signed_add_overflows(dst_reg->smax_value, smax_val)) {
     6b9:	mov    0x30(%rax),%r10   /* r10 = dst_reg->smax_value */
	s64 res = (s64)((u64)a + (u64)b);
     6bd:	lea    (%r10,%r8,1),%r11 /* r11 = dst_reg->smax_value + src_reg->smax_value */
	if (b < 0)
     6c1:	test   %r8,%r8
     6c4:	js     71e <scalar_min_max_add+0xae>
	if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
     6c6:	cmp    %r11,%r10 /* (dst_reg->smax_value + src_reg->smax_value) <= dst_reg->smax_value ? goto 723 */
     6c9:	jle    723 <scalar_min_max_add+0xb3>
	} else {
		dst_reg->smin_value += smin_val;
		dst_reg->smax_value += smax_val;
	}
     6cb:	mov    %rsi,0x28(%rax)
     ...
     6d5:	mov    %rdi,0x30(%rax)
     ...
	if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
     71e:	cmp    %r11,%r10
     721:	jl     6cb <scalar_min_max_add+0x5b>
		dst_reg->smin_value += smin_val;
     723:	mov    %r9,%rsi
		dst_reg->smax_value += smax_val;
     726:	mov    %r11,%rdi
     729:	jmp    6cb <scalar_min_max_add+0x5b>
		return res > a;
     72b:	cmp    %r9,%rdi
     72e:	setl   %r10b
     732:	jmp    69c <scalar_min_max_add+0x2c>
     737:	nopw   0x0(%rax,%rax,1)

Note: unlike adjust_ptr_min_max_vals() and scalar*_min_max_add(), it is
necessary to introduce intermediate variable in adjust_jmp_off() to keep
the functional behavior unchanged. Without an intermediate variable
imm/off will be altered even on overflow.

Suggested-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
Change since v2:
- Refactor use of signed_add*_overflows() in adjust_jmp_off() and remove
  signed_add16_overflow(), both added in commit 5337ac4c9b80 ("bpf: Fix
  the corner case with may_goto and jump to the 1st insn.")
---
 kernel/bpf/verifier.c | 114 +++++++++++++-----------------------------
 1 file changed, 34 insertions(+), 80 deletions(-)
diff mbox series

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index cf2eb07ddf28..0126c9c80c58 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12726,36 +12726,6 @@  static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	return 0;
 }
 
-static bool signed_add_overflows(s64 a, s64 b)
-{
-	/* Do the add in u64, where overflow is well-defined */
-	s64 res = (s64)((u64)a + (u64)b);
-
-	if (b < 0)
-		return res > a;
-	return res < a;
-}
-
-static bool signed_add32_overflows(s32 a, s32 b)
-{
-	/* Do the add in u32, where overflow is well-defined */
-	s32 res = (s32)((u32)a + (u32)b);
-
-	if (b < 0)
-		return res > a;
-	return res < a;
-}
-
-static bool signed_add16_overflows(s16 a, s16 b)
-{
-	/* Do the add in u16, where overflow is well-defined */
-	s16 res = (s16)((u16)a + (u16)b);
-
-	if (b < 0)
-		return res > a;
-	return res < a;
-}
-
 static bool signed_sub_overflows(s64 a, s64 b)
 {
 	/* Do the sub in u64, where overflow is well-defined */
@@ -13257,21 +13227,15 @@  static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 		 * added into the variable offset, and we copy the fixed offset
 		 * from ptr_reg.
 		 */
-		if (signed_add_overflows(smin_ptr, smin_val) ||
-		    signed_add_overflows(smax_ptr, smax_val)) {
+		if (check_add_overflow(smin_ptr, smin_val, &dst_reg->smin_value) ||
+		    check_add_overflow(smax_ptr, smax_val, &dst_reg->smax_value)) {
 			dst_reg->smin_value = S64_MIN;
 			dst_reg->smax_value = S64_MAX;
-		} else {
-			dst_reg->smin_value = smin_ptr + smin_val;
-			dst_reg->smax_value = smax_ptr + smax_val;
 		}
-		if (umin_ptr + umin_val < umin_ptr ||
-		    umax_ptr + umax_val < umax_ptr) {
+		if (check_add_overflow(umin_ptr, umin_val, &dst_reg->umin_value) ||
+		    check_add_overflow(umax_ptr, umax_val, &dst_reg->umax_value)) {
 			dst_reg->umin_value = 0;
 			dst_reg->umax_value = U64_MAX;
-		} else {
-			dst_reg->umin_value = umin_ptr + umin_val;
-			dst_reg->umax_value = umax_ptr + umax_val;
 		}
 		dst_reg->var_off = tnum_add(ptr_reg->var_off, off_reg->var_off);
 		dst_reg->off = ptr_reg->off;
@@ -13374,52 +13338,40 @@  static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 static void scalar32_min_max_add(struct bpf_reg_state *dst_reg,
 				 struct bpf_reg_state *src_reg)
 {
-	s32 smin_val = src_reg->s32_min_value;
-	s32 smax_val = src_reg->s32_max_value;
-	u32 umin_val = src_reg->u32_min_value;
-	u32 umax_val = src_reg->u32_max_value;
+	s32 *dst_smin = &dst_reg->s32_min_value;
+	s32 *dst_smax = &dst_reg->s32_max_value;
+	u32 *dst_umin = &dst_reg->u32_min_value;
+	u32 *dst_umax = &dst_reg->u32_max_value;
 
-	if (signed_add32_overflows(dst_reg->s32_min_value, smin_val) ||
-	    signed_add32_overflows(dst_reg->s32_max_value, smax_val)) {
-		dst_reg->s32_min_value = S32_MIN;
-		dst_reg->s32_max_value = S32_MAX;
-	} else {
-		dst_reg->s32_min_value += smin_val;
-		dst_reg->s32_max_value += smax_val;
+	if (check_add_overflow(*dst_smin, src_reg->s32_min_value, dst_smin) ||
+	    check_add_overflow(*dst_smax, src_reg->s32_max_value, dst_smax)) {
+		*dst_smin = S32_MIN;
+		*dst_smax = S32_MAX;
 	}
-	if (dst_reg->u32_min_value + umin_val < umin_val ||
-	    dst_reg->u32_max_value + umax_val < umax_val) {
-		dst_reg->u32_min_value = 0;
-		dst_reg->u32_max_value = U32_MAX;
-	} else {
-		dst_reg->u32_min_value += umin_val;
-		dst_reg->u32_max_value += umax_val;
+	if (check_add_overflow(*dst_umin, src_reg->u32_min_value, dst_umin) ||
+	    check_add_overflow(*dst_umax, src_reg->u32_max_value, dst_umax)) {
+		*dst_umin = 0;
+		*dst_umax = U32_MAX;
 	}
 }
 
 static void scalar_min_max_add(struct bpf_reg_state *dst_reg,
 			       struct bpf_reg_state *src_reg)
 {
-	s64 smin_val = src_reg->smin_value;
-	s64 smax_val = src_reg->smax_value;
-	u64 umin_val = src_reg->umin_value;
-	u64 umax_val = src_reg->umax_value;
+	s64 *dst_smin = &dst_reg->smin_value;
+	s64 *dst_smax = &dst_reg->smax_value;
+	u64 *dst_umin = &dst_reg->umin_value;
+	u64 *dst_umax = &dst_reg->umax_value;
 
-	if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
-	    signed_add_overflows(dst_reg->smax_value, smax_val)) {
-		dst_reg->smin_value = S64_MIN;
-		dst_reg->smax_value = S64_MAX;
-	} else {
-		dst_reg->smin_value += smin_val;
-		dst_reg->smax_value += smax_val;
+	if (check_add_overflow(*dst_smin, src_reg->smin_value, dst_smin) ||
+	    check_add_overflow(*dst_smax, src_reg->smax_value, dst_smax)) {
+		*dst_smin = S64_MIN;
+		*dst_smax = S64_MAX;
 	}
-	if (dst_reg->umin_value + umin_val < umin_val ||
-	    dst_reg->umax_value + umax_val < umax_val) {
-		dst_reg->umin_value = 0;
-		dst_reg->umax_value = U64_MAX;
-	} else {
-		dst_reg->umin_value += umin_val;
-		dst_reg->umax_value += umax_val;
+	if (check_add_overflow(*dst_umin, src_reg->umin_value, dst_umin) ||
+	    check_add_overflow(*dst_umax, src_reg->umax_value, dst_umax)) {
+		*dst_umin = 0;
+		*dst_umax = U64_MAX;
 	}
 }
 
@@ -18835,6 +18787,8 @@  static int adjust_jmp_off(struct bpf_prog *prog, u32 tgt_idx, u32 delta)
 {
 	struct bpf_insn *insn = prog->insnsi;
 	u32 insn_cnt = prog->len, i;
+	s32 imm;
+	s16 off;
 
 	for (i = 0; i < insn_cnt; i++, insn++) {
 		u8 code = insn->code;
@@ -18846,15 +18800,15 @@  static int adjust_jmp_off(struct bpf_prog *prog, u32 tgt_idx, u32 delta)
 		if (insn->code == (BPF_JMP32 | BPF_JA)) {
 			if (i + 1 + insn->imm != tgt_idx)
 				continue;
-			if (signed_add32_overflows(insn->imm, delta))
+			if (check_add_overflow(insn->imm, delta, &imm))
 				return -ERANGE;
-			insn->imm += delta;
+			insn->imm = imm;
 		} else {
 			if (i + 1 + insn->off != tgt_idx)
 				continue;
-			if (signed_add16_overflows(insn->off, delta))
+			if (check_add_overflow(insn->off, delta, &off))
 				return -ERANGE;
-			insn->off += delta;
+			insn->off = off;
 		}
 	}
 	return 0;