From: Harishankar Vishwanathan Date: Mon, 23 Jun 2025 04:03:56 +0000 (-0400) Subject: bpf, verifier: Improve precision for BPF_ADD and BPF_SUB X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=7a998a73162773d5d8dca4c9f46af26a499d8f19;p=thirdparty%2Flinux.git bpf, verifier: Improve precision for BPF_ADD and BPF_SUB This patch improves the precison of the scalar(32)_min_max_add and scalar(32)_min_max_sub functions, which update the u(32)min/u(32)_max ranges for the BPF_ADD and BPF_SUB instructions. We discovered this more precise operator using a technique we are developing for automatically synthesizing functions for updating tnums and ranges. According to the BPF ISA [1], "Underflow and overflow are allowed during arithmetic operations, meaning the 64-bit or 32-bit value will wrap". Our patch leverages the wrap-around semantics of unsigned overflow and underflow to improve precision. Below is an example of our patch for scalar_min_max_add; the idea is analogous for all four functions. There are three cases to consider when adding two u64 ranges [dst_umin, dst_umax] and [src_umin, src_umax]. Consider a value x in the range [dst_umin, dst_umax] and another value y in the range [src_umin, src_umax]. (a) No overflow: No addition x + y overflows. This occurs when even the largest possible sum, i.e., dst_umax + src_umax <= U64_MAX. (b) Partial overflow: Some additions x + y overflow. This occurs when the largest possible sum overflows (dst_umax + src_umax > U64_MAX), but the smallest possible sum does not overflow (dst_umin + src_umin <= U64_MAX). (c) Full overflow: All additions x + y overflow. This occurs when both the smallest possible sum and the largest possible sum overflow, i.e., both (dst_umin + src_umin) and (dst_umax + src_umax) are > U64_MAX. The current implementation conservatively sets the output bounds to unbounded, i.e, [umin=0, umax=U64_MAX], whenever there is *any* possibility of overflow, i.e, in cases (b) and (c). Otherwise it computes tight bounds as [dst_umin + src_umin, dst_umax + src_umax]: 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; } Our synthesis-based technique discovered a more precise operator. Particularly, in case (c), all possible additions x + y overflow and wrap around according to eBPF semantics, and the computation of the output range as [dst_umin + src_umin, dst_umax + src_umax] continues to work. Only in case (b), do we need to set the output bounds to unbounded, i.e., [0, U64_MAX]. Case (b) can be checked by seeing if the minimum possible sum does *not* overflow and the maximum possible sum *does* overflow, and when that happens, we set the output to unbounded: min_overflow = check_add_overflow(*dst_umin, src_reg->umin_value, dst_umin); max_overflow = check_add_overflow(*dst_umax, src_reg->umax_value, dst_umax); if (!min_overflow && max_overflow) { *dst_umin = 0; *dst_umax = U64_MAX; } Below is an example eBPF program and the corresponding log from the verifier. The current implementation of scalar_min_max_add() sets r3's bounds to [0, U64_MAX] at instruction 5: (0f) r3 += r3, due to conservative overflow handling. 0: R1=ctx() R10=fp0 0: (b7) r4 = 0 ; R4_w=0 1: (87) r4 = -r4 ; R4_w=scalar() 2: (18) r3 = 0xa000000000000000 ; R3_w=0xa000000000000000 4: (4f) r3 |= r4 ; R3_w=scalar(smin=0xa000000000000000,smax=-1,umin=0xa000000000000000,var_off=(0xa000000000000000; 0x5fffffffffffffff)) R4_w=scalar() 5: (0f) r3 += r3 ; R3_w=scalar() 6: (b7) r0 = 1 ; R0_w=1 7: (95) exit With our patch, r3's bounds after instruction 5 are set to a much more precise [0x4000000000000000,0xfffffffffffffffe]. ... 5: (0f) r3 += r3 ; R3_w=scalar(umin=0x4000000000000000,umax=0xfffffffffffffffe) 6: (b7) r0 = 1 ; R0_w=1 7: (95) exit The logic for scalar32_min_max_add is analogous. For the scalar(32)_min_max_sub functions, the reasoning is similar but applied to detecting underflow instead of overflow. We verified the correctness of the new implementations using Agni [3,4]. We since also discovered that a similar technique has been used to calculate output ranges for unsigned interval addition and subtraction in Hacker's Delight [2]. [1] https://docs.kernel.org/bpf/standardization/instruction-set.html [2] Hacker's Delight Ch.4-2, Propagating Bounds through Add’s and Subtract’s [3] https://github.com/bpfverif/agni [4] https://people.cs.rutgers.edu/~sn349/papers/sas24-preprint.pdf Co-developed-by: Matan Shachnai Signed-off-by: Matan Shachnai Co-developed-by: Srinivas Narayana Signed-off-by: Srinivas Narayana Co-developed-by: Santosh Nagarakatte Signed-off-by: Santosh Nagarakatte Signed-off-by: Harishankar Vishwanathan Acked-by: Eduard Zingerman Link: https://lore.kernel.org/r/20250623040359.343235-2-harishankar.vishwanathan@gmail.com Signed-off-by: Alexei Starovoitov --- diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 279a649332624..f403524bd2153 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -14605,14 +14605,25 @@ static void scalar32_min_max_add(struct bpf_reg_state *dst_reg, s32 *dst_smax = &dst_reg->s32_max_value; u32 *dst_umin = &dst_reg->u32_min_value; u32 *dst_umax = &dst_reg->u32_max_value; + u32 umin_val = src_reg->u32_min_value; + u32 umax_val = src_reg->u32_max_value; + bool min_overflow, max_overflow; 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 (check_add_overflow(*dst_umin, src_reg->u32_min_value, dst_umin) || - check_add_overflow(*dst_umax, src_reg->u32_max_value, dst_umax)) { + + /* If either all additions overflow or no additions overflow, then + * it is okay to set: dst_umin = dst_umin + src_umin, dst_umax = + * dst_umax + src_umax. Otherwise (some additions overflow), set + * the output bounds to unbounded. + */ + min_overflow = check_add_overflow(*dst_umin, umin_val, dst_umin); + max_overflow = check_add_overflow(*dst_umax, umax_val, dst_umax); + + if (!min_overflow && max_overflow) { *dst_umin = 0; *dst_umax = U32_MAX; } @@ -14625,14 +14636,25 @@ static void scalar_min_max_add(struct bpf_reg_state *dst_reg, s64 *dst_smax = &dst_reg->smax_value; u64 *dst_umin = &dst_reg->umin_value; u64 *dst_umax = &dst_reg->umax_value; + u64 umin_val = src_reg->umin_value; + u64 umax_val = src_reg->umax_value; + bool min_overflow, max_overflow; 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 (check_add_overflow(*dst_umin, src_reg->umin_value, dst_umin) || - check_add_overflow(*dst_umax, src_reg->umax_value, dst_umax)) { + + /* If either all additions overflow or no additions overflow, then + * it is okay to set: dst_umin = dst_umin + src_umin, dst_umax = + * dst_umax + src_umax. Otherwise (some additions overflow), set + * the output bounds to unbounded. + */ + min_overflow = check_add_overflow(*dst_umin, umin_val, dst_umin); + max_overflow = check_add_overflow(*dst_umax, umax_val, dst_umax); + + if (!min_overflow && max_overflow) { *dst_umin = 0; *dst_umax = U64_MAX; } @@ -14643,8 +14665,11 @@ static void scalar32_min_max_sub(struct bpf_reg_state *dst_reg, { 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; u32 umin_val = src_reg->u32_min_value; u32 umax_val = src_reg->u32_max_value; + bool min_underflow, max_underflow; if (check_sub_overflow(*dst_smin, src_reg->s32_max_value, dst_smin) || check_sub_overflow(*dst_smax, src_reg->s32_min_value, dst_smax)) { @@ -14652,14 +14677,18 @@ static void scalar32_min_max_sub(struct bpf_reg_state *dst_reg, *dst_smin = S32_MIN; *dst_smax = S32_MAX; } - if (dst_reg->u32_min_value < umax_val) { - /* Overflow possible, we know nothing */ - dst_reg->u32_min_value = 0; - dst_reg->u32_max_value = U32_MAX; - } else { - /* Cannot overflow (as long as bounds are consistent) */ - dst_reg->u32_min_value -= umax_val; - dst_reg->u32_max_value -= umin_val; + + /* If either all subtractions underflow or no subtractions + * underflow, it is okay to set: dst_umin = dst_umin - src_umax, + * dst_umax = dst_umax - src_umin. Otherwise (some subtractions + * underflow), set the output bounds to unbounded. + */ + min_underflow = check_sub_overflow(*dst_umin, umax_val, dst_umin); + max_underflow = check_sub_overflow(*dst_umax, umin_val, dst_umax); + + if (min_underflow && !max_underflow) { + *dst_umin = 0; + *dst_umax = U32_MAX; } } @@ -14668,8 +14697,11 @@ static void scalar_min_max_sub(struct bpf_reg_state *dst_reg, { 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; u64 umin_val = src_reg->umin_value; u64 umax_val = src_reg->umax_value; + bool min_underflow, max_underflow; if (check_sub_overflow(*dst_smin, src_reg->smax_value, dst_smin) || check_sub_overflow(*dst_smax, src_reg->smin_value, dst_smax)) { @@ -14677,14 +14709,18 @@ static void scalar_min_max_sub(struct bpf_reg_state *dst_reg, *dst_smin = S64_MIN; *dst_smax = S64_MAX; } - if (dst_reg->umin_value < umax_val) { - /* Overflow possible, we know nothing */ - dst_reg->umin_value = 0; - dst_reg->umax_value = U64_MAX; - } else { - /* Cannot overflow (as long as bounds are consistent) */ - dst_reg->umin_value -= umax_val; - dst_reg->umax_value -= umin_val; + + /* If either all subtractions underflow or no subtractions + * underflow, it is okay to set: dst_umin = dst_umin - src_umax, + * dst_umax = dst_umax - src_umin. Otherwise (some subtractions + * underflow), set the output bounds to unbounded. + */ + min_underflow = check_sub_overflow(*dst_umin, umax_val, dst_umin); + max_underflow = check_sub_overflow(*dst_umax, umin_val, dst_umax); + + if (min_underflow && !max_underflow) { + *dst_umin = 0; + *dst_umax = U64_MAX; } }