From: KaFai Wan Date: Mon, 3 Nov 2025 06:31:07 +0000 (+0800) Subject: bpf: Skip bounds adjustment for conditional jumps on same scalar register X-Git-Tag: v6.19-rc1~171^2~57^2~1 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=d43ad9da8052eda714caa38f243adbf32a8614cb;p=thirdparty%2Flinux.git bpf: Skip bounds adjustment for conditional jumps on same scalar register When conditional jumps are performed on the same scalar register (e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly attempts to adjust the register's min/max bounds. This leads to invalid range bounds and triggers a BUG warning. The problematic BPF program: 0: call bpf_get_prandom_u32 1: w8 = 0x80000000 2: r0 &= r8 3: if r0 > r0 goto The instruction 3 triggers kernel warning: 3: if r0 > r0 goto true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0) true_reg2: const tnum out of sync with range bounds u64=[0x0, 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] var_off=(0x0, 0x0) Comparing a register with itself should not change its bounds and for most comparison operations, comparing a register with itself has a known result (e.g., r0 == r0 is always true, r0 < r0 is always false). Fix this by: 1. Enhance is_scalar_branch_taken() to properly handle branch direction computation for same register comparisons across all BPF jump operations 2. Adds early return in reg_set_min_max() to avoid bounds adjustment for unknown branch directions (e.g., BPF_JSET) on the same register The fix ensures that unnecessary bounds adjustments are skipped, preventing the verifier bug while maintaining correct branch direction analysis. Reported-by: Kaiyan Mei Reported-by: Yinhao Hu Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/ Signed-off-by: KaFai Wan Acked-by: Eduard Zingerman Link: https://lore.kernel.org/r/20251103063108.1111764-2-kafai.wan@linux.dev Signed-off-by: Alexei Starovoitov --- diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 542e23fb19c7b..e4928846e7637 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -15993,6 +15993,30 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta s64 smin2 = is_jmp32 ? (s64)reg2->s32_min_value : reg2->smin_value; s64 smax2 = is_jmp32 ? (s64)reg2->s32_max_value : reg2->smax_value; + if (reg1 == reg2) { + switch (opcode) { + case BPF_JGE: + case BPF_JLE: + case BPF_JSGE: + case BPF_JSLE: + case BPF_JEQ: + return 1; + case BPF_JGT: + case BPF_JLT: + case BPF_JSGT: + case BPF_JSLT: + case BPF_JNE: + return 0; + case BPF_JSET: + if (tnum_is_const(t1)) + return t1.value != 0; + else + return (smin1 <= 0 && smax1 >= 0) ? -1 : 1; + default: + return -1; + } + } + switch (opcode) { case BPF_JEQ: /* constants, umin/umax and smin/smax checks would be @@ -16439,6 +16463,13 @@ static int reg_set_min_max(struct bpf_verifier_env *env, if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE) return 0; + /* We compute branch direction for same SCALAR_VALUE registers in + * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET) + * on the same registers, we don't need to adjust the min/max values. + */ + if (false_reg1 == false_reg2) + return 0; + /* fallthrough (FALSE) branch */ regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32); reg_bounds_sync(false_reg1);