Commit d43ad9da authored Nov 03, 2025 by KaFai Wan Committed by Alexei Starovoitov Nov 03, 2025

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 <exit>

The instruction 3 triggers kernel warning:
   3: if r0 > r0 goto <exit>
   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 <M202472210@hust.edu.cn>
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/


Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251103063108.1111764-2-kafai.wan@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent 5dae7453

kernel/bpf/verifier.c

+31 −0

Original line number	Diff line number	Diff line
		@@ -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);