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bpf: Skip bounds adjustment for conditional jumps on same scalar register

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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>
This commit is contained in:
KaFai Wan
2025-11-03 14:31:07 +08:00
committed by Alexei Starovoitov
parent 5dae7453ec
commit d43ad9da80
1 changed files with 31 additions and 0 deletions
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31
kernel/bpf/verifier.c
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@@ -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);