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 <[email protected]> Reported-by: Yinhao Hu <[email protected]> Closes: https://lore.kernel.org/all/[email protected]/ Signed-off-by: KaFai Wan <[email protected]> --- kernel/bpf/verifier.c | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 542e23fb19c7..a571263f4ebe 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -15995,6 +15995,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta switch (opcode) { case BPF_JEQ: + if (reg1 == reg2) + return 1; /* constants, umin/umax and smin/smax checks would be * redundant in this case because they all should match */ @@ -16021,6 +16023,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta } break; case BPF_JNE: + if (reg1 == reg2) + return 0; /* constants, umin/umax and smin/smax checks would be * redundant in this case because they all should match */ @@ -16047,6 +16051,12 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta } break; case BPF_JSET: + if (reg1 == reg2) { + if (tnum_is_const(t1)) + return t1.value != 0; + else + return (smin1 <= 0 && smax1 >= 0) ? -1 : 1; + } if (!is_reg_const(reg2, is_jmp32)) { swap(reg1, reg2); swap(t1, t2); @@ -16059,48 +16069,64 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta return 0; break; case BPF_JGT: + if (reg1 == reg2) + return 0; if (umin1 > umax2) return 1; else if (umax1 <= umin2) return 0; break; case BPF_JSGT: + if (reg1 == reg2) + return 0; if (smin1 > smax2) return 1; else if (smax1 <= smin2) return 0; break; case BPF_JLT: + if (reg1 == reg2) + return 0; if (umax1 < umin2) return 1; else if (umin1 >= umax2) return 0; break; case BPF_JSLT: + if (reg1 == reg2) + return 0; if (smax1 < smin2) return 1; else if (smin1 >= smax2) return 0; break; case BPF_JGE: + if (reg1 == reg2) + return 1; if (umin1 >= umax2) return 1; else if (umax1 < umin2) return 0; break; case BPF_JSGE: + if (reg1 == reg2) + return 1; if (smin1 >= smax2) return 1; else if (smax1 < smin2) return 0; break; case BPF_JLE: + if (reg1 == reg2) + return 1; if (umax1 <= umin2) return 1; else if (umin1 > umax2) return 0; break; case BPF_JSLE: + if (reg1 == reg2) + return 1; if (smax1 <= smin2) return 1; else if (smin1 > smax2) @@ -16439,6 +16465,13 @@ 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); -- 2.43.0
