Fix the case where min/max adjustments on a conditional statement cross u32 boundaries. The case where this bug was found has the following scenario (see full bpf prog below): insn 2: (bf) r0 = r2 ... insn 4: (3d) if r3 >= r0 goto pc+1 ... insn 9: (67) r2 <<= 7 ... insn 13: (65) if r7 s> 2 goto pc-12 # back to insn 2 Since this is a loop, at some later iteration, both u32_min_value/u32_max_value of r0 converge to 0 due to the shift of r2 on insn 9 and later the assignment to r0 on insn 2. When evaluating the branch FALSE on the conditional jump of insn 4, since it needs to respect r0 > r3, it elevates umin of r0 by 1: r0.umin = r3.umin + 1 This is done by this part of the code: static void regs_refine_cond_op(...) { ... case BPF_JLT: ... u64 new_umin = max(reg1->umin_value + 1, reg2->umin_value); But the problem is that u32_min_value/u32_max_value are zero, so it needs to be adjusted by reg_bounds_sync(), which calls __reg_deduce_mixed_bounds() that performs the following tightening: new_umin = (reg->umin_value & ~0xffffffffULL) | reg->u32_min_value; ... reg->umin_value = max_t(u64, reg->umin_value, new_umin); Here it was hitting the situation of: u64=[0x1,0xfffffff800000000] u32=[0x0,0x0] Leading to: new_umin = 0 reg->umin_value = max(1, 0) = 1 Which is inconsistent because: (u32)reg->umin_value > reg->u32_max_value. That propagates until it reaches a: verifier bug: REG INVARIANTS VIOLATION (true_reg2): range bounds violation u64=[0x0, 0x7800000000] s64=[0x0, 0xffffffffffffffff] u32=[0x80000000, 0x0] s32=[0x0, 0xffffffff] var_off=(0x0, 0x7800000000) Fix __reg_deduce_mixed_bounds() to detect the case when the u32 boundaries are crossed and advance umin to the next 32-bit block (or retreat umax to the previous one). Example: If we have: u64=[0x1,0xfffffff800000000] u32=[0x0,0x0] Go to the next 32-bit block: u64=[0x100000000,0xfffffff800000000] u32=[0x0,0x0] This way we keep the consistency on the lower 32 bits boundaries. The same applies for the max value, but go to the previous 32-bit block. Full BPF prog for reference: ============================ NOTE: this was extracted from a reproducer of Syzbot from another bug report. insn 0: (61) r2 = *(u32 *)(r1 + 48) insn 1: (61) r3 = *(u32 *)(r1 + 76) insn 2: (bf) r0 = r2 insn 3: (16) if w0 == 21502727 goto pc+16 insn 4: (3d) if r3 >= r0 goto pc+1 insn 5: (0f) r2 += r0 insn 6: (bc) w6 = (s16)w2 insn 7: (bf) r7 = (s32)r6 insn 8: (16) if w2 == 524047 goto pc+0 insn 9: (67) r2 <<= 7 insn 10: (36) if w6 >= -268376562 goto pc+0 insn 11: (bf) r5 = r0 insn 12: (0f) r5 += r6 insn 13: (65) if r7 s> 2 goto pc-12 insn 14: (07) r7 += 4194380 insn 15: (1f) r5 -= r7 insn 16: (bf) r4 = r5 insn 17: (07) r5 += -458749 insn 18: (ad) if r3 < r4 goto pc+1 insn 19: (95) exit insn 20: (05) goto pc+0 insn 21: (95) exit insn 22: (4d) if r11 & r9 goto pc-28203 insn 23: (99) if w3 >= -2094934247 goto pc-30763 The reproducer can be found at: https://syzkaller.appspot.com/text?tag=ReproC&x=16773406580000 Fixes: c51d5ad6543c ("bpf: improve deduction of 64-bit bounds from 32-bit bounds") Signed-off-by: Helen Koike --- Hi all, I'm not familiar with the verifier code base or discussions around it, but I tried to do my best, I searched a bit if this was already discussed, but the discussions I found didn't seem related. Please let me know if this case was already discussed and/or if I should follow a different path into solving this. Tested on bpf/master branch. Thanks in advance. --- kernel/bpf/verifier.c | 26 +++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a965b2c45bbe..ddac09c8a9e5 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2702,9 +2702,29 @@ static void __reg_deduce_mixed_bounds(struct bpf_reg_state *reg) __u64 new_umin, new_umax; __s64 new_smin, new_smax; - /* u32 -> u64 tightening, it's always well-formed */ - new_umin = (reg->umin_value & ~0xffffffffULL) | reg->u32_min_value; - new_umax = (reg->umax_value & ~0xffffffffULL) | reg->u32_max_value; + /* + * If (u32)umin > u32_max, no value in the current upper-32-bit block + * satisfies [u32_min, u32_max] while being >= umin; advance umin to + * the next block. Otherwise apply standard u32->u64 tightening. + */ + if ((u32)reg->umin_value > reg->u32_max_value) + new_umin = (reg->umin_value & ~0xffffffffULL) + (1ULL << 32) | + reg->u32_min_value; + else + new_umin = (reg->umin_value & ~0xffffffffULL) | + reg->u32_min_value; + + /* + * Symmetrically, if (u32)umax < u32_min, retreat umax to the + * previous block. Otherwise apply standard u32->u64 tightening. + */ + if ((u32)reg->umax_value < reg->u32_min_value) + new_umax = (reg->umax_value & ~0xffffffffULL) - (1ULL << 32) | + reg->u32_max_value; + else + new_umax = (reg->umax_value & ~0xffffffffULL) | + reg->u32_max_value; + reg->umin_value = max_t(u64, reg->umin_value, new_umin); reg->umax_value = min_t(u64, reg->umax_value, new_umax); /* u32 -> s64 tightening, u32 range embedded into s64 preserves range validity */ -- 2.53.0