On 3/8/2023 6:04 AM, Xu Kuohai wrote:
The following XDP prog is accepted by verifier.
0: (61) r2 = *(u32 *)(r1 +0) ; R2_w=pkt(off=0,r=0,imm=0) 1: (61) r3 = *(u32 *)(r1 +4) ; R3_w=pkt_end(off=0,imm=0) 2: (bf) r1 = r2 3: (07) r1 += 1 4: (2d) if r1 > r3 goto pc+6 5: (71) r1 = *(u8 *)(r2 +0) ; R1_w=scalar(umax=255,var_off=(0x0; 0xff)) 6: (b4) w0 = 0x7fffff10 7: (0c) w1 += w0 ; R1_w=scalar(umin=0x7fffff10,umax=0x8000000f,var_off=(0x0; 0xffffffff)) 8: (b4) w0 = 0x80000000 9: (04) w0 += 1 10: (ae) if w0 < w1 goto pc-2 11: (b7) r0 = 0 12: (95) exit
while the following 64-bit version is rejected.
0: (61) r2 = *(u32 *)(r1 +0) ; R2_w=pkt(off=0,r=0,imm=0) 1: (61) r3 = *(u32 *)(r1 +4) ; R3_w=pkt_end(off=0,imm=0) 2: (bf) r1 = r2 3: (07) r1 += 1 4: (2d) if r1 > r3 goto pc+8 5: (71) r1 = *(u8 *)(r2 +0) ; R1_w=scalar(umax=255,var_off=(0x0; 0xff)) 6: (18) r0 = 0x7fffffffffffff10 8: (0f) r1 += r0 ; R1_w=scalar(umin=0x7fffffffffffff10,umax=0x800000000000000f) 9: (18) r0 = 0x8000000000000000 11: (07) r0 += 1 12: (ad) if r0 < r1 goto pc-2 13: (b7) r0 = 0 14: (95) exit
The verifier log says:
[...]
from 12 to 11: R0_w=-9223372036854775794 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) 11: (07) r0 += 1 ; R0_w=-9223372036854775793 12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) 13: safe
from 12 to 11: R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) 11: (07) r0 += 1 ; R0_w=-9223372036854775792 12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775792 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) 13: safe
[...]
The loop crosses termination condition r0 == r1.umax, and does not stop.
The reason is that when the verifier enumerates to r1.umin == r1.umax, the value 0x800000000000000f of r1.umin is greater than U32_MAX, so __reg_combine_64_into_32 sets the u32 range of r1 to [0, U32_MAX] instead of marking r1 as a constant, making is_branch_taken() in check_cond_jmp_op() be skipped.
To fix it, update 32-bit bounds when the lower 32-bit value is not wrapping, even if the 64-bit value is beyond the range of [0, U32_MAX] or [S32_MIN, S32_MAX].
Signed-off-by: Xu Kuohai xukuohai@huaweicloud.com
Oops, missing fixes tag, will resend
kernel/bpf/verifier.c | 27 +++++++++++---------------- 1 file changed, 11 insertions(+), 16 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index b2116ca78d9a..64c9ee3857ec 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2013,26 +2013,21 @@ static void __reg_combine_32_into_64(struct bpf_reg_state *reg) reg_bounds_sync(reg); } -static bool __reg64_bound_s32(s64 a) -{
- return a >= S32_MIN && a <= S32_MAX;
-}
-static bool __reg64_bound_u32(u64 a) -{
- return a >= U32_MIN && a <= U32_MAX;
-}
- static void __reg_combine_64_into_32(struct bpf_reg_state *reg) {
- s64 smin = reg->smin_value;
- s64 smax = reg->smax_value;
- u64 umin = reg->umin_value;
- u64 umax = reg->umax_value;
- __mark_reg32_unbounded(reg);
- if (__reg64_bound_s32(reg->smin_value) && __reg64_bound_s32(reg->smax_value)) {
reg->s32_min_value = (s32)reg->smin_value;reg->s32_max_value = (s32)reg->smax_value;
- if ((u64)(smax - smin) <= (u64)U32_MAX && (s32)smin <= (s32)smax) {
reg->s32_min_value = (s32)smin;reg->s32_max_value = (s32)smax;
- if (__reg64_bound_u32(reg->umin_value) && __reg64_bound_u32(reg->umax_value)) {
reg->u32_min_value = (u32)reg->umin_value;reg->u32_max_value = (u32)reg->umax_value;
- if (umax - umin <= U32_MAX && (u32)umin <= (u32)umax) {
reg->u32_min_value = (u32)umin;reg->u32_max_value = (u32)umax;