__reg64_deduce_bounds currently improves the s64 range using the u64 range and vice versa, but only if it doesn't cross the sign boundary. This patch improves __reg64_deduce_bounds to cover the case where the s64 range crosses the sign boundary but overlaps with the u64 range on only one end. In that case, we can improve both ranges. Consider the following example, with the s64 range crossing the sign boundary: 0 U64_MAX | [xxxxxxxxxxxxxx u64 range xxxxxxxxxxxxxx] | |----------------------------|----------------------------| |xxxxx s64 range xxxxxxxxx] [xxxxxxx| 0 S64_MAX S64_MIN -1 The u64 range overlaps only with positive portion of the s64 range. We can thus derive the following new s64 and u64 ranges. 0 U64_MAX | [xxxxxx u64 range xxxxx] | |----------------------------|----------------------------| | [xxxxxx s64 range xxxxx] | 0 S64_MAX S64_MIN -1 The same logic can probably apply to the s32/u32 ranges, but this patch doesn't implement that change. In addition to the selftests, this change was also tested with Agni, the formal verification tool for the range analysis [1]. Link: https://github.com/bpfverif/agni [1] Acked-by: Eduard Zingerman Acked-by: Shung-Hsi Yu Signed-off-by: Paul Chaignon --- kernel/bpf/verifier.c | 52 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 52 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index e2fcea860755..f0a41f1596b6 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2523,6 +2523,58 @@ static void __reg64_deduce_bounds(struct bpf_reg_state *reg) if ((u64)reg->smin_value <= (u64)reg->smax_value) { reg->umin_value = max_t(u64, reg->smin_value, reg->umin_value); reg->umax_value = min_t(u64, reg->smax_value, reg->umax_value); + } else { + /* If the s64 range crosses the sign boundary, then it's split + * between the beginning and end of the U64 domain. In that + * case, we can derive new bounds if the u64 range overlaps + * with only one end of the s64 range. + * + * In the following example, the u64 range overlaps only with + * positive portion of the s64 range. + * + * 0 U64_MAX + * | [xxxxxxxxxxxxxx u64 range xxxxxxxxxxxxxx] | + * |----------------------------|----------------------------| + * |xxxxx s64 range xxxxxxxxx] [xxxxxxx| + * 0 S64_MAX S64_MIN -1 + * + * We can thus derive the following new s64 and u64 ranges. + * + * 0 U64_MAX + * | [xxxxxx u64 range xxxxx] | + * |----------------------------|----------------------------| + * | [xxxxxx s64 range xxxxx] | + * 0 S64_MAX S64_MIN -1 + * + * If they overlap in two places, we can't derive anything + * because reg_state can't represent two ranges per numeric + * domain. + * + * 0 U64_MAX + * | [xxxxxxxxxxxxxxxxx u64 range xxxxxxxxxxxxxxxxx] | + * |----------------------------|----------------------------| + * |xxxxx s64 range xxxxxxxxx] [xxxxxxxxxx| + * 0 S64_MAX S64_MIN -1 + * + * The first condition below corresponds to the first diagram + * above. + */ + if (reg->umax_value < (u64)reg->smin_value) { + reg->smin_value = (s64)reg->umin_value; + reg->umax_value = min_t(u64, reg->umax_value, reg->smax_value); + } else if ((u64)reg->smax_value < reg->umin_value) { + /* This second condition considers the case where the u64 range + * overlaps with the negative portion of the s64 range: + * + * 0 U64_MAX + * | [xxxxxxxxxxxxxx u64 range xxxxxxxxxxxxxx] | + * |----------------------------|----------------------------| + * |xxxxxxxxx] [xxxxxxxxxxxx s64 range | + * 0 S64_MAX S64_MIN -1 + */ + reg->smax_value = (s64)reg->umax_value; + reg->umin_value = max_t(u64, reg->umin_value, reg->smin_value); + } } } -- 2.43.0 This patch updates the range refinement logic in the reg_bound test to match the new logic from the previous commit. Without this change, tests would fail because we end with more precise ranges than the tests expect. Acked-by: Eduard Zingerman Signed-off-by: Paul Chaignon --- .../testing/selftests/bpf/prog_tests/reg_bounds.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c b/tools/testing/selftests/bpf/prog_tests/reg_bounds.c index 39d42271cc46..e261b0e872db 100644 --- a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c +++ b/tools/testing/selftests/bpf/prog_tests/reg_bounds.c @@ -465,6 +465,20 @@ static struct range range_refine(enum num_t x_t, struct range x, enum num_t y_t, return range_improve(x_t, x, x_swap); } + if (!t_is_32(x_t) && !t_is_32(y_t) && x_t != y_t) { + if (x_t == S64 && x.a > x.b) { + if (x.b < y.a && x.a <= y.b) + return range(x_t, x.a, y.b); + if (x.a > y.b && x.b >= y.a) + return range(x_t, y.a, x.b); + } else if (x_t == U64 && y.a > y.b) { + if (y.b < x.a && y.a <= x.b) + return range(x_t, y.a, x.b); + if (y.a > x.b && y.b >= x.a) + return range(x_t, x.a, y.b); + } + } + /* otherwise, plain range cast and intersection works */ return range_improve(x_t, x, y_cast); } -- 2.43.0 This patch adds coverage for the new cross-sign 64bits range refinement logic. The three tests cover the cases when the u64 and s64 ranges overlap (1) in the negative portion of s64, (2) in the positive portion of s64, and (3) in both portions. The first test is a simplified version of a BPF program generated by syzkaller that caused an invariant violation [1]. It looks like syzkaller could not extract the reproducer itself (and therefore didn't report it to the mailing list), but I was able to extract it from the console logs of a crash. The principle is similar to the invariant violation described in 6279846b9b25 ("bpf: Forget ranges when refining tnum after JSET"): the verifier walks a dead branch, uses the condition to refine ranges, and ends up with inconsistent ranges. In this case, the dead branch is when we fallthrough on both jumps. The new refinement logic improves the bounds such that the second jump is properly detected as always-taken and the verifier doesn't end up walking a dead branch. The second and third tests are inspired by the first, but rely on condition jumps to prepare the bounds instead of ALU instructions. An R10 write is used to trigger a verifier error when the bounds can't be refined. Link: https://syzkaller.appspot.com/bug?extid=c711ce17dd78e5d4fdcf [1] Signed-off-by: Paul Chaignon --- .../selftests/bpf/progs/verifier_bounds.c | 118 ++++++++++++++++++ 1 file changed, 118 insertions(+) diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c index 63b533ca4933..dd4e3e9f41d3 100644 --- a/tools/testing/selftests/bpf/progs/verifier_bounds.c +++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c @@ -1550,4 +1550,122 @@ l0_%=: r0 = 0; \ : __clobber_all); } +/* This test covers the bounds deduction on 64bits when the s64 and u64 ranges + * overlap on the negative side. At instruction 7, the ranges look as follows: + * + * 0 umin=0xfffffcf1 umax=0xff..ff6e U64_MAX + * | [xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx] | + * |----------------------------|------------------------------| + * |xxxxxxxxxx] [xxxxxxxxxxxx| + * 0 smax=0xeffffeee smin=-655 -1 + * + * We should therefore deduce the following new bounds: + * + * 0 u64=[0xff..ffd71;0xff..ff6e] U64_MAX + * | [xxx] | + * |----------------------------|------------------------------| + * | [xxx] | + * 0 s64=[-655;-146] -1 + * + * Without the deduction cross sign boundary, we end up with an invariant + * violation error. + */ +SEC("socket") +__description("bounds deduction cross sign boundary, negative overlap") +__success __log_level(2) __flag(BPF_F_TEST_REG_INVARIANTS) +__msg("7: (1f) r0 -= r6 {{.*}} R0=scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e,var_off=(0xfffffffffffffc00; 0x3ff))") +__retval(0) +__naked void bounds_deduct_negative_overlap(void) +{ + asm volatile(" \ + call %[bpf_get_prandom_u32]; \ + w3 = w0; \ + w6 = (s8)w0; \ + r0 = (s8)r0; \ + if w6 >= 0xf0000000 goto l0_%=; \ + r0 += r6; \ + r6 += 400; \ + r0 -= r6; \ + if r3 < r0 goto l0_%=; \ +l0_%=: r0 = 0; \ + exit; \ +" : + : __imm(bpf_get_prandom_u32) + : __clobber_all); +} + +/* This test covers the bounds deduction on 64bits when the s64 and u64 ranges + * overlap on the positive side. At instruction 3, the ranges look as follows: + * + * 0 umin=0 umax=0xfffffffffffffeff U64_MAX + * [xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx] | + * |----------------------------|------------------------------| + * |xxxxxxxx] [xxxxxxxx| + * 0 smax=127 smin=-128 -1 + * + * We should therefore deduce the following new bounds: + * + * 0 u64=[0;127] U64_MAX + * [xxxxxxxx] | + * |----------------------------|------------------------------| + * [xxxxxxxx] | + * 0 s64=[0;127] -1 + * + * Without the deduction cross sign boundary, the program is rejected due to + * the frame pointer write. + */ +SEC("socket") +__description("bounds deduction cross sign boundary, positive overlap") +__success __log_level(2) __flag(BPF_F_TEST_REG_INVARIANTS) +__msg("3: (2d) if r0 > r1 {{.*}} R0_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=127,var_off=(0x0; 0x7f))") +__retval(0) +__naked void bounds_deduct_positive_overlap(void) +{ + asm volatile(" \ + call %[bpf_get_prandom_u32]; \ + r0 = (s8)r0; \ + r1 = 0xffffffffffffff00; \ + if r0 > r1 goto l0_%=; \ + if r0 < 128 goto l0_%=; \ + r10 = 0; \ +l0_%=: r0 = 0; \ + exit; \ +" : + : __imm(bpf_get_prandom_u32) + : __clobber_all); +} + +/* This test is the same as above, but the s64 and u64 ranges overlap in two + * places. At instruction 3, the ranges look as follows: + * + * 0 umin=0 umax=0xffffffffffffff80 U64_MAX + * [xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx] | + * |----------------------------|------------------------------| + * |xxxxxxxx] [xxxxxxxx| + * 0 smax=127 smin=-128 -1 + * + * 0xffffffffffffff80 = (u64)-128. We therefore can't deduce anything new and + * the program should fail due to the frame pointer write. + */ +SEC("socket") +__description("bounds deduction cross sign boundary, two overlaps") +__failure __flag(BPF_F_TEST_REG_INVARIANTS) +__msg("3: (2d) if r0 > r1 {{.*}} R0_w=scalar(smin=smin32=-128,smax=smax32=127,umax=0xffffffffffffff80)") +__msg("frame pointer is read only") +__naked void bounds_deduct_two_overlaps(void) +{ + asm volatile(" \ + call %[bpf_get_prandom_u32]; \ + r0 = (s8)r0; \ + r1 = 0xffffffffffffff80; \ + if r0 > r1 goto l0_%=; \ + if r0 < 128 goto l0_%=; \ + r10 = 0; \ +l0_%=: r0 = 0; \ + exit; \ +" : + : __imm(bpf_get_prandom_u32) + : __clobber_all); +} + char _license[] SEC("license") = "GPL"; -- 2.43.0 The improvement of the u64/s64 range refinement fixed the invariant violation that was happening on this test for BPF_JSLT when crossing the sign boundary. After this patch, we have one test remaining with a known invariant violation. It's the same test as fixed here but for 32 bits ranges. Signed-off-by: Paul Chaignon --- tools/testing/selftests/bpf/progs/verifier_bounds.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c index dd4e3e9f41d3..85e488b27756 100644 --- a/tools/testing/selftests/bpf/progs/verifier_bounds.c +++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c @@ -1066,7 +1066,7 @@ l0_%=: r0 = 0; \ SEC("xdp") __description("bound check with JMP_JSLT for crossing 64-bit signed boundary") __success __retval(0) -__flag(!BPF_F_TEST_REG_INVARIANTS) /* known invariants violation */ +__flag(BPF_F_TEST_REG_INVARIANTS) __naked void crossing_64_bit_signed_boundary_2(void) { asm volatile (" \ -- 2.43.0