From: Harishankar Vishwanathan This commit introduces tnum_step(), a function that, when given t, and a number z returns the smallest member of t larger than z. The number z must be greater or equal to the smallest member of t and less than the largest member of t. The first step is to compute j, a number that keeps all of t's known bits, and matches all unknown bits to z's bits. Since j is a member of the t, it is already a candidate for result. However, we want our result to be (minimally) greater than z. There are only two possible cases: (1) Case j <= z. In this case, we want to increase the value of j and make it > z. (2) Case j > z. In this case, we want to decrease the value of j while keeping it > z. (Case 1) j <= z t = xx11x0x0 z = 10111101 (189) j = 10111000 (184) ^ k (Case 1.1) Let's first consider the case where j < z. We will address j == z later. Since z > j, there had to be a bit position that was 1 in z and a 0 in j, beyond which all positions of higher significance are equal in j and z. Further, this position could not have been unknown in a, because the unknown positions of a match z. This position had to be a 1 in z and known 0 in t. Let k be position of the most significant 1-to-0 flip. In our example, k = 3 (starting the count at 1 at the least significant bit). Setting (to 1) the unknown bits of t in positions of significance smaller than k will not produce a result > z. Hence, we must set/unset the unknown bits at positions of significance higher than k. Specifically, we look for the next larger combination of 1s and 0s to place in those positions, relative to the combination that exists in z. We can achieve this by concatenating bits at unknown positions of t into an integer, adding 1, and writing the bits of that result back into the corresponding bit positions previously extracted from z. >From our example, considering only positions of significance greater than k: t = xx..x z = 10..1 + 1 ----- 11..0 This is the exact combination 1s and 0s we need at the unknown bits of t in positions of significance greater than k. Further, our result must only increase the value minimally above z. Hence, unknown bits in positions of significance smaller than k should remain 0. We finally have, result = 11110000 (240) (Case 1.2) Now consider the case when j = z, for example t = 1x1x0xxx z = 10110100 (180) j = 10110100 (180) Matching the unknown bits of the t to the bits of z yielded exactly z. To produce a number greater than z, we must set/unset the unknown bits in t, and *all* the unknown bits of t candidates for being set/unset. We can do this similar to Case 1.1, by adding 1 to the bits extracted from the masked bit positions of z. Essentially, this case is equivalent to Case 1.1, with k = 0. t = 1x1x0xxx z = .0.1.100 + 1 --------- .0.1.101 This is the exact combination of bits needed in the unknown positions of t. After recalling the known positions of t, we get result = 10110101 (181) (Case 2) j > z t = x00010x1 z = 10000010 (130) j = 10001011 (139) ^ k Since j > z, there had to be a bit position which was 0 in z, and a 1 in j, beyond which all positions of higher significance are equal in j and z. This position had to be a 0 in z and known 1 in t. Let k be the position of the most significant 0-to-1 flip. In our example, k = 4. Because of the 0-to-1 flip at position k, a member of t can become greater than z if the bits in positions greater than k are themselves >= to z. To make that member *minimally* greater than z, the bits in positions greater than k must be exactly = z. Hence, we simply match all of t's unknown bits in positions more significant than k to z's bits. In positions less significant than k, we set all t's unknown bits to 0 to retain minimality. In our example, in positions of greater significance than k (=4), t=x000. These positions are matched with z (1000) to produce 1000. In positions of lower significance than k, t=10x1. All unknown bits are set to 0 to produce 1001. The final result is: result = 10001001 (137) This concludes the computation for a result > z that is a member of t. The procedure for tnum_step() in this commit implements the idea described above. As a proof of correctness, we verified the algorithm against a logical specification of tnum_step. The specification asserts the following about the inputs t, z and output res that: 1. res is a member of t, and 2. res is strictly greater than z, and 3. there does not exist another value res2 such that 3a. res2 is also a member of t, and 3b. res2 is greater than z 3c. res2 is smaller than res We checked the implementation against this logical specification using an SMT solver. The verification formula in SMTLIB format is available at [1]. The verification returned an "unsat": indicating that no input assignment exists for which the implementation and the specification produce different outputs. In addition, we also automatically generated the logical encoding of the C implementation using Agni [2] and verified it against the same specification. This verification also returned an "unsat", confirming that the implementation is equivalent to the specification. The formula for this check is also available at [3]. Link: https://pastebin.com/raw/2eRWbiit [1] Link: https://github.com/bpfverif/agni [2] Link: https://pastebin.com/raw/EztVbBJ2 [3] Signed-off-by: Paul Chaignon Co-developed-by: Srinivas Narayana Signed-off-by: Srinivas Narayana Co-developed-by: Santosh Nagarakatte Signed-off-by: Santosh Nagarakatte Signed-off-by: Harishankar Vishwanathan --- include/linux/tnum.h | 3 +++ kernel/bpf/tnum.c | 52 ++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 55 insertions(+) diff --git a/include/linux/tnum.h b/include/linux/tnum.h index fa4654ffb621..ca2cfec8de08 100644 --- a/include/linux/tnum.h +++ b/include/linux/tnum.h @@ -131,4 +131,7 @@ static inline bool tnum_subreg_is_const(struct tnum a) return !(tnum_subreg(a)).mask; } +/* Returns the smallest member of t larger than z */ +u64 tnum_step(struct tnum t, u64 z); + #endif /* _LINUX_TNUM_H */ diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c index 26fbfbb01700..55197c9fadc2 100644 --- a/kernel/bpf/tnum.c +++ b/kernel/bpf/tnum.c @@ -269,3 +269,55 @@ struct tnum tnum_bswap64(struct tnum a) { return TNUM(swab64(a.value), swab64(a.mask)); } + +/* Given tnum t, and a number z such that tmin <= z < tmax, where tmin + * is the smallest member of the t (= t.value) and tmax is the largest + * member of t (= t.value | t.mask), returns the smallest member of t + * larger than z. + * + * For example, + * t = x11100x0 + * z = 11110001 (241) + * result = 11110010 (242) + * + * Note: if this function is called with z >= tmax, it just returns + * early with tmax; if this function is called with z < tmin, the + * algorithm already returns tmin. + */ +u64 tnum_step(struct tnum t, u64 z) +{ + u64 tmax, j, p, q, r, s, v, u, w, res; + u8 k; + + tmax = t.value | t.mask; + + /* if z >= largest member of t, return largest member of t */ + if (z >= tmax) + return tmax; + + /* keep t's known bits, and match all unknown bits to z */ + j = t.value | (z & t.mask); + + if (j > z) { + p = ~z & t.value & ~t.mask; + k = fls64(p); /* k is the most-significant 0-to-1 flip */ + q = U64_MAX << k; + r = q & z; /* positions > k matched to z */ + s = ~q & t.value; /* positions <= k matched to t.value */ + v = r | s; + res = v; + } else { + p = z & ~t.value & ~t.mask; + k = fls64(p); /* k is the most-significant 1-to-0 flip */ + q = U64_MAX << k; + r = q & t.mask & z; /* unknown positions > k, matched to z */ + s = q & ~t.mask; /* known positions > k, set to 1 */ + v = r | s; + /* add 1 to unknown positions > k to make value greater than z */ + u = v + (1ULL << k); + /* extract bits in unknown positions > k from u, rest from t.value */ + w = (u & t.mask) | t.value; + res = w; + } + return res; +} -- 2.43.0 We're hitting an invariant violation in Cilium that sometimes leads to BPF programs being rejected and Cilium failing to start [1]. The following extract from verifier logs shows what's happening: from 201 to 236: R1=0 R6=ctx() R7=1 R9=scalar(smin=umin=smin32=umin32=3584,smax=umax=smax32=umax32=3840,var_off=(0xe00; 0x100)) R10=fp0 236: R1=0 R6=ctx() R7=1 R9=scalar(smin=umin=smin32=umin32=3584,smax=umax=smax32=umax32=3840,var_off=(0xe00; 0x100)) R10=fp0 ; if (magic == MARK_MAGIC_HOST || magic == MARK_MAGIC_OVERLAY || magic == MARK_MAGIC_ENCRYPT) @ bpf_host.c:1337 236: (16) if w9 == 0xe00 goto pc+45 ; R9=scalar(smin=umin=smin32=umin32=3585,smax=umax=smax32=umax32=3840,var_off=(0xe00; 0x100)) 237: (16) if w9 == 0xf00 goto pc+1 verifier bug: REG INVARIANTS VIOLATION (false_reg1): range bounds violation u64=[0xe01, 0xe00] s64=[0xe01, 0xe00] u32=[0xe01, 0xe00] s32=[0xe01, 0xe00] var_off=(0xe00, 0x0) We reach instruction 236 with two possible values for R9, 0xe00 and 0xf00. This is perfectly reflected in the tnum, but of course the ranges are less accurate and cover [0xe00; 0xf00]. Taking the fallthrough path at instruction 236 allows the verifier to reduce the range to [0xe01; 0xf00]. The tnum is however not updated. With these ranges, at instruction 237, the verifier is not able to deduce that R9 is always equal to 0xf00. Hence the fallthrough pass is explored first, the verifier refines the bounds using the assumption that R9 != 0xf00, and ends up with an invariant violation. This pattern of impossible branch + bounds refinement is common to all invariant violations seen so far. The long-term solution is likely to rely on the refinement + invariant violation check to detect dead branches, as started by Eduard. To fix the current issue, we need something with less refactoring that we can backport. This patch uses the tnum_step helper introduced in the previous patch to detect the above situation. In particular, three cases are now detected in the bounds refinement: 1. The u64 range and the tnum only overlap in umin. u64: ---[xxxxxx]----- tnum: --xx----------x- 2. The u64 range and the tnum only overlap in the maximum value represented by the tnum, called tmax. u64: ---[xxxxxx]----- tnum: xx-----x-------- 3. The u64 range and the tnum only overlap u64: ---[xxxxxx]----- tnum: xx----x-------x- To detect these three cases, we call tnum_step(tnum, umin), which returns the largest member of the tnum greater than umin, called tnum_next here. We're in case (1) if umin uis part of the tnum and tnum_next is greater than umax. We're in case (2) if umin is not part of the tnum and tnum_next is equal to tmax. Finally, we're in case (3) if tnum_next is inferior or equal to umax AND calling tnum_step a second time gives us a value past umax. This change implements these three cases. With it, the above bytecode looks as follows: 0: (85) call bpf_get_prandom_u32#7 ; R0=scalar() 1: (47) r0 |= 3584 ; R0=scalar(smin=0x8000000000000e00,umin=umin32=3584,smin32=0x80000e00,var_off=(0xe00; 0xfffffffffffff1ff)) 2: (57) r0 &= 3840 ; R0=scalar(smin=umin=smin32=umin32=3584,smax=umax=smax32=umax32=3840,var_off=(0xe00; 0x100)) 3: (15) if r0 == 0xe00 goto pc+2 ; R0=3840 4: (15) if r0 == 0xf00 goto pc+1 4: R0=3840 6: (95) exit In addition to the new selftests, this change was also verified with Agni [3]. For the record, the raw SMT is available at [4]. The property it verifies is that: If a concrete value x is contained in all input abstract values, after __update_reg_bounds, it will continue to be contained in all output abstract values. Link: https://github.com/cilium/cilium/issues/44216 [1] Link: https://pchaigno.github.io/test-verifier-complexity.html [2] Link: https://github.com/bpfverif/agni [3] Link: https://pastebin.com/raw/9BJAi6Rw [4] Co-developed-by: Harishankar Vishwanathan Signed-off-by: Harishankar Vishwanathan Signed-off-by: Paul Chaignon --- kernel/bpf/verifier.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index dbaafb64d3bd..c4478c874616 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2379,6 +2379,8 @@ static void __update_reg32_bounds(struct bpf_reg_state *reg) static void __update_reg64_bounds(struct bpf_reg_state *reg) { + u64 tnum_next; + /* min signed is max(sign bit) | min(other bits) */ reg->smin_value = max_t(s64, reg->smin_value, reg->var_off.value | (reg->var_off.mask & S64_MIN)); @@ -2388,6 +2390,20 @@ static void __update_reg64_bounds(struct bpf_reg_state *reg) reg->umin_value = max(reg->umin_value, reg->var_off.value); reg->umax_value = min(reg->umax_value, reg->var_off.value | reg->var_off.mask); + + /* Check if u64 and tnum overlap in a single value */ + tnum_next = tnum_step(reg->var_off, reg->umin_value); + if ((reg->umin_value & ~reg->var_off.mask) == reg->var_off.value) { + /* The only overlap is umin */ + if (tnum_next > reg->umax_value) + ___mark_reg_known(reg, reg->umin_value); + } else if (tnum_next == (reg->var_off.value | reg->var_off.mask)) { + /* The only overlap is tmax */ + ___mark_reg_known(reg, tnum_next); + } else if (tnum_next <= reg->umax_value && + tnum_step(reg->var_off, tnum_next) > reg->umax_value) { + ___mark_reg_known(reg, tnum_next); + } } static void __update_reg_bounds(struct bpf_reg_state *reg) -- 2.43.0 This patch introduces selftests to cover the new bounds refinement logic introduced in the previous patch. Without the previous patch, the first two tests fail because of the invariant violation they trigger. The last test fails because the R10 access is not detected as dead code. In addition, all tests fail because of R0 having a non-constant value in the verifier logs. Signed-off-by: Paul Chaignon --- .../selftests/bpf/progs/verifier_bounds.c | 91 +++++++++++++++++++ 1 file changed, 91 insertions(+) diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c index 560531404bce..41dd249faadd 100644 --- a/tools/testing/selftests/bpf/progs/verifier_bounds.c +++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c @@ -1863,4 +1863,95 @@ l1_%=: r0 = 1; \ : __clobber_all); } +/* This test covers the bounds deduction when the u64 range and the tnum + * overlap only at umax. After instruction 3, the ranges look as follows: + * + * 0 umin=0xe01 umax=0xf00 U64_MAX + * | [xxxxxxxxxxxxxx] | + * |----------------------------|------------------------------| + * | x x | tnum values + * + * The verifier can therefore deduce that the R0=0xf00=3840. + */ +SEC("socket") +__description("bounds refinement with single-value tnum on umax") +__msg("3: (15) if r0 == 0xe00 {{.*}} R0=3840") +__success __log_level(2) +__flag(BPF_F_TEST_REG_INVARIANTS) +__naked void bounds_refinement_tnum_umax(void *ctx) +{ + asm volatile(" \ + call %[bpf_get_prandom_u32]; \ + r0 |= 0xe00; \ + r0 &= 0xf00; \ + if r0 == 0xe00 goto +2; \ + if r0 == 0xf00 goto +1; \ + r0 = 0; \ + exit; \ +" : + : __imm(bpf_get_prandom_u32) + : __clobber_all); +} + +/* This test covers the bounds deduction when the u64 range and the tnum + * overlap only at umin. After instruction 3, the ranges look as follows: + * + * 0 umin=0xe00 umax=0xeff U64_MAX + * | [xxxxxxxxxxxxxx] | + * |----------------------------|------------------------------| + * | x x | tnum values + * + * The verifier can therefore deduce that the R0=0xe00=3584. + */ +SEC("socket") +__description("bounds refinement with single-value tnum on umin") +__msg("3: (15) if r0 == 0xf00 {{.*}} R0=3584") +__success __log_level(2) +__flag(BPF_F_TEST_REG_INVARIANTS) +__naked void bounds_refinement_tnum_umin(void *ctx) +{ + asm volatile(" \ + call %[bpf_get_prandom_u32]; \ + r0 |= 0xe00; \ + r0 &= 0xf00; \ + if r0 == 0xf00 goto +2; \ + if r0 == 0xe00 goto +1; \ + r0 = 0; \ + exit; \ +" : + : __imm(bpf_get_prandom_u32) + : __clobber_all); +} + +/* This test covers the bounds deduction when the only possible tnum value is + * in the middle of the u64 range. After instruction 3, the ranges look as + * follows: + * + * 0 umin=0x7cf umax=0x7df U64_MAX + * | [xxxxxxxxxxxxxx] | + * |----------------------------|------------------------------| + * | x x x x x| tnum values + * + * The verifier can therefore deduce that the R0=0x7d0=2000. Instruction 5 is + * therefore dead code. + */ +SEC("socket") +__description("bounds refinement with single-value tnum in middle of range") +__msg("3: (a5) if r0 < 0x7cf {{.*}} R0=2000") +__success __log_level(2) +__naked void bounds_refinement_tnum_middle(void *ctx) +{ + asm volatile(" \ + call %[bpf_get_prandom_u32]; \ + if r0 & 0x0f goto +4; \ + if r0 > 0x7df goto +3; \ + if r0 < 0x7cf goto +2; \ + if r0 == 0x7d0 goto +1; \ + r10 = 0; \ + exit; \ +" : + : __imm(bpf_get_prandom_u32) + : __clobber_all); +} + char _license[] SEC("license") = "GPL"; -- 2.43.0 The reg_bounds_crafted tests validate the verifier's range analysis logic. They focus on the actual ranges and thus ignore the tnum. As a consequence, they carry the assumption that the tested cases can be reproduced in userspace without using the tnum information. Unfortunately, the previous change the refinement logic breaks that assumption for one test case: (u64)2147483648 (u32) [4294967294; 0x100000000] The tested bytecode is shown below. Without our previous improvement, on the false branch of the condition, R7 is only known to have u64 range [0xfffffffe; 0x100000000]. With our improvement, and using the tnum information, we can deduce that R7 equals 0x100000000. 19: (bc) w0 = w6 ; R6=0x80000000 20: (bc) w0 = w7 ; R7=scalar(smin=umin=0xfffffffe,smax=umax=0x100000000,smin32=-2,smax32=0,var_off=(0x0; 0x1ffffffff)) 21: (be) if w6 <= w7 goto pc+3 ; R6=0x80000000 R7=0x100000000 R7's tnum is (0; 0x1ffffffff). On the false branch, regs_refine_cond_op refines R7's u32 range to [0; 0x7fffffff]. Then, __reg32_deduce_bounds refines the s32 range to 0 using u32 and finally also sets u32=0. From this, __reg_bound_offset improves the tnum to (0; 0x100000000). Finally, our previous patch uses this new tnum to deduce that it only intersect with u64=[0xfffffffe; 0x100000000] in a single value: 0x100000000. Because the verifier uses the tnum to reach this constant value, the selftest is unable to reproduce it by only simulating ranges. The solution implemented in this patch is to change the test case such that there is more than one overlap value between u64 and the tnum. The max. u64 value is thus changed from 0x100000000 to 0x300000000. Signed-off-by: Paul Chaignon --- tools/testing/selftests/bpf/prog_tests/reg_bounds.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c b/tools/testing/selftests/bpf/prog_tests/reg_bounds.c index d93a0c7b1786..0322f817d07b 100644 --- a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c +++ b/tools/testing/selftests/bpf/prog_tests/reg_bounds.c @@ -2091,7 +2091,7 @@ static struct subtest_case crafted_cases[] = { {U64, S64, {0, 0xffffffffULL}, {0x7fffffff, 0x7fffffff}}, {U64, U32, {0, 0x100000000}, {0, 0}}, - {U64, U32, {0xfffffffe, 0x100000000}, {0x80000000, 0x80000000}}, + {U64, U32, {0xfffffffe, 0x300000000}, {0x80000000, 0x80000000}}, {U64, S32, {0, 0xffffffff00000000ULL}, {0, 0}}, /* these are tricky cases where lower 32 bits allow to tighten 64 -- 2.43.0