Calls like bpf_loop() or bpf_for_each_map_elem() introduce loops that are not explicitly present in the control-flow graph. The verifier processes such calls by repeatedly interpreting the callback function body within the same verification path (until the current state converges with a previous state). Such loops require a bpf_scc_visit instance in order to allow the accumulation of the state graph backedges. Otherwise, certain checkpoint states created within the bodies of such loops will have incomplete precision marks. See the next patch for an example of a program that leads to the verifier accepting an unsafe program. Fixes: 96c6aa4c63af ("bpf: compute SCCs in program control flow graph") Fixes: c9e31900b54c ("bpf: propagate read/precision marks over state graph backedges") Reported-by: Breno Leitao Signed-off-by: Eduard Zingerman --- kernel/bpf/verifier.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 2de1a736ef69514fcf599de498aae56eaf24fe33..0baae7828af220accd4086b9bad270e745f4aff9 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -19830,8 +19830,10 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) } } if (bpf_calls_callback(env, insn_idx)) { - if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) + if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) { + loop = true; goto hit; + } goto skip_inf_loop_check; } /* attempt to detect infinite loop to avoid unnecessary doomed work */ @@ -25071,15 +25073,18 @@ static int compute_scc(struct bpf_verifier_env *env) } /* * Assign SCC number only if component has two or more elements, - * or if component has a self reference. + * or if component has a self reference, or if instruction is a + * callback calling function (implicit loop). */ - assign_scc = stack[stack_sz - 1] != w; - for (j = 0; j < succ->cnt; ++j) { + assign_scc = stack[stack_sz - 1] != w; /* two or more elements? */ + for (j = 0; j < succ->cnt; ++j) { /* self reference? */ if (succ->items[j] == w) { assign_scc = true; break; } } + if (bpf_calls_callback(env, w)) /* implicit loop? */ + assign_scc = true; /* Pop component elements from stack */ do { t = stack[--stack_sz]; -- 2.52.0 Test for state graph backedges accumulation for SCCs formed by bpf_loop(). Equivalent to the following C program: int main(void) { 1: fp[-8] = bpf_get_prandom_u32(); 2: fp[-16] = -32; // used in a memory access below 3: bpf_loop(7, loop_cb4, fp, 0); 4: return 0; } int loop_cb4(int i, void *ctx) { 5: if (unlikely(ctx[-8] > bpf_get_prandom_u32())) 6: *(u64 *)(fp + ctx[-16]) = 42; // aligned access expected 7: if (unlikely(fp[-8] > bpf_get_prandom_u32())) 8: ctx[-16] = -31; // makes said access unaligned 9: return 0; } If state graph backedges are not accumulated properly at the SCC formed by loop_cb4() call from bpf_loop(), the state {ctx[-16]=-32} injected at instruction 9 on verification path 1,2,3,5,7,9,4 would be considered fully verified and would lack precision mark for ctx[-16]. This would lead to early pruning of verification path 1,2,3,5,7,8,9 in state {ctx[-16]=-31}, which in turn leads to the incorrect assumption that the above program is safe. Signed-off-by: Eduard Zingerman --- tools/testing/selftests/bpf/progs/iters.c | 75 +++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) diff --git a/tools/testing/selftests/bpf/progs/iters.c b/tools/testing/selftests/bpf/progs/iters.c index 7dd92a303bf6b3f0fc2962f6ce6cc453350561e3..69061f0309579eada74e5f2a68640470ff94a8b3 100644 --- a/tools/testing/selftests/bpf/progs/iters.c +++ b/tools/testing/selftests/bpf/progs/iters.c @@ -1926,4 +1926,79 @@ static int loop1_wrapper(void) ); } +/* + * This is similar to a test case absent_mark_in_the_middle_state(), + * but adapted for use with bpf_loop(). + */ +SEC("raw_tp") +__flag(BPF_F_TEST_STATE_FREQ) +__failure __msg("math between fp pointer and register with unbounded min value is not allowed") +__naked void absent_mark_in_the_middle_state4(void) +{ + /* + * Equivalent to a C program below: + * + * int main(void) { + * fp[-8] = bpf_get_prandom_u32(); + * fp[-16] = -32; // used in a memory access below + * bpf_loop(7, loop_cb4, fp, 0); + * return 0; + * } + * + * int loop_cb4(int i, void *ctx) { + * if (unlikely(ctx[-8] > bpf_get_prandom_u32())) + * *(u64 *)(fp + ctx[-16]) = 42; // aligned access expected + * if (unlikely(fp[-8] > bpf_get_prandom_u32())) + * ctx[-16] = -31; // makes said access unaligned + * return 0; + * } + */ + asm volatile ( + "call %[bpf_get_prandom_u32];" + "r8 = r0;" + "*(u64 *)(r10 - 8) = r0;" + "*(u64 *)(r10 - 16) = -32;" + "r1 = 7;" + "r2 = loop_cb4 ll;" + "r3 = r10;" + "r4 = 0;" + "call %[bpf_loop];" + "r0 = 0;" + "exit;" + : + : __imm(bpf_loop), + __imm(bpf_get_prandom_u32) + : __clobber_all + ); +} + +__used __naked +static void loop_cb4(void) +{ + asm volatile ( + "r9 = r2;" + "r8 = *(u64 *)(r9 - 8);" + "r6 = *(u64 *)(r9 - 16);" + "call %[bpf_get_prandom_u32];" + "if r0 > r8 goto use_fp16_%=;" + "1:" + "call %[bpf_get_prandom_u32];" + "if r0 > r8 goto update_fp16_%=;" + "2:" + "r0 = 0;" + "exit;" + "use_fp16_%=:" + "r1 = r10;" + "r1 += r6;" + "*(u64 *)(r1 + 0) = 42;" + "goto 1b;" + "update_fp16_%=:" + "*(u64 *)(r9 - 16) = -31;" + "goto 2b;" + : + : __imm(bpf_get_prandom_u32) + : __clobber_all + ); +} + char _license[] SEC("license") = "GPL"; -- 2.52.0