This is a test case minimized from a syzbot reproducer from [1]. The test case triggers verifier.c:maybe_exit_scc() w/o preceding call to verifier.c:maybe_enter_scc() on a speculative symbolic execution path. Here is verifier log for the test case: Live regs before insn: 0: .......... (b7) r0 = 100 1 1: 0......... (7b) *(u64 *)(r10 -512) = r0 1 2: 0......... (b5) if r0 <= 0x0 goto pc-2 3: 0......... (95) exit 0: R1=ctx() R10=fp0 0: (b7) r0 = 100 ; R0_w=100 1: (7b) *(u64 *)(r10 -512) = r0 ; R0_w=100 R10=fp0 fp-512_w=100 2: (b5) if r0 <= 0x0 goto pc-2 mark_precise: ... 2: R0_w=100 3: (95) exit from 2 to 1 (speculative execution): R0_w=scalar() R1=ctx() R10=fp0 fp-512_w=100 1: R0_w=scalar() R1=ctx() R10=fp0 fp-512_w=100 1: (7b) *(u64 *)(r10 -512) = r0 processed 5 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0 - Non-speculative execution path 0-3 does not allocate any checkpoints (and hence does not call maybe_enter_scc()), and schedules a speculative jump from 2 to 1. - Speculative execution path stops immediately because of an infinite loop detection and triggers verifier.c:update_branch_counts() -> maybe_exit_scc() calls. [1] https://lore.kernel.org/bpf/68c85acd.050a0220.2ff435.03a4.GAE@google.com/ Signed-off-by: Eduard Zingerman --- .../selftests/bpf/progs/verifier_loops1.c | 21 +++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/tools/testing/selftests/bpf/progs/verifier_loops1.c b/tools/testing/selftests/bpf/progs/verifier_loops1.c index e07b43b78fd2..fbdde80e7b90 100644 --- a/tools/testing/selftests/bpf/progs/verifier_loops1.c +++ b/tools/testing/selftests/bpf/progs/verifier_loops1.c @@ -283,4 +283,25 @@ exit_%=: \ : __clobber_all); } +/* + * This test case triggered a bug in verifier.c:maybe_exit_scc(). + * Speculative execution path reaches stack access instruction, + * stops and triggers maybe_exit_scc() w/o accompanying maybe_enter_scc() call. + */ +SEC("socket") +__arch_x86_64 +__caps_unpriv(CAP_BPF) +__naked void maybe_exit_scc_bug1(void) +{ + asm volatile ( + "r0 = 100;" +"1:" + /* Speculative execution path reaches and stops here. */ + "*(u64 *)(r10 - 512) = r0;" + /* Condition is always false, but verifier speculatively executes the true branch. */ + "if r0 <= 0x0 goto 1b;" + "exit;" + ::: __clobber_all); +} + char _license[] SEC("license") = "GPL"; -- 2.51.0