From: Björn Töpel <bjorn@xxxxxxxxxxxx>
A BPF call instruction can be, correctly, marked with zext_dst set to
true. An example of this can be found in the BPF selftests
progs/bpf_cubic.c:
...
extern __u32 tcp_reno_undo_cwnd(struct sock *sk) __ksym;
__u32 BPF_STRUCT_OPS(bpf_cubic_undo_cwnd, struct sock *sk)
{
return tcp_reno_undo_cwnd(sk);
}
...
which compiles to:
0: r1 = *(u64 *)(r1 + 0x0)
1: call -0x1
2: exit
The call will be marked as zext_dst set to true, and for some backends
(bpf_jit_needs_zext() returns true) expanded to:
0: r1 = *(u64 *)(r1 + 0x0)
1: call -0x1
2: w0 = w0
3: exit
The opt_subreg_zext_lo32_rnd_hi32() function which is responsible for
the zext patching, relies on insn_def_regno() to fetch the register to
zero-extend. However, this function does not handle call instructions
correctly, and opt_subreg_zext_lo32_rnd_hi32() fails the verification.
Make sure that R0 is correctly resolved for (BPF_JMP | BPF_CALL)
instructions.
Fixes: 83a2881903f3 ("bpf: Account for BPF_FETCH in insn_has_def32()")
Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
---
I'm not super happy about the additional special case -- first
cmpxchg, and now call. :-( A more elegant/generic solution is welcome!
---
kernel/bpf/verifier.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 264b3dc714cc..4f9660eafc72 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -13386,6 +13386,9 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
if (!bpf_jit_needs_zext() && !is_cmpxchg_insn(&insn))
continue;
+ if (insn.code == (BPF_JMP | BPF_CALL))
+ load_reg = BPF_REG_0;
+
if (WARN_ON(load_reg == -1)) {
verbose(env, "verifier bug. zext_dst is set, but no reg is defined\n");
return -EFAULT;
base-commit: 01f856ae6d0ca5ad0505b79bf2d22d7ca439b2a1
--
2.37.2
