[PATCH 6/6] ebpf: allow BPF_REG_X in src_reg conditional jumps

[Tycho Andersen <tycho.andersen@canonical.com>]

The classic converter generates conditional jumps with:

if (BPF_SRC(fp->code) == BPF_K && (int) fp->k < 0) {
        ...
} else {
        insn->dst_reg = BPF_REG_A;
        insn->src_reg = BPF_REG_X;
        insn->imm = fp->k;
        bpf_src = BPF_SRC(fp->code);
}

but here, we enforce that the src_reg == BPF_REG_0. We should also allow
BPF_REG_X since that's what the converter generates; this enables us to
load eBPF programs that were generated by the converter.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Kees Cook <keescook@chromium.org>
CC: Will Drewry <wad@chromium.org>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Serge E. Hallyn <serge.hallyn@ubuntu.com>
CC: Alexei Starovoitov <ast@kernel.org>
CC: Daniel Borkmann <daniel@iogearbox.net>
---
 kernel/bpf/verifier.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 039d866..2fff8cd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1080,7 +1080,12 @@ static int check_cond_jmp_op(struct verifier_env *env,
 		if (err)
 			return err;
 	} else {
-		if (insn->src_reg != BPF_REG_0) {
+		switch (insn->src_reg) {
+		case BPF_REG_0:
+		/* the classic converter generates BPF_JMP with src_reg of X */
+		case BPF_REG_X:
+			break;
+		default:
 			verbose("BPF_JMP uses reserved fields\n");
 			return -EINVAL;
 		}
-- 
2.1.4