bpf: Skip invalid kfunc call in backtrack_insn

The verifier skips invalid kfunc call in check_kfunc_call(), which
would be captured in fixup_kfunc_call() if such insn is not eliminated
by dead code elimination. However, this can lead to the following
warning in backtrack_insn(), also see [1]:

  ------------[ cut here ]------------
  verifier backtracking bug
  WARNING: CPU: 6 PID: 8646 at kernel/bpf/verifier.c:2756 backtrack_insn
  kernel/bpf/verifier.c:2756
	__mark_chain_precision kernel/bpf/verifier.c:3065
	mark_chain_precision kernel/bpf/verifier.c:3165
	adjust_reg_min_max_vals kernel/bpf/verifier.c:10715
	check_alu_op kernel/bpf/verifier.c:10928
	do_check kernel/bpf/verifier.c:13821 [inline]
	do_check_common kernel/bpf/verifier.c:16289
  [...]

So make backtracking conservative with this by returning ENOTSUPP.

  [1] https://lore.kernel.org/bpf/CACkBjsaXNceR8ZjkLG=dT3P=4A8SBsg0Z5h5PWLryF5=ghKq=g@mail.gmail.com/

Reported-by: syzbot+4da3ff23081bafe74fc2@syzkaller.appspotmail.com
Signed-off-by: Hao Sun <sunhao.th@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230104014709.9375-1-sunhao.th@gmail.com

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 85f96c1..c4c0985 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2748,6 +2748,12 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx,
                         */
                        if (insn->src_reg == 0 && is_callback_calling_function(insn->imm))
                                return -ENOTSUPP;
+                       /* kfunc with imm==0 is invalid and fixup_kfunc_call will
+                        * catch this error later. Make backtracking conservative
+                        * with ENOTSUPP.
+                        */
+                       if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL && insn->imm == 0)
+                               return -ENOTSUPP;
                        /* regular helper call sets R0 */
                        *reg_mask &= ~1;
                        if (*reg_mask & 0x3f) {