[PATCH 5.15 120/120] bpf: Skip invalid kfunc call in backtrack_insn

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From: Hao Sun <sunhao.th@xxxxxxxxx>

commit d3178e8a434b58678d99257c0387810a24042fb6 upstream.

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@xxxxxxxxxxxxxx/

Reported-by: syzbot+4da3ff23081bafe74fc2@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
Link: https://lore.kernel.org/bpf/20230104014709.9375-1-sunhao.th@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/verifier.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2261,6 +2261,12 @@ static int backtrack_insn(struct bpf_ver
 		if (opcode == BPF_CALL) {
 			if (insn->src_reg == BPF_PSEUDO_CALL)
 				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) {





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