[PATCH] bpf: skip invalid kfunc call in backtrack_insn

[Hao Sun]

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(), alse 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/

Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4a25375ebb0d..04887b1e4178 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2750,6 +2750,12 @@ 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) {

base-commit: acd3b7768048fe338248cdf43ccfbf8c084a6bc1
-- 
2.39.0