bpf: reject BPF_PSEUDO_FUNC reference to the main program

fixups.c:jit_subprogs() rewrites BPF_PSEUDO_FUNC loads to contain real
function addresses. This function is invoked from bpf_jit_subprogs()
only when env->subprog_cnt > 1. Meaning that for any program like
below:

  int main(void *ctx) {
    void *ptr = main;
    ...
    bpf_timer_set_callback(..., ptr);
    ...
  }

The 'ptr' won't be ever converted to contain an address.
In combination with e.g. bpf_timer_set_callback() this would lead to a
function call at a bogus address.

Instead of complicating the implementation, just assume that no useful
program needs main to be a sync or async callback and reject
BPF_PSEUDO_FUNC loads for the main subprogram.

Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260902233658.1186477-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e64035683795..7d8ddb1bee00 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -17089,6 +17089,15 @@ static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
 			verbose(env, "callback function not static\n");
 			return -EINVAL;
 		}
+		/*
+		 * When env->subprog_cnt == 1 this instruction won't be rewritten
+		 * to hold a real function address. Assume that no usable program
+		 * combines e.g. main and timer callback and just reject here.
+		 */
+		if (subprogno == 0) {
+			verbose(env, "callback function cannot be the main program\n");
+			return -EINVAL;
+		}
 
 		dst_reg->type = PTR_TO_FUNC;
 		dst_reg->subprogno = subprogno;