[PATCH bpf v1 03/10] bpf: Reject tail calls directly from callback frames

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

A tail call from a non-zero frame is modeled as a return from that frame.
The verifier makes R0 unknown and calls prepare_func_exit() for the taken
branch.

When the current frame is a synchronous callback, prepare_func_exit()
enforces the callback return-value contract and marks R0 precise. Since the
tail-call path synthesized R0 rather than deriving it from an instruction,
precision backtracking reaches the callback-calling instruction with R0
still requested and triggers the "callback unexpected regs" verifier bug.
A CAP_BPF task can therefore cause a WARN and an -EFAULT BPF_PROG_LOAD.

Tail calls reachable from callbacks are already rejected later by
check_max_stack_depth(). Reject a tail call made directly by a callback
before constructing the inconsistent return state, using the existing
diagnostic. Tail calls from ordinary subprograms keep their current
behavior.

Fixes: e3245f899043 ("bpf: properly verify tail call behavior")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7d8ddb1bee00..f540279ff4ab 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11228,6 +11228,17 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		if (env->cur_state->curframe) {
 			struct bpf_verifier_state *branch;
 
+			/*
+			 * A taken tail call is modeled as a return from the current
+			 * frame. A callback frame cannot be left that way because
+			 * prepare_func_exit() would apply its return contract to the
+			 * unknown R0 synthesized below. Stack-depth validation rejects
+			 * this construct anyway.
+			 */
+			if (cur_func(env)->in_callback_fn) {
+				verbose(env, "cannot tail call within callback\n");
+				return -EINVAL;
+			}
 			mark_reg_scratched(env, BPF_REG_0);
 			branch = push_stack(env, env->insn_idx + 1, env->insn_idx, false);
 			if (IS_ERR(branch))
-- 
2.53.0