[PATCH bpf-next v2 06/13] bpf: Reject callbacks returning more than 8 bytes

A callback handed to a helper or a kfunc (bpf_loop(),
bpf_timer_set_callback(), bpf_for_each_map_elem(), ...) is invoked
through bpf_callback_t, and an exception callback is invoked by
bpf_throw() through

  u64 (*bpf_exception_cb)(u64 cookie, u64 sp, u64 bp, u64, u64);

Both prototypes yield a single u64 in R0, and neither caller has any
notion of a second return register, so a callback returning a value in
the R0:R2 pair would have the upper half of its return value silently
dropped.

Reject both at load time:

 - check_ld_imm(): a callback is materialized as PTR_TO_FUNC by an
   ld_imm64 pointing at its subprogram, so the subprogram's return
   convention can be checked where the callback pointer is created,
   before it ever reaches a helper or kfunc argument.

 - do_check_common(): an exception callback is not referenced by a
   PTR_TO_FUNC, it is named by a BTF decl_tag and verified on its own,
   so check it as its frame is set up, next to the existing "cannot
   return void" and single-argument checks.

Signed-off-by: Yonghong Song <[email protected]>
---
 kernel/bpf/verifier.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 282aee7fc44c..5584178a0e1c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -16426,6 +16426,11 @@ static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
 			verbose(env, "callback function not static\n");
 			return -EINVAL;
 		}
+		if (bpf_ret_reg_pair(env, subprogno)) {
+			verbose(env,
+				"callback function with >8-byte return value is not supported\n");
+			return -EINVAL;
+		}
 
 		dst_reg->type = PTR_TO_FUNC;
 		dst_reg->subprogno = subprogno;
@@ -18650,6 +18655,12 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
 				ret = -EINVAL;
 				goto out;
 			}
+			if (bpf_ret_reg_pair(env, subprog)) {
+				verbose(env,
+					"exception cb cannot return value larger than 8 bytes\n");
+				ret = -EINVAL;
+				goto out;
+			}
 
 			/* Also ensure the callback only has a single scalar argument. */
 			if (sub->arg_cnt != 1 || sub->args[0].arg_type != ARG_ANYTHING) {
-- 
2.53.0-Meta