[PATCH bpf-next 3/4] bpf: Keep untrusted PTR_TO_MEM read-only on RCU invalidation

invalidate_rcu_protected_refs() turns MEM_RCU pointers into PTR_UNTRUSTED
ones once the RCU read-side critical section ends.

For a PTR_TO_BTF_ID base that is fine, but for a PTR_TO_MEM base the
result has to carry MEM_RDONLY as well, since the rest of the verifier
relies on the two coming as a pair (e.g. bpf_convert_ctx_accesses()).

A writable untrusted PTR_TO_MEM would otherwise end up as a plain load
or store to memory without fault protection. No in-tree kfunc produces
the combination today, thus this is mainly hardening.

Signed-off-by: Daniel Borkmann <[email protected]>
---
 kernel/bpf/verifier.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8ef9418733ca..0c0dd53118d5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9054,6 +9054,9 @@ static void invalidate_rcu_protected_refs(struct bpf_verifier_env *env)
 		if (reg->type & MEM_RCU) {
 			reg->type &= ~(MEM_RCU | PTR_MAYBE_NULL);
 			reg->type |= PTR_UNTRUSTED;
+			/* An untrusted PTR_TO_MEM has to be MEM_RDONLY. */
+			if (base_type(reg->type) == PTR_TO_MEM)
+				reg->type |= MEM_RDONLY;
 		}
 	}));
 }
-- 
2.43.0