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

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2026-08-13 at 22:40 +0200, Daniel Borkmann wrote:
> 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 <daniel@iogearbox.net>
> ---
>  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;
>  		}
>  	}));
>  }

And said extension to bpf_convert_ctx_accesses() would cover this case automatically.