From: Dave Marchevsky Date: Mon, 21 Aug 2023 19:33:07 +0000 (-0700) Subject: bpf: Use bpf_mem_free_rcu when bpf_obj_dropping refcounted nodes X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=7e26cd12ad1c;p=linux.git bpf: Use bpf_mem_free_rcu when bpf_obj_dropping refcounted nodes This is the final fix for the use-after-free scenario described in commit 7793fc3babe9 ("bpf: Make bpf_refcount_acquire fallible for non-owning refs"). That commit, by virtue of changing bpf_refcount_acquire's refcount_inc to a refcount_inc_not_zero, fixed the "refcount incr on 0" splat. The not_zero check in refcount_inc_not_zero, though, still occurs on memory that could have been free'd and reused, so the commit didn't properly fix the root cause. This patch actually fixes the issue by free'ing using the recently-added bpf_mem_free_rcu, which ensures that the memory is not reused until RCU grace period has elapsed. If that has happened then there are no non-owning references alive that point to the recently-free'd memory, so it can be safely reused. Signed-off-by: Dave Marchevsky Acked-by: Yonghong Song Link: https://lore.kernel.org/r/20230821193311.3290257-4-davemarchevsky@fb.com Signed-off-by: Alexei Starovoitov --- diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index eb91cae0612ae..945a85e25ac5e 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1913,7 +1913,11 @@ void __bpf_obj_drop_impl(void *p, const struct btf_record *rec) if (rec) bpf_obj_free_fields(rec, p); - bpf_mem_free(&bpf_global_ma, p); + + if (rec && rec->refcount_off >= 0) + bpf_mem_free_rcu(&bpf_global_ma, p); + else + bpf_mem_free(&bpf_global_ma, p); } __bpf_kfunc void bpf_obj_drop_impl(void *p__alloc, void *meta__ign)