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SubjectRe: [PATCH 08/17] net: convert sk_filter.refcnt from atomic_t to refcount_t
On 03/16/2017 04:28 PM, Elena Reshetova wrote:
> refcount_t type and corresponding API should be
> used instead of atomic_t when the variable is used as
> a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free
> situations.
>
> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: David Windsor <dwindsor@gmail.com>
> ---
> include/linux/filter.h | 3 ++-
> net/core/filter.c | 7 ++++---
> 2 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 8053c38..20247e7 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -7,6 +7,7 @@
> #include <stdarg.h>
>
> #include <linux/atomic.h>
> +#include <linux/refcount.h>
> #include <linux/compat.h>
> #include <linux/skbuff.h>
> #include <linux/linkage.h>
> @@ -431,7 +432,7 @@ struct bpf_prog {
> };
>
> struct sk_filter {
> - atomic_t refcnt;
> + refcount_t refcnt;
> struct rcu_head rcu;
> struct bpf_prog *prog;
> };
> diff --git a/net/core/filter.c b/net/core/filter.c
> index ebaeaf2..62267e2 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -928,7 +928,7 @@ static void sk_filter_release_rcu(struct rcu_head *rcu)
> */
> static void sk_filter_release(struct sk_filter *fp)
> {
> - if (atomic_dec_and_test(&fp->refcnt))
> + if (refcount_dec_and_test(&fp->refcnt))
> call_rcu(&fp->rcu, sk_filter_release_rcu);
> }
>
> @@ -950,7 +950,7 @@ bool sk_filter_charge(struct sock *sk, struct sk_filter *fp)
> /* same check as in sock_kmalloc() */
> if (filter_size <= sysctl_optmem_max &&
> atomic_read(&sk->sk_omem_alloc) + filter_size < sysctl_optmem_max) {
> - atomic_inc(&fp->refcnt);
> + refcount_inc(&fp->refcnt);
> atomic_add(filter_size, &sk->sk_omem_alloc);
> return true;
> }
> @@ -1179,12 +1179,13 @@ static int __sk_attach_prog(struct bpf_prog *prog, struct sock *sk)
> return -ENOMEM;
>
> fp->prog = prog;
> - atomic_set(&fp->refcnt, 0);
> + refcount_set(&fp->refcnt, 1);
>
> if (!sk_filter_charge(sk, fp)) {
> kfree(fp);
> return -ENOMEM;
> }
> + refcount_set(&fp->refcnt, 1);

Regarding the two subsequent refcount_set(, 1) that look a bit strange
due to the sk_filter_charge() having refcount_inc() I presume ... can't
the refcount API handle such corner case? Or alternatively the let the
sk_filter_charge() handle it, for example:

bool __sk_filter_charge(struct sock *sk, struct sk_filter *fp)
{
u32 filter_size = bpf_prog_size(fp->prog->len);

/* same check as in sock_kmalloc() */
if (filter_size <= sysctl_optmem_max &&
atomic_read(&sk->sk_omem_alloc) + filter_size < sysctl_optmem_max) {
atomic_add(filter_size, &sk->sk_omem_alloc);
return true;
}
return false;
}

And this goes to filter.h:

bool __sk_filter_charge(struct sock *sk, struct sk_filter *fp);

bool sk_filter_charge(struct sock *sk, struct sk_filter *fp)
{
bool ret = __sk_filter_charge(sk, fp);
if (ret)
refcount_inc(&fp->refcnt);
return ret;
}

... and let __sk_attach_prog() call __sk_filter_charge() and only fo
the second refcount_set()?

> old_fp = rcu_dereference_protected(sk->sk_filter,
> lockdep_sock_is_held(sk));
>

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