Possible read of uninitialized array values in reuseport_select_sock?

Possible read of uninitialized array values in reuseport_select_sock?

[Baptiste Lepers]

Hello,

While reading the code of net/core/sock_reuseport.c, I think I found a
possible race in reuseport_select_sock. The current code has the
following pattern:

   socks = READ_ONCE(reuse->num_socks);
   smp_rmb(); // paired with reuseport_add_sock to make sure
reuse->socks[i < num_socks] are initialized
   while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
        if (i >= reuse->num_socks) // should be "socks" and not
"reuse->num_socks"?

The barrier seems to be here to make sure that all items of
reuse->socks are initialized before being read, but the barrier only
protects indexes < socks, not indexes < reuse->num_socks.

I have a patch ready to fix this issue, but I wanted to confirm that
the current code is indeed incorrect (if the code is correct for a
non-obvious reason, I'd be happy to add some comments to document the
behavior).


Here is the diff of the patch I was planning to submit:

diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
index bbdd3c7b6cb5..b065f0a103ed 100644
--- a/net/core/sock_reuseport.c
+++ b/net/core/sock_reuseport.c
@@ -293,7 +293,7 @@ struct sock *reuseport_select_sock(struct sock *sk,
             i = j = reciprocal_scale(hash, socks);
             while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
                 i++;
-                if (i >= reuse->num_socks)
+                if (i >= socks)
                     i = 0;
                 if (i == j)
                     goto out;


Thanks,
Baptiste.

Re: Possible read of uninitialized array values in reuseport_select_sock?

[Willem de Bruijn]

On Wed, Jan 6, 2021 at 10:54 PM Baptiste Lepers
<baptiste.lepers@gmail.com> wrote:
>
> Hello,
>
> While reading the code of net/core/sock_reuseport.c, I think I found a
> possible race in reuseport_select_sock. The current code has the
> following pattern:
>
>    socks = READ_ONCE(reuse->num_socks);
>    smp_rmb(); // paired with reuseport_add_sock to make sure
> reuse->socks[i < num_socks] are initialized
>    while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
>         if (i >= reuse->num_socks) // should be "socks" and not
> "reuse->num_socks"?
>
> The barrier seems to be here to make sure that all items of
> reuse->socks are initialized before being read, but the barrier only
> protects indexes < socks, not indexes < reuse->num_socks.
>
> I have a patch ready to fix this issue, but I wanted to confirm that
> the current code is indeed incorrect (if the code is correct for a
> non-obvious reason, I'd be happy to add some comments to document the
> behavior).
>
>
> Here is the diff of the patch I was planning to submit:
>
> diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> index bbdd3c7b6cb5..b065f0a103ed 100644
> --- a/net/core/sock_reuseport.c
> +++ b/net/core/sock_reuseport.c
> @@ -293,7 +293,7 @@ struct sock *reuseport_select_sock(struct sock *sk,
>              i = j = reciprocal_scale(hash, socks);
>              while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
>                  i++;
> -                if (i >= reuse->num_socks)
> +                if (i >= socks)
>                      i = 0;
>                  if (i == j)
>                      goto out;
>
>
> Thanks,
> Baptiste.

Thanks for the clear description. Yes, I believe you're correct.

Re: Possible read of uninitialized array values in reuseport_select_sock?

[Baptiste Lepers]

Thanks. I will submit a patch.

Baptiste.

On Thu, Jan 7, 2021 at 3:57 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> On Wed, Jan 6, 2021 at 10:54 PM Baptiste Lepers
> <baptiste.lepers@gmail.com> wrote:
> >
> > Hello,
> >
> > While reading the code of net/core/sock_reuseport.c, I think I found a
> > possible race in reuseport_select_sock. The current code has the
> > following pattern:
> >
> >    socks = READ_ONCE(reuse->num_socks);
> >    smp_rmb(); // paired with reuseport_add_sock to make sure
> > reuse->socks[i < num_socks] are initialized
> >    while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
> >         if (i >= reuse->num_socks) // should be "socks" and not
> > "reuse->num_socks"?
> >
> > The barrier seems to be here to make sure that all items of
> > reuse->socks are initialized before being read, but the barrier only
> > protects indexes < socks, not indexes < reuse->num_socks.
> >
> > I have a patch ready to fix this issue, but I wanted to confirm that
> > the current code is indeed incorrect (if the code is correct for a
> > non-obvious reason, I'd be happy to add some comments to document the
> > behavior).
> >
> >
> > Here is the diff of the patch I was planning to submit:
> >
> > diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> > index bbdd3c7b6cb5..b065f0a103ed 100644
> > --- a/net/core/sock_reuseport.c
> > +++ b/net/core/sock_reuseport.c
> > @@ -293,7 +293,7 @@ struct sock *reuseport_select_sock(struct sock *sk,
> >              i = j = reciprocal_scale(hash, socks);
> >              while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
> >                  i++;
> > -                if (i >= reuse->num_socks)
> > +                if (i >= socks)
> >                      i = 0;
> >                  if (i == j)
> >                      goto out;
> >
> >
> > Thanks,
> > Baptiste.
>
> Thanks for the clear description. Yes, I believe you're correct.