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SubjectRe: netlink: Add barrier to netlink_connect for theoretical case
On Thu, Sep 24, 2015 at 08:24:56PM -0700, Linus Torvalds wrote:
>
> The above looks very suspicious.

You're right Linus. I've added the READ_ONCE there. The reason I
kept the conditional is because the helper is always called in a
context where the result is used as part of an if statement. The
assembly actually looks sane, e.g., for netlink_bind:

3a06: 41 0f b6 94 24 e8 02 movzbl 0x2e8(%r12),%edx
3a0d: 00 00
3a0f: 84 d2 test %dl,%dl
3a11: 0f 85 97 00 00 00 jne 3aae <netlink_bind+0x12e>
3a17: 49 83 bc 24 98 03 00 cmpq $0x0,0x398(%r12)
3a1e: 00 00

...

3aae: 41 8b 84 24 a0 02 00 mov 0x2a0(%r12),%eax
3ab5: 00
3ab6: 41 39 46 04 cmp %eax,0x4(%r14)
3aba: 0f 84 57 ff ff ff je 3a17 <netlink_bind+0x97>
3ac0: b8 ea ff ff ff mov $0xffffffea,%eax
3ac5: eb d1 jmp 3a98 <netlink_bind+0x118>

So there is no unnecessary jumping around. I checked the other
two call sites and they look the same. I'm on a fairly old compiler
though (4.7.2) so it is possible that newer gcc's may do silly things.

Thanks,

---8<---
If a netlink_connect call is followed by a netlink_getname call
the portid returned may not be up-to-date. This patch adds a
barrier for that case.

As all nlk->bound dereferences now have barriers this patch also
adds a netlink_bound helper to encapsulate the barrier, as was
suggested by Tejun. Also as suggested by Linus, the lockless
read of nlk->bound is now protected with READ_ONCE to ensure that
the compiler doesn't do double reads that may screw up our use
of the barrier.

Fixes: da314c9923fe ("netlink: Replace rhash_portid with bound")
Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 2c15fae..dd0a294 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -125,6 +125,17 @@ static inline u32 netlink_group_mask(u32 group)
return group ? 1 << (group - 1) : 0;
}

+static inline bool netlink_bound(struct netlink_sock *nlk)
+{
+ bool bound = READ_ONCE(nlk->bound);
+
+ /* Ensure nlk is hashed and visible. */
+ if (bound)
+ smp_rmb();
+
+ return bound;
+}
+
int netlink_add_tap(struct netlink_tap *nt)
{
if (unlikely(nt->dev->type != ARPHRD_NETLINK))
@@ -1524,14 +1535,10 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
return err;
}

- bound = nlk->bound;
- if (bound) {
- /* Ensure nlk->portid is up-to-date. */
- smp_rmb();
-
+ bound = netlink_bound(nlk);
+ if (bound)
if (nladdr->nl_pid != nlk->portid)
return -EINVAL;
- }

if (nlk->netlink_bind && groups) {
int group;
@@ -1547,9 +1554,6 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
}
}

- /* No need for barriers here as we return to user-space without
- * using any of the bound attributes.
- */
if (!bound) {
err = nladdr->nl_pid ?
netlink_insert(sk, nladdr->nl_pid) :
@@ -1598,10 +1602,7 @@ static int netlink_connect(struct socket *sock, struct sockaddr *addr,
!netlink_allowed(sock, NL_CFG_F_NONROOT_SEND))
return -EPERM;

- /* No need for barriers here as we return to user-space without
- * using any of the bound attributes.
- */
- if (!nlk->bound)
+ if (!netlink_bound(nlk))
err = netlink_autobind(sock);

if (err == 0) {
@@ -2442,13 +2443,10 @@ static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
dst_group = nlk->dst_group;
}

- if (!nlk->bound) {
+ if (!netlink_bound(nlk)) {
err = netlink_autobind(sock);
if (err)
goto out;
- } else {
- /* Ensure nlk is hashed and visible. */
- smp_rmb();
}

/* It's a really convoluted way for userland to ask for mmaped
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt


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