Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[David Miller]

From: David Miller 
Date: Sat, 11 Nov 2017 23:21:01 +0900 (KST)

> Aha, that's what I missed.  Indeed, it cannot happen.

Applied and queued up for -stable.

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[Jason A. Donenfeld]

On Sat, Nov 11, 2017 at 11:18 PM, Johannes Berg
 wrote:
>
>> > If you're handling this by forcing another read() to procude the
>> > NLMSG_DONE, then you have no reason to WARN_ON() here.
>> >
>> > In fact you are adding a WARN_ON() which is trivially triggerable by
>> > any user.
>>
>> I added this in my suggestion for how this could work, but I don't
>> think you're right, since we previously check if there's enough space.
>
> Or perhaps I should say this differently:
>
> Forcing another read happens through the
>
> skb_tailroom(skb) < nlmsg_total_size(...)
>
> check, so the nlmsg_put_answer() can't really fail.
>
>
> Handling nlmsg_put_answer() failures by forcing another read would have
> required jumping to the existing if code with a goto, or restructuring
> the whole thing completely somehow, and I didn't see how to do that.

Exactly. And if something _does_ go wrong in our logic, and we can't
add NLMSG_DONE, we really do want people to report this to us, since
dumps must always end that way. We'd probably have caught this a
number of years ago when userspace developers were twiddling with
their receive buffers if we had had the WARN_ON. Nice suggestion from
Johannes.

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[David Miller]

From: Johannes Berg 
Date: Sat, 11 Nov 2017 15:15:21 +0100

> On Sat, 2017-11-11 at 23:09 +0900, David Miller wrote:
>> From: "Jason A. Donenfeld" 
>> Date: Thu,  9 Nov 2017 13:04:44 +0900
>> 
>> > @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
>> >   return 0;
>> >   }
>> >  
>> > - nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), 
>> > NLM_F_MULTI);
>> > - if (!nlh)
>> > + nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
>> > +sizeof(nlk->dump_done_errno), NLM_F_MULTI);
>> > + if (WARN_ON(!nlh))
>> >   goto errout_skb;
>> 
>> If you're handling this by forcing another read() to procude the
>> NLMSG_DONE, then you have no reason to WARN_ON() here.
>> 
>> In fact you are adding a WARN_ON() which is trivially triggerable by
>> any user.
> 
> I added this in my suggestion for how this could work, but I don't
> think you're right, since we previously check if there's enough space.
> The patch is missing the full context, but this is:
 ...
> So unless the nlmsg_total_size() vs. nlmsg_put_answer() suddenly gets a
> different idea of how much space is needed, nlh shouldn't ever be NULL
> once we get here.

Aha, that's what I missed.  Indeed, it cannot happen.

My bad.

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[Johannes Berg]

> > If you're handling this by forcing another read() to procude the
> > NLMSG_DONE, then you have no reason to WARN_ON() here.
> > 
> > In fact you are adding a WARN_ON() which is trivially triggerable by
> > any user.
> 
> I added this in my suggestion for how this could work, but I don't
> think you're right, since we previously check if there's enough space.

Or perhaps I should say this differently:

Forcing another read happens through the

skb_tailroom(skb) < nlmsg_total_size(...)

check, so the nlmsg_put_answer() can't really fail.


Handling nlmsg_put_answer() failures by forcing another read would have
required jumping to the existing if code with a goto, or restructuring
the whole thing completely somehow, and I didn't see how to do that.

johannes

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[Johannes Berg]

On Sat, 2017-11-11 at 23:09 +0900, David Miller wrote:
> From: "Jason A. Donenfeld" 
> Date: Thu,  9 Nov 2017 13:04:44 +0900
> 
> > @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
> >   return 0;
> >   }
> >  
> > - nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
> > - if (!nlh)
> > + nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
> > +sizeof(nlk->dump_done_errno), NLM_F_MULTI);
> > + if (WARN_ON(!nlh))
> >   goto errout_skb;
> 
> If you're handling this by forcing another read() to procude the
> NLMSG_DONE, then you have no reason to WARN_ON() here.
> 
> In fact you are adding a WARN_ON() which is trivially triggerable by
> any user.

I added this in my suggestion for how this could work, but I don't
think you're right, since we previously check if there's enough space.
The patch is missing the full context, but this is:


+   if (nlk->dump_done_errno > 0 ||
+   skb_tailroom(skb) < nlmsg_total_size(sizeof(nlk->dump_done_errno))) 
{
mutex_unlock(nlk->cb_mutex);
 
if (sk_filter(sk, skb))
kfree_skb(skb);
else
__netlink_sendskb(sk, skb);
return 0;
}

-   nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
-   if (!nlh)
+   nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
+  sizeof(nlk->dump_done_errno), NLM_F_MULTI);
+   if (WARN_ON(!nlh))


So unless the nlmsg_total_size() vs. nlmsg_put_answer() suddenly gets a
different idea of how much space is needed, nlh shouldn't ever be NULL
once we get here.

johannes

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[David Miller]

From: "Jason A. Donenfeld" 
Date: Thu,  9 Nov 2017 13:04:44 +0900

> @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
>   return 0;
>   }
>  
> - nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
> - if (!nlh)
> + nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
> +sizeof(nlk->dump_done_errno), NLM_F_MULTI);
> + if (WARN_ON(!nlh))
>   goto errout_skb;

If you're handling this by forcing another read() to procude the
NLMSG_DONE, then you have no reason to WARN_ON() here.

In fact you are adding a WARN_ON() which is trivially triggerable by
any user.

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[Jason A. Donenfeld]

On Sat, Nov 11, 2017 at 11:37 AM, David Miller  wrote:
> Jason I'm already pushing my luck as-is with the pull request I made
> yesterday.
>
> I've seen your original requst to get this in, you don't have to say
> it multiple times.
>
> We can get this into the merge window and submit it for -stable, so
> please relax.

Whoops, sorry! Okay, no problem.

Jason

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[David Miller]

From: "Jason A. Donenfeld" 
Date: Sat, 11 Nov 2017 11:26:12 +0900

> IIRC, 4.14 comes tomorrow-ish? If possible, it would be nice to get
> this in 4.14 before then, so it doesn't have to take time to trickle
> down through stable.

Jason I'm already pushing my luck as-is with the pull request I made
yesterday.

I've seen your original requst to get this in, you don't have to say
it multiple times.

We can get this into the merge window and submit it for -stable, so
please relax.

Thank you.

Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[Jason A. Donenfeld]

IIRC, 4.14 comes tomorrow-ish? If possible, it would be nice to get
this in 4.14 before then, so it doesn't have to take time to trickle
down through stable.

Jason

On Thu, Nov 9, 2017 at 1:04 PM, Jason A. Donenfeld  wrote:
> The way people generally use netlink_dump is that they fill in the skb
> as much as possible, breaking when nla_put returns an error. Then, they
> get called again and start filling out the next skb, and again, and so
> forth. The mechanism at work here is the ability for the iterative
> dumping function to detect when the skb is filled up and not fill it
> past the brim, waiting for a fresh skb for the rest of the data.
>
> However, if the attributes are small and nicely packed, it is possible
> that a dump callback function successfully fills in attributes until the
> skb is of size 4080 (libmnl's default page-sized receive buffer size).
> The dump function completes, satisfied, and then, if it happens to be
> that this is actually the last skb, and no further ones are to be sent,
> then netlink_dump will add on the NLMSG_DONE part:
>
>   nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
>
> It is very important that netlink_dump does this, of course. However, in
> this example, that call to nlmsg_put_answer will fail, because the
> previous filling by the dump function did not leave it enough room. And
> how could it possibly have done so? All of the nla_put variety of
> functions simply check to see if the skb has enough tailroom,
> independent of the context it is in.
>
> In order to keep the important assumptions of all netlink dump users, it
> is therefore important to give them an skb that has this end part of the
> tail already reserved, so that the call to nlmsg_put_answer does not
> fail. Otherwise, library authors are forced to find some bizarre sized
> receive buffer that has a large modulo relative to the common sizes of
> messages received, which is ugly and buggy.
>
> This patch thus saves the NLMSG_DONE for an additional message, for the
> case that things are dangerously close to the brim. This requires
> keeping track of the errno from ->dump() across calls.
>
> Signed-off-by: Jason A. Donenfeld 
> ---
> Can we get this into 4.14? Is there still time? It should also be queued
> up for stable.
>
> Changes v3->v4:
>  - I like long lines. The kernel does not. Wrapped at 80 now.
>
>  net/netlink/af_netlink.c | 17 +++--
>  net/netlink/af_netlink.h |  1 +
>  2 files changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index b93148e8e9fb..15c99dfa3d72 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -2136,7 +2136,7 @@ static int netlink_dump(struct sock *sk)
> struct sk_buff *skb = NULL;
> struct nlmsghdr *nlh;
> struct module *module;
> -   int len, err = -ENOBUFS;
> +   int err = -ENOBUFS;
> int alloc_min_size;
> int alloc_size;
>
> @@ -2183,9 +2183,11 @@ static int netlink_dump(struct sock *sk)
> skb_reserve(skb, skb_tailroom(skb) - alloc_size);
> netlink_skb_set_owner_r(skb, sk);
>
> -   len = cb->dump(skb, cb);
> +   if (nlk->dump_done_errno > 0)
> +   nlk->dump_done_errno = cb->dump(skb, cb);
>
> -   if (len > 0) {
> +   if (nlk->dump_done_errno > 0 ||
> +   skb_tailroom(skb) < 
> nlmsg_total_size(sizeof(nlk->dump_done_errno))) {
> mutex_unlock(nlk->cb_mutex);
>
> if (sk_filter(sk, skb))
> @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
> return 0;
> }
>
> -   nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
> -   if (!nlh)
> +   nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
> +  sizeof(nlk->dump_done_errno), NLM_F_MULTI);
> +   if (WARN_ON(!nlh))
> goto errout_skb;
>
> nl_dump_check_consistent(cb, nlh);
>
> -   memcpy(nlmsg_data(nlh), , sizeof(len));
> +   memcpy(nlmsg_data(nlh), >dump_done_errno,
> +  sizeof(nlk->dump_done_errno));
>
> if (sk_filter(sk, skb))
> kfree_skb(skb);
> @@ -2273,6 +2277,7 @@ int __netlink_dump_start(struct sock *ssk, struct 
> sk_buff *skb,
> }
>
> nlk->cb_running = true;
> +   nlk->dump_done_errno = INT_MAX;
>
> mutex_unlock(nlk->cb_mutex);
>
> diff --git a/net/netlink/af_netlink.h b/net/netlink/af_netlink.h
> index 028188597eaa..962de7b3c023 100644
> --- a/net/netlink/af_netlink.h
> +++ b/net/netlink/af_netlink.h
> @@ -34,6 +34,7 @@ struct netlink_sock {
> wait_queue_head_t   wait;
> boolbound;
> boolcb_running;
> +   int dump_done_errno;
> struct netlink_callback cb;
> struct mutex*cb_mutex;
> struct mutex

[PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[Jason A. Donenfeld]

The way people generally use netlink_dump is that they fill in the skb
as much as possible, breaking when nla_put returns an error. Then, they
get called again and start filling out the next skb, and again, and so
forth. The mechanism at work here is the ability for the iterative
dumping function to detect when the skb is filled up and not fill it
past the brim, waiting for a fresh skb for the rest of the data.

However, if the attributes are small and nicely packed, it is possible
that a dump callback function successfully fills in attributes until the
skb is of size 4080 (libmnl's default page-sized receive buffer size).
The dump function completes, satisfied, and then, if it happens to be
that this is actually the last skb, and no further ones are to be sent,
then netlink_dump will add on the NLMSG_DONE part:

  nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);

It is very important that netlink_dump does this, of course. However, in
this example, that call to nlmsg_put_answer will fail, because the
previous filling by the dump function did not leave it enough room. And
how could it possibly have done so? All of the nla_put variety of
functions simply check to see if the skb has enough tailroom,
independent of the context it is in.

In order to keep the important assumptions of all netlink dump users, it
is therefore important to give them an skb that has this end part of the
tail already reserved, so that the call to nlmsg_put_answer does not
fail. Otherwise, library authors are forced to find some bizarre sized
receive buffer that has a large modulo relative to the common sizes of
messages received, which is ugly and buggy.

This patch thus saves the NLMSG_DONE for an additional message, for the
case that things are dangerously close to the brim. This requires
keeping track of the errno from ->dump() across calls.

Signed-off-by: Jason A. Donenfeld 
---
Can we get this into 4.14? Is there still time? It should also be queued
up for stable.

Changes v3->v4:
 - I like long lines. The kernel does not. Wrapped at 80 now.

 net/netlink/af_netlink.c | 17 +++--
 net/netlink/af_netlink.h |  1 +
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index b93148e8e9fb..15c99dfa3d72 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2136,7 +2136,7 @@ static int netlink_dump(struct sock *sk)
struct sk_buff *skb = NULL;
struct nlmsghdr *nlh;
struct module *module;
-   int len, err = -ENOBUFS;
+   int err = -ENOBUFS;
int alloc_min_size;
int alloc_size;
 
@@ -2183,9 +2183,11 @@ static int netlink_dump(struct sock *sk)
skb_reserve(skb, skb_tailroom(skb) - alloc_size);
netlink_skb_set_owner_r(skb, sk);
 
-   len = cb->dump(skb, cb);
+   if (nlk->dump_done_errno > 0)
+   nlk->dump_done_errno = cb->dump(skb, cb);
 
-   if (len > 0) {
+   if (nlk->dump_done_errno > 0 ||
+   skb_tailroom(skb) < nlmsg_total_size(sizeof(nlk->dump_done_errno))) 
{
mutex_unlock(nlk->cb_mutex);
 
if (sk_filter(sk, skb))
@@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
return 0;
}
 
-   nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
-   if (!nlh)
+   nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
+  sizeof(nlk->dump_done_errno), NLM_F_MULTI);
+   if (WARN_ON(!nlh))
goto errout_skb;
 
nl_dump_check_consistent(cb, nlh);
 
-   memcpy(nlmsg_data(nlh), , sizeof(len));
+   memcpy(nlmsg_data(nlh), >dump_done_errno,
+  sizeof(nlk->dump_done_errno));
 
if (sk_filter(sk, skb))
kfree_skb(skb);
@@ -2273,6 +2277,7 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff 
*skb,
}
 
nlk->cb_running = true;
+   nlk->dump_done_errno = INT_MAX;
 
mutex_unlock(nlk->cb_mutex);
 
diff --git a/net/netlink/af_netlink.h b/net/netlink/af_netlink.h
index 028188597eaa..962de7b3c023 100644
--- a/net/netlink/af_netlink.h
+++ b/net/netlink/af_netlink.h
@@ -34,6 +34,7 @@ struct netlink_sock {
wait_queue_head_t   wait;
boolbound;
boolcb_running;
+   int dump_done_errno;
struct netlink_callback cb;
struct mutex*cb_mutex;
struct mutexcb_def_mutex;
-- 
2.15.0