| Message ID | 1703081351-85579-2-git-send-email-alibuda@linux.alibaba.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | netfilter: bpf: support prog update | expand |
On Wed, Dec 20, 2023 at 6:09 AM D. Wythe <alibuda@linux.alibaba.com> wrote: > > From: "D. Wythe" <alibuda@linux.alibaba.com> > > To support the prog update, we need to ensure that the prog seen > within the hook is always valid. Considering that hooks are always > protected by rcu_read_lock(), which provide us the ability to > access the prog under rcu. > > Signed-off-by: D. Wythe <alibuda@linux.alibaba.com> > --- > net/netfilter/nf_bpf_link.c | 63 ++++++++++++++++++++++++++++++++++----------- > 1 file changed, 48 insertions(+), 15 deletions(-) > > diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c > index e502ec0..9bc91d1 100644 > --- a/net/netfilter/nf_bpf_link.c > +++ b/net/netfilter/nf_bpf_link.c > @@ -8,17 +8,8 @@ > #include <net/netfilter/nf_bpf_link.h> > #include <uapi/linux/netfilter_ipv4.h> > > -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb, > - const struct nf_hook_state *s) > -{ > - const struct bpf_prog *prog = bpf_prog; > - struct bpf_nf_ctx ctx = { > - .state = s, > - .skb = skb, > - }; > - > - return bpf_prog_run(prog, &ctx); > -} > +/* protect link update in parallel */ > +static DEFINE_MUTEX(bpf_nf_mutex); > > struct bpf_nf_link { > struct bpf_link link; > @@ -26,8 +17,20 @@ struct bpf_nf_link { > struct net *net; > u32 dead; > const struct nf_defrag_hook *defrag_hook; > + struct rcu_head head; I have to point out the same issues as before, but will ask them differently... Why do you think above rcu_head is necessary? > }; > > +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb, > + const struct nf_hook_state *s) > +{ > + const struct bpf_nf_link *nf_link = bpf_link; > + struct bpf_nf_ctx ctx = { > + .state = s, > + .skb = skb, > + }; > + return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx); > +} > + > #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6) > static const struct nf_defrag_hook * > get_proto_defrag_hook(struct bpf_nf_link *link, > @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link) > static void bpf_nf_link_dealloc(struct bpf_link *link) > { > struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); > - > - kfree(nf_link); > + kfree_rcu(nf_link, head); Why is this needed ? Have you looked at tcx_link_lops ? > } > > static int bpf_nf_link_detach(struct bpf_link *link) > @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link, > static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog, > struct bpf_prog *old_prog) > { > - return -EOPNOTSUPP; > + struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); > + int err = 0; > + > + mutex_lock(&bpf_nf_mutex); Why do you need this mutex? What race does it solve? > + > + if (nf_link->dead) { > + err = -EPERM; > + goto out; > + } > + > + /* target old_prog mismatch */ > + if (old_prog && link->prog != old_prog) { > + err = -EPERM; > + goto out; > + } > + > + old_prog = link->prog; > + if (old_prog == new_prog) { > + /* don't need update */ > + bpf_prog_put(new_prog); > + goto out; > + } > + > + old_prog = xchg(&link->prog, new_prog); > + bpf_prog_put(old_prog); > +out: > + mutex_unlock(&bpf_nf_mutex); > + return err; > } > > static const struct bpf_link_ops bpf_nf_link_lops = { > @@ -226,7 +255,11 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog) > > link->hook_ops.hook = nf_hook_run_bpf; > link->hook_ops.hook_ops_type = NF_HOOK_OP_BPF; > - link->hook_ops.priv = prog; > + > + /* bpf_nf_link_release & bpf_nf_link_dealloc() can ensures that link remains > + * valid at all times within nf_hook_run_bpf(). > + */ > + link->hook_ops.priv = link; > > link->hook_ops.pf = attr->link_create.netfilter.pf; > link->hook_ops.priority = attr->link_create.netfilter.priority; > -- > 1.8.3.1 >
On 12/21/23 5:11 AM, Alexei Starovoitov wrote: > On Wed, Dec 20, 2023 at 6:09 AM D. Wythe <alibuda@linux.alibaba.com> wrote: >> From: "D. Wythe" <alibuda@linux.alibaba.com> >> >> To support the prog update, we need to ensure that the prog seen >> within the hook is always valid. Considering that hooks are always >> protected by rcu_read_lock(), which provide us the ability to >> access the prog under rcu. >> >> Signed-off-by: D. Wythe <alibuda@linux.alibaba.com> >> --- >> net/netfilter/nf_bpf_link.c | 63 ++++++++++++++++++++++++++++++++++----------- >> 1 file changed, 48 insertions(+), 15 deletions(-) >> >> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c >> index e502ec0..9bc91d1 100644 >> --- a/net/netfilter/nf_bpf_link.c >> +++ b/net/netfilter/nf_bpf_link.c >> @@ -8,17 +8,8 @@ >> #include <net/netfilter/nf_bpf_link.h> >> #include <uapi/linux/netfilter_ipv4.h> >> >> -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb, >> - const struct nf_hook_state *s) >> -{ >> - const struct bpf_prog *prog = bpf_prog; >> - struct bpf_nf_ctx ctx = { >> - .state = s, >> - .skb = skb, >> - }; >> - >> - return bpf_prog_run(prog, &ctx); >> -} >> +/* protect link update in parallel */ >> +static DEFINE_MUTEX(bpf_nf_mutex); >> >> struct bpf_nf_link { >> struct bpf_link link; >> @@ -26,8 +17,20 @@ struct bpf_nf_link { >> struct net *net; >> u32 dead; >> const struct nf_defrag_hook *defrag_hook; >> + struct rcu_head head; > I have to point out the same issues as before, but > will ask them differently... > > Why do you think above rcu_head is necessary? > >> }; >> >> +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb, >> + const struct nf_hook_state *s) >> +{ >> + const struct bpf_nf_link *nf_link = bpf_link; >> + struct bpf_nf_ctx ctx = { >> + .state = s, >> + .skb = skb, >> + }; >> + return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx); >> +} >> + >> #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6) >> static const struct nf_defrag_hook * >> get_proto_defrag_hook(struct bpf_nf_link *link, >> @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link) >> static void bpf_nf_link_dealloc(struct bpf_link *link) >> { >> struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); >> - >> - kfree(nf_link); >> + kfree_rcu(nf_link, head); > Why is this needed ? > Have you looked at tcx_link_lops ? Introducing rcu_head/kfree_rcu is to address the situation where the netfilter hooks might still access the link after bpf_nf_link_dealloc. nf_hook_run_bpf const struct bpf_nf_link *nf_link = bpf_link; bpf_nf_link_release nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops); bpf_nf_link_dealloc free(link) bpf_prog_run(link->prog); I had checked the tcx_link_lops ,it's seems it use the synchronize_rcu() to solve the same problem, which is also the way we used in the first version. https://lore.kernel.org/bpf/1702467945-38866-1-git-send-email-alibuda@linux.alibaba.com/ However, we have received some opposing views, believing that this is a bit overkill, so we decided to use kfree_rcu. https://lore.kernel.org/bpf/20231213222415.GA13818@breakpoint.cc/ >> } >> >> static int bpf_nf_link_detach(struct bpf_link *link) >> @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link, >> static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog, >> struct bpf_prog *old_prog) >> { >> - return -EOPNOTSUPP; >> + struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); >> + int err = 0; >> + >> + mutex_lock(&bpf_nf_mutex); > Why do you need this mutex? > What race does it solve? To avoid user update a link with differ prog at the same time. I noticed that sys_bpf() doesn't seem to prevent being invoked by user at the same time. Have I missed something? Best wishes, D. Wythe >> + >> + if (nf_link->dead) { >> + err = -EPERM; >> + goto out; >> + } >> + >> + /* target old_prog mismatch */ >> + if (old_prog && link->prog != old_prog) { >> + err = -EPERM; >> + goto out; >> + } >> + >> + old_prog = link->prog; >> + if (old_prog == new_prog) { >> + /* don't need update */ >> + bpf_prog_put(new_prog); >> + goto out; >> + } >> + >> + old_prog = xchg(&link->prog, new_prog); >> + bpf_prog_put(old_prog); >> +out: >> + mutex_unlock(&bpf_nf_mutex); >> + return err; >> } >> >> static const struct bpf_link_ops bpf_nf_link_lops = { >> @@ -226,7 +255,11 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog) >> >> link->hook_ops.hook = nf_hook_run_bpf; >> link->hook_ops.hook_ops_type = NF_HOOK_OP_BPF; >> - link->hook_ops.priv = prog; >> + >> + /* bpf_nf_link_release & bpf_nf_link_dealloc() can ensures that link remains >> + * valid at all times within nf_hook_run_bpf(). >> + */ >> + link->hook_ops.priv = link; >> >> link->hook_ops.pf = attr->link_create.netfilter.pf; >> link->hook_ops.priority = attr->link_create.netfilter.priority; >> -- >> 1.8.3.1 >>
On Thu, Dec 21, 2023 at 11:06 PM D. Wythe <alibuda@linux.alibaba.com> wrote: > > > > On 12/21/23 5:11 AM, Alexei Starovoitov wrote: > > On Wed, Dec 20, 2023 at 6:09 AM D. Wythe <alibuda@linux.alibaba.com> wrote: > >> From: "D. Wythe" <alibuda@linux.alibaba.com> > >> > >> To support the prog update, we need to ensure that the prog seen > >> within the hook is always valid. Considering that hooks are always > >> protected by rcu_read_lock(), which provide us the ability to > >> access the prog under rcu. > >> > >> Signed-off-by: D. Wythe <alibuda@linux.alibaba.com> > >> --- > >> net/netfilter/nf_bpf_link.c | 63 ++++++++++++++++++++++++++++++++++----------- > >> 1 file changed, 48 insertions(+), 15 deletions(-) > >> > >> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c > >> index e502ec0..9bc91d1 100644 > >> --- a/net/netfilter/nf_bpf_link.c > >> +++ b/net/netfilter/nf_bpf_link.c > >> @@ -8,17 +8,8 @@ > >> #include <net/netfilter/nf_bpf_link.h> > >> #include <uapi/linux/netfilter_ipv4.h> > >> > >> -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb, > >> - const struct nf_hook_state *s) > >> -{ > >> - const struct bpf_prog *prog = bpf_prog; > >> - struct bpf_nf_ctx ctx = { > >> - .state = s, > >> - .skb = skb, > >> - }; > >> - > >> - return bpf_prog_run(prog, &ctx); > >> -} > >> +/* protect link update in parallel */ > >> +static DEFINE_MUTEX(bpf_nf_mutex); > >> > >> struct bpf_nf_link { > >> struct bpf_link link; > >> @@ -26,8 +17,20 @@ struct bpf_nf_link { > >> struct net *net; > >> u32 dead; > >> const struct nf_defrag_hook *defrag_hook; > >> + struct rcu_head head; > > I have to point out the same issues as before, but > > will ask them differently... > > > > Why do you think above rcu_head is necessary? > > > >> }; > >> > >> +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb, > >> + const struct nf_hook_state *s) > >> +{ > >> + const struct bpf_nf_link *nf_link = bpf_link; > >> + struct bpf_nf_ctx ctx = { > >> + .state = s, > >> + .skb = skb, > >> + }; > >> + return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx); > >> +} > >> + > >> #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6) > >> static const struct nf_defrag_hook * > >> get_proto_defrag_hook(struct bpf_nf_link *link, > >> @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link) > >> static void bpf_nf_link_dealloc(struct bpf_link *link) > >> { > >> struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); > >> - > >> - kfree(nf_link); > >> + kfree_rcu(nf_link, head); > > Why is this needed ? > > Have you looked at tcx_link_lops ? > > Introducing rcu_head/kfree_rcu is to address the situation where the > netfilter hooks might > still access the link after bpf_nf_link_dealloc. Why do you think so? > > nf_hook_run_bpf > const struct > bpf_nf_link *nf_link = bpf_link; > > bpf_nf_link_release > nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops); > > bpf_nf_link_dealloc > free(link) > bpf_prog_run(link->prog); > > > I had checked the tcx_link_lops ,it's seems it use the synchronize_rcu() > to solve the Where do you see such code in tcx_link_lops ? > same problem, which is also the way we used in the first version. > > https://lore.kernel.org/bpf/1702467945-38866-1-git-send-email-alibuda@linux.alibaba.com/ > > However, we have received some opposing views, believing that this is a > bit overkill, > so we decided to use kfree_rcu. > > https://lore.kernel.org/bpf/20231213222415.GA13818@breakpoint.cc/ > > >> } > >> > >> static int bpf_nf_link_detach(struct bpf_link *link) > >> @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link, > >> static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog, > >> struct bpf_prog *old_prog) > >> { > >> - return -EOPNOTSUPP; > >> + struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); > >> + int err = 0; > >> + > >> + mutex_lock(&bpf_nf_mutex); > > Why do you need this mutex? > > What race does it solve? > > To avoid user update a link with differ prog at the same time. I noticed > that sys_bpf() > doesn't seem to prevent being invoked by user at the same time. Have I > missed something? You're correct that sys_bpf() doesn't lock anything. But what are you serializing in this bpf_nf_link_update() ? What will happen if multiple bpf_nf_link_update() without mutex run on different CPUs in parallel ?
On 12/23/23 6:23 AM, Alexei Starovoitov wrote: > On Thu, Dec 21, 2023 at 11:06 PM D. Wythe <alibuda@linux.alibaba.com> wrote: >> >> >> On 12/21/23 5:11 AM, Alexei Starovoitov wrote: >>> On Wed, Dec 20, 2023 at 6:09 AM D. Wythe <alibuda@linux.alibaba.com> wrote: >>>> From: "D. Wythe" <alibuda@linux.alibaba.com> >>>> >>>> To support the prog update, we need to ensure that the prog seen >>>> within the hook is always valid. Considering that hooks are always >>>> protected by rcu_read_lock(), which provide us the ability to >>>> access the prog under rcu. >>>> >>>> Signed-off-by: D. Wythe <alibuda@linux.alibaba.com> >>>> --- >>>> net/netfilter/nf_bpf_link.c | 63 ++++++++++++++++++++++++++++++++++----------- >>>> 1 file changed, 48 insertions(+), 15 deletions(-) >>>> >>>> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c >>>> index e502ec0..9bc91d1 100644 >>>> --- a/net/netfilter/nf_bpf_link.c >>>> +++ b/net/netfilter/nf_bpf_link.c >>>> @@ -8,17 +8,8 @@ >>>> #include <net/netfilter/nf_bpf_link.h> >>>> #include <uapi/linux/netfilter_ipv4.h> >>>> >>>> -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb, >>>> - const struct nf_hook_state *s) >>>> -{ >>>> - const struct bpf_prog *prog = bpf_prog; >>>> - struct bpf_nf_ctx ctx = { >>>> - .state = s, >>>> - .skb = skb, >>>> - }; >>>> - >>>> - return bpf_prog_run(prog, &ctx); >>>> -} >>>> +/* protect link update in parallel */ >>>> +static DEFINE_MUTEX(bpf_nf_mutex); >>>> >>>> struct bpf_nf_link { >>>> struct bpf_link link; >>>> @@ -26,8 +17,20 @@ struct bpf_nf_link { >>>> struct net *net; >>>> u32 dead; >>>> const struct nf_defrag_hook *defrag_hook; >>>> + struct rcu_head head; >>> I have to point out the same issues as before, but >>> will ask them differently... >>> >>> Why do you think above rcu_head is necessary? >>> >>>> }; >>>> >>>> +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb, >>>> + const struct nf_hook_state *s) >>>> +{ >>>> + const struct bpf_nf_link *nf_link = bpf_link; >>>> + struct bpf_nf_ctx ctx = { >>>> + .state = s, >>>> + .skb = skb, >>>> + }; >>>> + return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx); >>>> +} >>>> + >>>> #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6) >>>> static const struct nf_defrag_hook * >>>> get_proto_defrag_hook(struct bpf_nf_link *link, >>>> @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link) >>>> static void bpf_nf_link_dealloc(struct bpf_link *link) >>>> { >>>> struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); >>>> - >>>> - kfree(nf_link); >>>> + kfree_rcu(nf_link, head); >>> Why is this needed ? >>> Have you looked at tcx_link_lops ? >> Introducing rcu_head/kfree_rcu is to address the situation where the >> netfilter hooks might >> still access the link after bpf_nf_link_dealloc. > Why do you think so? > Hi Alexei, IMMO, nf_unregister_net_hook does not wait for the completion of the execution of the hook that is being removed, instead, it allocates a new array without the very hook to replace the old arrayvia rcu_assign_pointer() (in __nf_hook_entries_try_shrink), then it use call_rcu() to release the old one. You can find more details in commit 8c873e2199700c2de7dbd5eedb9d90d5f109462b. In other words, when nf_unregister_net_hook returns, there may still be contexts executing hooks on the old array, which means that the `link` may still be accessed after nf_unregister_net_hook returns. And that's the reason why we use kfree_rcu() to release the `link`. >> nf_hook_run_bpf >> const struct >> bpf_nf_link *nf_link = bpf_link; >> >> bpf_nf_link_release >> nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops); >> >> bpf_nf_link_dealloc >> free(link) >> bpf_prog_run(link->prog); >> >> >> I had checked the tcx_link_lops ,it's seems it use the synchronize_rcu() >> to solve the > Where do you see such code in tcx_link_lops ? I'm not certain if the reason that it choose to use synchronize_rcu()is the same as mine, but I did see it here: tcx_link_release() -> tcx_entry_sync() static inline void tcx_entry_sync(void) { /* bpf_mprog_entry got a/b swapped, therefore ensure that * there are no inflight users on the old one anymore. */ synchronize_rcu(); } >> same problem, which is also the way we used in the first version. >> >> https://lore.kernel.org/bpf/1702467945-38866-1-git-send-email-alibuda@linux.alibaba.com/ >> >> However, we have received some opposing views, believing that this is a >> bit overkill, >> so we decided to use kfree_rcu. >> >> https://lore.kernel.org/bpf/20231213222415.GA13818@breakpoint.cc/ >> >>>> } >>>> >>>> static int bpf_nf_link_detach(struct bpf_link *link) >>>> @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link, >>>> static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog, >>>> struct bpf_prog *old_prog) >>>> { >>>> - return -EOPNOTSUPP; >>>> + struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); >>>> + int err = 0; >>>> + >>>> + mutex_lock(&bpf_nf_mutex); >>> Why do you need this mutex? >>> What race does it solve? >> To avoid user update a link with differ prog at the same time. I noticed >> that sys_bpf() >> doesn't seem to prevent being invoked by user at the same time. Have I >> missed something? > You're correct that sys_bpf() doesn't lock anything. > But what are you serializing in this bpf_nf_link_update() ? > What will happen if multiple bpf_nf_link_update() > without mutex run on different CPUs in parallel ? I must admit that it is indeed feasible if we eliminate the mutex and use cmpxchg to swap the prog (we need to ensure that there is only one bpf_prog_put() on the old prog). However, when cmpxchg fails, it means that this context has not outcompeted the other one, and we have to return a failure. Maybe something like this: if (!cmpxchg(&link->prog, old_prog, new_prog)) { /* already replaced by another link_update */ return -xxx; } As a comparison, The version with the mutex wouldn't encounter this error, every update would succeed. I think that it's too harsh for the user to receive a failure in that case since they haven't done anything wrong. Best wishes, D. Wythe
On Wed, Dec 27, 2023 at 12:20 AM D. Wythe <alibuda@linux.alibaba.com> wrote: > > > Hi Alexei, > > > IMMO, nf_unregister_net_hook does not wait for the completion of the > execution of the hook that is being removed, > instead, it allocates a new array without the very hook to replace the > old arrayvia rcu_assign_pointer() (in __nf_hook_entries_try_shrink), > then it use call_rcu() to release the old one. > > You can find more details in commit > 8c873e2199700c2de7dbd5eedb9d90d5f109462b. > > In other words, when nf_unregister_net_hook returns, there may still be > contexts executing hooks on the > old array, which means that the `link` may still be accessed after > nf_unregister_net_hook returns. > > And that's the reason why we use kfree_rcu() to release the `link`. > >> nf_hook_run_bpf > >> const struct > >> bpf_nf_link *nf_link = bpf_link; > >> > >> bpf_nf_link_release > >> nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops); > >> > >> bpf_nf_link_dealloc > >> free(link) > >> bpf_prog_run(link->prog); Got it. Sounds like it's an existing bug. If so it should be an independent patch with Fixes tag. Also please craft a test case to demonstrate UAF. > > I must admit that it is indeed feasible if we eliminate the mutex and > use cmpxchg to swap the prog (we need to ensure that there is only one > bpf_prog_put() on the old prog). > However, when cmpxchg fails, it means that this context has not > outcompeted the other one, and we have to return a failure. Maybe > something like this: > > if (!cmpxchg(&link->prog, old_prog, new_prog)) { > /* already replaced by another link_update */ > return -xxx; > } > > As a comparison, The version with the mutex wouldn't encounter this > error, every update would succeed. I think that it's too harsh for the > user to receive a failure > in that case since they haven't done anything wrong. Disagree. The mutex doesn't prevent this issue. There is always a race. It happens when link_update.old_prog_fd and BPF_F_REPLACE were specified. One user space passes an FD of the old prog and another user space doing the same. They both race and one of them gets if (old_prog && link->prog != old_prog) { err = -EPERM; it's no different with dropping the mutex and doing: if (old_prog) { if (!cmpxchg(&link->prog, old_prog, new_prog)) -EPERM } else { old_prog = xchg(&link->prog, new_prog); }
On 12/28/23 3:00 AM, Alexei Starovoitov wrote: > On Wed, Dec 27, 2023 at 12:20 AM D. Wythe <alibuda@linux.alibaba.com> wrote: >> >> Hi Alexei, >> >> >> IMMO, nf_unregister_net_hook does not wait for the completion of the >> execution of the hook that is being removed, >> instead, it allocates a new array without the very hook to replace the >> old arrayvia rcu_assign_pointer() (in __nf_hook_entries_try_shrink), >> then it use call_rcu() to release the old one. >> >> You can find more details in commit >> 8c873e2199700c2de7dbd5eedb9d90d5f109462b. >> >> In other words, when nf_unregister_net_hook returns, there may still be >> contexts executing hooks on the >> old array, which means that the `link` may still be accessed after >> nf_unregister_net_hook returns. >> >> And that's the reason why we use kfree_rcu() to release the `link`. >>>> nf_hook_run_bpf >>>> const struct >>>> bpf_nf_link *nf_link = bpf_link; >>>> >>>> bpf_nf_link_release >>>> nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops); >>>> >>>> bpf_nf_link_dealloc >>>> free(link) >>>> bpf_prog_run(link->prog); > Got it. > Sounds like it's an existing bug. If so it should be an independent > patch with Fixes tag. > > Also please craft a test case to demonstrate UAF. > It is not an existing bug... Accessing the link within the hook was something I introduced here to support updates
diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c index e502ec0..9bc91d1 100644 --- a/net/netfilter/nf_bpf_link.c +++ b/net/netfilter/nf_bpf_link.c @@ -8,17 +8,8 @@ #include <net/netfilter/nf_bpf_link.h> #include <uapi/linux/netfilter_ipv4.h> -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb, - const struct nf_hook_state *s) -{ - const struct bpf_prog *prog = bpf_prog; - struct bpf_nf_ctx ctx = { - .state = s, - .skb = skb, - }; - - return bpf_prog_run(prog, &ctx); -} +/* protect link update in parallel */ +static DEFINE_MUTEX(bpf_nf_mutex); struct bpf_nf_link { struct bpf_link link; @@ -26,8 +17,20 @@ struct bpf_nf_link { struct net *net; u32 dead; const struct nf_defrag_hook *defrag_hook; + struct rcu_head head; }; +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb, + const struct nf_hook_state *s) +{ + const struct bpf_nf_link *nf_link = bpf_link; + struct bpf_nf_ctx ctx = { + .state = s, + .skb = skb, + }; + return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx); +} + #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6) static const struct nf_defrag_hook * get_proto_defrag_hook(struct bpf_nf_link *link, @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link) static void bpf_nf_link_dealloc(struct bpf_link *link) { struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); - - kfree(nf_link); + kfree_rcu(nf_link, head); } static int bpf_nf_link_detach(struct bpf_link *link) @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link, static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog, struct bpf_prog *old_prog) { - return -EOPNOTSUPP; + struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); + int err = 0; + + mutex_lock(&bpf_nf_mutex); + + if (nf_link->dead) { + err = -EPERM; + goto out; + } + + /* target old_prog mismatch */ + if (old_prog && link->prog != old_prog) { + err = -EPERM; + goto out; + } + + old_prog = link->prog; + if (old_prog == new_prog) { + /* don't need update */ + bpf_prog_put(new_prog); + goto out; + } + + old_prog = xchg(&link->prog, new_prog); + bpf_prog_put(old_prog); +out: + mutex_unlock(&bpf_nf_mutex); + return err; } static const struct bpf_link_ops bpf_nf_link_lops = { @@ -226,7 +255,11 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog) link->hook_ops.hook = nf_hook_run_bpf; link->hook_ops.hook_ops_type = NF_HOOK_OP_BPF; - link->hook_ops.priv = prog; + + /* bpf_nf_link_release & bpf_nf_link_dealloc() can ensures that link remains + * valid at all times within nf_hook_run_bpf(). + */ + link->hook_ops.priv = link; link->hook_ops.pf = attr->link_create.netfilter.pf; link->hook_ops.priority = attr->link_create.netfilter.priority;