| Message ID | 20240409201047.1032217-1-mhal@rbox.co (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 47d8ac011fe1c9251070e1bd64cb10b48193ec51 |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [net,v2] af_unix: Fix garbage collector racing against connect() | expand |
From: Michal Luczaj <mhal@rbox.co> Date: Tue, 9 Apr 2024 22:09:39 +0200 > Garbage collector does not take into account the risk of embryo getting > enqueued during the garbage collection. If such embryo has a peer that > carries SCM_RIGHTS, two consecutive passes of scan_children() may see a > different set of children. Leading to an incorrectly elevated inflight > count, and then a dangling pointer within the gc_inflight_list. > > sockets are AF_UNIX/SOCK_STREAM > S is an unconnected socket > L is a listening in-flight socket bound to addr, not in fdtable > V's fd will be passed via sendmsg(), gets inflight count bumped > > connect(S, addr) sendmsg(S, [V]); close(V) __unix_gc() > ---------------- ------------------------- ----------- > > NS = unix_create1() > skb1 = sock_wmalloc(NS) > L = unix_find_other(addr) > unix_state_lock(L) > unix_peer(S) = NS > // V count=1 inflight=0 > > NS = unix_peer(S) > skb2 = sock_alloc() > skb_queue_tail(NS, skb2[V]) > > // V became in-flight > // V count=2 inflight=1 > > close(V) > > // V count=1 inflight=1 > // GC candidate condition met > > for u in gc_inflight_list: > if (total_refs == inflight_refs) > add u to gc_candidates > > // gc_candidates={L, V} > > for u in gc_candidates: > scan_children(u, dec_inflight) > > // embryo (skb1) was not > // reachable from L yet, so V's > // inflight remains unchanged > __skb_queue_tail(L, skb1) > unix_state_unlock(L) > for u in gc_candidates: > if (u.inflight) > scan_children(u, inc_inflight_move_tail) > > // V count=1 inflight=2 (!) > > If there is a GC-candidate listening socket, lock/unlock its state. This > makes GC wait until the end of any ongoing connect() to that socket. After > flipping the lock, a possibly SCM-laden embryo is already enqueued. And if > there is another embryo coming, it can not possibly carry SCM_RIGHTS. At > this point, unix_inflight() can not happen because unix_gc_lock is already > taken. Inflight graph remains unaffected. > > Fixes: 1fd05ba5a2f2 ("[AF_UNIX]: Rewrite garbage collector, fixes race.") > Signed-off-by: Michal Luczaj <mhal@rbox.co> Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com> Thanks! > --- > v2: > - Adhere to reverse xmas tree variable ordering > - Expand commit message > - Drop the reproducer > > v1: https://lore.kernel.org/netdev/20240408161336.612064-1-mhal@rbox.co/ > > net/unix/garbage.c | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/net/unix/garbage.c b/net/unix/garbage.c > index fa39b6265238..6433a414acf8 100644 > --- a/net/unix/garbage.c > +++ b/net/unix/garbage.c > @@ -274,11 +274,22 @@ static void __unix_gc(struct work_struct *work) > * receive queues. Other, non candidate sockets _can_ be > * added to queue, so we must make sure only to touch > * candidates. > + * > + * Embryos, though never candidates themselves, affect which > + * candidates are reachable by the garbage collector. Before > + * being added to a listener's queue, an embryo may already > + * receive data carrying SCM_RIGHTS, potentially making the > + * passed socket a candidate that is not yet reachable by the > + * collector. It becomes reachable once the embryo is > + * enqueued. Therefore, we must ensure that no SCM-laden > + * embryo appears in a (candidate) listener's queue between > + * consecutive scan_children() calls. > */ > list_for_each_entry_safe(u, next, &gc_inflight_list, link) { > + struct sock *sk = &u->sk; > long total_refs; > > - total_refs = file_count(u->sk.sk_socket->file); > + total_refs = file_count(sk->sk_socket->file); > > WARN_ON_ONCE(!u->inflight); > WARN_ON_ONCE(total_refs < u->inflight); > @@ -286,6 +297,11 @@ static void __unix_gc(struct work_struct *work) > list_move_tail(&u->link, &gc_candidates); > __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags); > __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags); > + > + if (sk->sk_state == TCP_LISTEN) { > + unix_state_lock(sk); > + unix_state_unlock(sk); > + } > } > } > > -- > 2.44.0 >
Hello: This patch was applied to netdev/net.git (main) by Paolo Abeni <pabeni@redhat.com>: On Tue, 9 Apr 2024 22:09:39 +0200 you wrote: > Garbage collector does not take into account the risk of embryo getting > enqueued during the garbage collection. If such embryo has a peer that > carries SCM_RIGHTS, two consecutive passes of scan_children() may see a > different set of children. Leading to an incorrectly elevated inflight > count, and then a dangling pointer within the gc_inflight_list. > > sockets are AF_UNIX/SOCK_STREAM > S is an unconnected socket > L is a listening in-flight socket bound to addr, not in fdtable > V's fd will be passed via sendmsg(), gets inflight count bumped > > [...] Here is the summary with links: - [net,v2] af_unix: Fix garbage collector racing against connect() https://git.kernel.org/netdev/net/c/47d8ac011fe1 You are awesome, thank you!
On Tue, 9 Apr 2024 16:26:59 -0700 Kuniyuki Iwashima wrote: > > Fixes: 1fd05ba5a2f2 ("[AF_UNIX]: Rewrite garbage collector, fixes race.") > > Signed-off-by: Michal Luczaj <mhal@rbox.co> > > Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com> Hi Kuniyuki! This problem goes away with your GC rework in net-next, right? I should keep the net-next code when merging?
From: Jakub Kicinski <kuba@kernel.org> Date: Thu, 11 Apr 2024 14:30:35 -0700 > On Tue, 9 Apr 2024 16:26:59 -0700 Kuniyuki Iwashima wrote: > > > Fixes: 1fd05ba5a2f2 ("[AF_UNIX]: Rewrite garbage collector, fixes race.") > > > Signed-off-by: Michal Luczaj <mhal@rbox.co> > > > > Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com> > > Hi Kuniyuki! This problem goes away with your GC rework in net-next, > right? I should keep the net-next code when merging? Hi Jakub! Yes, the issue doesn't exist in the new GC, so this change should be removed when merging net-next.
diff --git a/net/unix/garbage.c b/net/unix/garbage.c index fa39b6265238..6433a414acf8 100644 --- a/net/unix/garbage.c +++ b/net/unix/garbage.c @@ -274,11 +274,22 @@ static void __unix_gc(struct work_struct *work) * receive queues. Other, non candidate sockets _can_ be * added to queue, so we must make sure only to touch * candidates. + * + * Embryos, though never candidates themselves, affect which + * candidates are reachable by the garbage collector. Before + * being added to a listener's queue, an embryo may already + * receive data carrying SCM_RIGHTS, potentially making the + * passed socket a candidate that is not yet reachable by the + * collector. It becomes reachable once the embryo is + * enqueued. Therefore, we must ensure that no SCM-laden + * embryo appears in a (candidate) listener's queue between + * consecutive scan_children() calls. */ list_for_each_entry_safe(u, next, &gc_inflight_list, link) { + struct sock *sk = &u->sk; long total_refs; - total_refs = file_count(u->sk.sk_socket->file); + total_refs = file_count(sk->sk_socket->file); WARN_ON_ONCE(!u->inflight); WARN_ON_ONCE(total_refs < u->inflight); @@ -286,6 +297,11 @@ static void __unix_gc(struct work_struct *work) list_move_tail(&u->link, &gc_candidates); __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags); __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags); + + if (sk->sk_state == TCP_LISTEN) { + unix_state_lock(sk); + unix_state_unlock(sk); + } } }
Garbage collector does not take into account the risk of embryo getting enqueued during the garbage collection. If such embryo has a peer that carries SCM_RIGHTS, two consecutive passes of scan_children() may see a different set of children. Leading to an incorrectly elevated inflight count, and then a dangling pointer within the gc_inflight_list. sockets are AF_UNIX/SOCK_STREAM S is an unconnected socket L is a listening in-flight socket bound to addr, not in fdtable V's fd will be passed via sendmsg(), gets inflight count bumped connect(S, addr) sendmsg(S, [V]); close(V) __unix_gc() ---------------- ------------------------- ----------- NS = unix_create1() skb1 = sock_wmalloc(NS) L = unix_find_other(addr) unix_state_lock(L) unix_peer(S) = NS // V count=1 inflight=0 NS = unix_peer(S) skb2 = sock_alloc() skb_queue_tail(NS, skb2[V]) // V became in-flight // V count=2 inflight=1 close(V) // V count=1 inflight=1 // GC candidate condition met for u in gc_inflight_list: if (total_refs == inflight_refs) add u to gc_candidates // gc_candidates={L, V} for u in gc_candidates: scan_children(u, dec_inflight) // embryo (skb1) was not // reachable from L yet, so V's // inflight remains unchanged __skb_queue_tail(L, skb1) unix_state_unlock(L) for u in gc_candidates: if (u.inflight) scan_children(u, inc_inflight_move_tail) // V count=1 inflight=2 (!) If there is a GC-candidate listening socket, lock/unlock its state. This makes GC wait until the end of any ongoing connect() to that socket. After flipping the lock, a possibly SCM-laden embryo is already enqueued. And if there is another embryo coming, it can not possibly carry SCM_RIGHTS. At this point, unix_inflight() can not happen because unix_gc_lock is already taken. Inflight graph remains unaffected. Fixes: 1fd05ba5a2f2 ("[AF_UNIX]: Rewrite garbage collector, fixes race.") Signed-off-by: Michal Luczaj <mhal@rbox.co> --- v2: - Adhere to reverse xmas tree variable ordering - Expand commit message - Drop the reproducer v1: https://lore.kernel.org/netdev/20240408161336.612064-1-mhal@rbox.co/ net/unix/garbage.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)