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net: unix: fix inflight counting bug in garbage collector

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Previously I assumed that the receive queues of candidates don't
change during the GC.  This is only half true, nothing can be received
from the queues (see comment in unix_gc()), but buffers could be added
through the other half of the socket pair, which may still have file
descriptors referring to it.

This can result in inc_inflight_move_tail() erronously increasing the
"inflight" counter for a unix socket for which dec_inflight() wasn't
previously called.  This in turn can trigger the "BUG_ON(total_refs <
inflight_refs)" in a later garbage collection run.

Fix this by only manipulating the "inflight" counter for sockets which
are candidates themselves.  Duplicating the file references in
unix_attach_fds() is also needed to prevent a socket becoming a
candidate for GC while the skb that contains it is not yet queued.

Reported-by: Andrea Bittau <a.bittau@cs.ucl.ac.uk>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Miklos Szeredi
2008-11-09 15:23:57 +01:00
committed by Linus Torvalds
parent 058e3739f6
commit 6209344f5a
3 changed files with 62 additions and 19 deletions
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49
net/unix/garbage.c
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@@ -186,8 +186,17 @@ static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
*/
struct sock *sk = unix_get_socket(*fp++);
if (sk) {
hit = true;
func(unix_sk(sk));
struct unix_sock *u = unix_sk(sk);
/*
* Ignore non-candidates, they could
* have been added to the queues after
* starting the garbage collection
*/
if (u->gc_candidate) {
hit = true;
func(u);
}
}
}
if (hit && hitlist != NULL) {
@@ -249,11 +258,11 @@ static void inc_inflight_move_tail(struct unix_sock *u)
{
atomic_long_inc(&u->inflight);
/*
* If this is still a candidate, move it to the end of the
* list, so that it's checked even if it was already passed
* over
* If this still might be part of a cycle, move it to the end
* of the list, so that it's checked even if it was already
* passed over
*/
if (u->gc_candidate)
if (u->gc_maybe_cycle)
list_move_tail(&u->link, &gc_candidates);
}
@@ -267,6 +276,7 @@ void unix_gc(void)
struct unix_sock *next;
struct sk_buff_head hitlist;
struct list_head cursor;
LIST_HEAD(not_cycle_list);
spin_lock(&unix_gc_lock);
@@ -282,10 +292,14 @@ void unix_gc(void)
*
* Holding unix_gc_lock will protect these candidates from
* being detached, and hence from gaining an external
* reference. This also means, that since there are no
* possible receivers, the receive queues of these sockets are
* static during the GC, even though the dequeue is done
* before the detach without atomicity guarantees.
* reference. Since there are no possible receivers, all
* buffers currently on the candidates' queues stay there
* during the garbage collection.
*
* We also know that no new candidate can be added onto the
* receive queues. Other, non candidate sockets _can_ be
* added to queue, so we must make sure only to touch
* candidates.
*/
list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
long total_refs;
@@ -299,6 +313,7 @@ void unix_gc(void)
if (total_refs == inflight_refs) {
list_move_tail(&u->link, &gc_candidates);
u->gc_candidate = 1;
u->gc_maybe_cycle = 1;
}
}
@@ -325,13 +340,23 @@ void unix_gc(void)
list_move(&cursor, &u->link);
if (atomic_long_read(&u->inflight) > 0) {
list_move_tail(&u->link, &gc_inflight_list);
u->gc_candidate = 0;
list_move_tail(&u->link, &not_cycle_list);
u->gc_maybe_cycle = 0;
scan_children(&u->sk, inc_inflight_move_tail, NULL);
}
}
list_del(&cursor);
/*
* not_cycle_list contains those sockets which do not make up a
* cycle. Restore these to the inflight list.
*/
while (!list_empty(&not_cycle_list)) {
u = list_entry(not_cycle_list.next, struct unix_sock, link);
u->gc_candidate = 0;
list_move_tail(&u->link, &gc_inflight_list);
}
/*
* Now gc_candidates contains only garbage. Restore original
* inflight counters for these as well, and remove the skbuffs