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[SRU,Focal,1/1] ipc: replace costly bailout check in sysvipc_find_ipc()

Message ID 20230217171349.55737-3-yuxuan.luo@canonical.com
State New
Headers show
Series CVE-2021-3669 | expand

Commit Message

Yuxuan Luo Feb. 17, 2023, 5:13 p.m. UTC 
From: Rafael Aquini <aquini@redhat.com>

sysvipc_find_ipc() was left with a costly way to check if the offset
position fed to it is bigger than the total number of IPC IDs in use.  So
much so that the time it takes to iterate over /proc/sysvipc/* files grows
exponentially for a custom benchmark that creates "N" SYSV shm segments
and then times the read of /proc/sysvipc/shm (milliseconds):

    12 msecs to read   1024 segs from /proc/sysvipc/shm
    18 msecs to read   2048 segs from /proc/sysvipc/shm
    65 msecs to read   4096 segs from /proc/sysvipc/shm
   325 msecs to read   8192 segs from /proc/sysvipc/shm
  1303 msecs to read  16384 segs from /proc/sysvipc/shm
  5182 msecs to read  32768 segs from /proc/sysvipc/shm

The root problem lies with the loop that computes the total amount of ids
in use to check if the "pos" feeded to sysvipc_find_ipc() grew bigger than
"ids->in_use".  That is a quite inneficient way to get to the maximum
index in the id lookup table, specially when that value is already
provided by struct ipc_ids.max_idx.

This patch follows up on the optimization introduced via commit
15df03c879836 ("sysvipc: make get_maxid O(1) again") and gets rid of the
aforementioned costly loop replacing it by a simpler checkpoint based on
ipc_get_maxidx() returned value, which allows for a smooth linear increase
in time complexity for the same custom benchmark:

     2 msecs to read   1024 segs from /proc/sysvipc/shm
     2 msecs to read   2048 segs from /proc/sysvipc/shm
     4 msecs to read   4096 segs from /proc/sysvipc/shm
     9 msecs to read   8192 segs from /proc/sysvipc/shm
    19 msecs to read  16384 segs from /proc/sysvipc/shm
    39 msecs to read  32768 segs from /proc/sysvipc/shm

Link: https://lkml.kernel.org/r/20210809203554.1562989-1-aquini@redhat.com
Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Waiman Long <llong@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 20401d1058f3f841f35a594ac2fc1293710e55b9)
CVE-2021-3669
Signed-off-by: Yuxuan Luo <yuxuan.luo@canonical.com>
---
 ipc/util.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)
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Patch

diff --git a/ipc/util.c b/ipc/util.c
index 09c3bd9f8e768..aaf28b14a030e 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -754,21 +754,13 @@  struct pid_namespace *ipc_seq_pid_ns(struct seq_file *s)
 static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
 					      loff_t *new_pos)
 {
-	struct kern_ipc_perm *ipc;
-	int total, id;
-
-	total = 0;
-	for (id = 0; id < pos && total < ids->in_use; id++) {
-		ipc = idr_find(&ids->ipcs_idr, id);
-		if (ipc != NULL)
-			total++;
-	}
+	struct kern_ipc_perm *ipc = NULL;
+	int max_idx = ipc_get_maxidx(ids);
 
-	ipc = NULL;
-	if (total >= ids->in_use)
+	if (max_idx == -1 || pos > max_idx)
 		goto out;
 
-	for (; pos < ipc_mni; pos++) {
+	for (; pos <= max_idx; pos++) {
 		ipc = idr_find(&ids->ipcs_idr, pos);
 		if (ipc != NULL) {
 			rcu_read_lock();