From: Amir Goldstein <amir73il@gmail.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
linux-unionfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: [PATCH]i[RESEND] kernel/acct.c: fix locking order when switching acct files
Date: Sat, 29 Sep 2018 08:51:33 +0300 [thread overview]
Message-ID: <20180929055133.28955-1-amir73il@gmail.com> (raw)
This looks like an old bug, pre-dating the "Fixes" commit, but the
"Fixes" commit did not handle it properly.
The bug recently surfaced as a lockdep possible deadlock warning
with commit d1d04ef8572b ("ovl: stack file ops").
When acct_on() replaces one acct file with another, it takes sb_writers
lock on new file sb and calls acct_pin_kill(old) before releasing the
sb_writers lock.
If new file is on the same fs as old file, acct_pin_kill(old) fail to
file_start_write_trylock() and skip writing the old file, because
sb_writers (of new) is already taken by acct_on().
If new file is not on same fs as old file, this ordering violation
creates an unneeded dependency between new sb_writers and old sb_writers,
which may later be reported as possible deadlock.
This could result in an actual deadlock if acct file is replaced from
an old file in overlayfs over "real fs" to a new file in "real fs".
acct_on() takes freeze protection on "real fs" and tries to write to
overlayfs file. overlayfs is not freeze protected so do_acct_process()
can carry on with __kernel_write() to overlayfs file, which would
try to take freeze protection on "real fs" and deadlock.
Reproducer of lockdep possible deadlock warning:
./run --ov -s # unionmount-testsuite
touch /mnt/x /upper/y
accton /mnt/x
accton /upper/y
======================================================
WARNING: possible circular locking dependency detected
4.19.0-rc1-xfstests #3424 Not tainted
------------------------------------------------------
accton/1390 is trying to acquire lock:
00000000e0585aa5 (&acct->lock#2){+.+.}, at: acct_pin_kill+0x1b/0x76
but task is already holding lock:
000000003692505a (sb_writers#6){.+.+}, at: mnt_want_write+0x1d/0x42
Reported-by: syzbot+695726bc473f9c36a4b6@syzkaller.appspotmail.com
Fixes: 59eda0e07f43 ("new fs_pin killing logics")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
Al,
Welcome back. It would be nice to get an ACK (or an Applied) on this
patch. Fixes label claims to fix your commit, but I believe the bug was
already there before your commit.
As you can see, I have a reproducer to demonstrate the manifestation of
the bug in the case of switching acct file from overlayfs to real fs.
This started to manifest with overlayfs stacked f_op.
I have made another claim which seems obvious from the code about
changing acct file on same fs, but did not bother to write a reproducer
for that case.
Thanks,
Amir.
kernel/acct.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/acct.c b/kernel/acct.c
index addf7732fb56..c09355a7ae46 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -251,8 +251,8 @@ static int acct_on(struct filename *pathname)
rcu_read_lock();
old = xchg(&ns->bacct, &acct->pin);
mutex_unlock(&acct->lock);
- pin_kill(old);
mnt_drop_write(mnt);
+ pin_kill(old);
mntput(mnt);
return 0;
}
--
2.17.1
next reply other threads:[~2018-09-29 12:18 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-29 5:51 Amir Goldstein [this message]
2018-10-17 19:39 ` [PATCH][RESEND] kernel/acct.c: fix locking order when switching acct files Amir Goldstein
2019-02-26 14:22 ` Amir Goldstein
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180929055133.28955-1-amir73il@gmail.com \
--to=amir73il@gmail.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-unionfs@vger.kernel.org \
--cc=miklos@szeredi.hu \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).