linux-kernel - [PATCH 4.9 01/21] kernel/acct.c: fix the acct->needcheck check in check_free

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Message-Id: <20180108125920.176340344@linuxfoundation.org>
Date:   Mon,  8 Jan 2018 13:59:21 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, TSUKADA Koutaro <tsukada@...ade.co.jp>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.9 01/21] kernel/acct.c: fix the acct->needcheck check in check_free_space()

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@...hat.com>

commit 4d9570158b6260f449e317a5f9ed030c2504a615 upstream.

As Tsukada explains, the time_is_before_jiffies(acct->needcheck) check
is very wrong, we need time_is_after_jiffies() to make sys_acct() work.

Ignoring the overflows, the code should "goto out" if needcheck >
jiffies, while currently it checks "needcheck < jiffies" and thus in the
likely case check_free_space() does nothing until jiffies overflow.

In particular this means that sys_acct() is simply broken, acct_on()
sets acct->needcheck = jiffies and expects that check_free_space()
should set acct->active = 1 after the free-space check, but this won't
happen if jiffies increments in between.

This was broken by commit 32dc73086015 ("get rid of timer in
kern/acct.c") in 2011, then another (correct) commit 795a2f22a8ea
("acct() should honour the limits from the very beginning") made the
problem more visible.

Link: http://lkml.kernel.org/r/20171213133940.GA6554@redhat.com
Fixes: 32dc73086015 ("get rid of timer in kern/acct.c")
Reported-by: TSUKADA Koutaro <tsukada@...ade.co.jp>
Suggested-by: TSUKADA Koutaro <tsukada@...ade.co.jp>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/acct.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -99,7 +99,7 @@ static int check_free_space(struct bsd_a
 {
 	struct kstatfs sbuf;

-	if (time_is_before_jiffies(acct->needcheck))
+	if (time_is_after_jiffies(acct->needcheck))
 		goto out;

 	/* May block */