Patch "kernel/acct.c: fix the acct->needcheck check in check_free_space()" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

    kernel/acct.c: fix the acct->needcheck check in check_free_space()

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kernel-acct.c-fix-the-acct-needcheck-check-in-check_free_space.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Oleg Nesterov <[email protected]>
Date: Thu, 4 Jan 2018 16:17:49 -0800
Subject: kernel/acct.c: fix the acct->needcheck check in check_free_space()

From: Oleg Nesterov <[email protected]>

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.

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

@@ -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 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/kernel-acct.c-fix-the-acct-needcheck-check-in-check_free_space.patch