linux-kernel - [67/94] random: fix accounting race condition with lockless irq entropy

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Message-ID: <lsq.1369712993.140194207@decadent.org.uk>
Date:	Tue, 28 May 2013 04:49:53 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Greg KH" <greg@...ah.com>, "Jiri Kosina" <jkosina@...e.cz>,
	"Theodore Ts'o" <tytso@....edu>
Subject: [67/94] random: fix accounting race condition with lockless irq
 entropy_count update

3.2.46-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Jiri Kosina <jkosina@...e.cz>

commit 10b3a32d292c21ea5b3ad5ca5975e88bb20b8d68 upstream.

Commit 902c098a3663 ("random: use lockless techniques in the interrupt
path") turned IRQ path from being spinlock protected into lockless
cmpxchg-retry update.

That commit removed r->lock serialization between crediting entropy bits
from IRQ context and accounting when extracting entropy on userspace
read path, but didn't turn the r->entropy_count reads/updates in
account() to use cmpxchg as well.

It has been observed, that under certain circumstances this leads to
read() on /dev/urandom to return 0 (EOF), as r->entropy_count gets
corrupted and becomes negative, which in turn results in propagating 0
all the way from account() to the actual read() call.

Convert the accounting code to be the proper lockless counterpart of
what has been partially done by 902c098a3663.

Signed-off-by: Jiri Kosina <jkosina@...e.cz>
Cc: Theodore Ts'o <tytso@....edu>
Cc: Greg KH <greg@...ah.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/char/random.c |   24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -889,16 +889,24 @@ static size_t account(struct entropy_sto
 	if (r->entropy_count / 8 < min + reserved) {
 		nbytes = 0;
 	} else {
+		int entropy_count, orig;
+retry:
+		entropy_count = orig = ACCESS_ONCE(r->entropy_count);
 		/* If limited, never pull more than available */
-		if (r->limit && nbytes + reserved >= r->entropy_count / 8)
-			nbytes = r->entropy_count/8 - reserved;
+		if (r->limit && nbytes + reserved >= entropy_count / 8)
+			nbytes = entropy_count/8 - reserved;
 
-		if (r->entropy_count / 8 >= nbytes + reserved)
-			r->entropy_count -= nbytes*8;
-		else
-			r->entropy_count = reserved;
+		if (entropy_count / 8 >= nbytes + reserved) {
+			entropy_count -= nbytes*8;
+			if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
+				goto retry;
+		} else {
+			entropy_count = reserved;
+			if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
+				goto retry;
+		}
 
-		if (r->entropy_count < random_write_wakeup_thresh) {
+		if (entropy_count < random_write_wakeup_thresh) {
 			wake_up_interruptible(&random_write_wait);
 			kill_fasync(&fasync, SIGIO, POLL_OUT);
 		}

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