[016/141] random: fix accounting race condition with lockless irq entropy_count update

Steven Rostedt
2013-07-03T18:56:59Z

[-- Attachment #1: 0016-random-fix-accounting-race-condition-with-lockless-i.patch --]
[-- Type: text/plain, Size: 2702 bytes --]

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

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

From: Jiri Kosina <jkosina@suse.cz>

[ Upstream commit 10b3a32d292c21ea5b3ad5ca5975e88bb20b8d68 ]

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@suse.cz>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Greg KH <greg@kroah.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
drivers/char/random.c | 24 ++++++++++++++++--------
1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index b86eae9..35cfbb0 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -867,16 +867,24 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
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->entropy_count / 8 >= nbytes + reserved)
- r->entropy_count -= nbytes*8;
- else
- r->entropy_count = reserved;
+ if (r->limit && nbytes + reserved >= entropy_count / 8)
+ nbytes = entropy_count/8 - 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);
}
--
1.7.10.4