This is a note to let you know that I've just added the patch titled
random: fix accounting race condition with lockless irq entropy_count update
to the 3.9-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:
random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update.patch
and it can be found in the queue-3.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 10b3a32d292c21ea5b3ad5ca5975e88bb20b8d68 Mon Sep 17 00:00:00 2001
From: Jiri Kosina <jkosina@xxxxxxx>
Date: Fri, 24 May 2013 15:55:33 -0700
Subject: random: fix accounting race condition with lockless irq entropy_count update
From: Jiri Kosina <jkosina@xxxxxxx>
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@xxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/char/random.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -865,16 +865,24 @@ 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)
wakeup_write = 1;
}
Patches currently in stable-queue which might be from jkosina@xxxxxxx are
queue-3.9/random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update.patch
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