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[3.8.y.z,extended,stable] Patch "random: fix accounting race condition with lockless irq entropy_count" has been added to staging queue

Message ID 1369946098-13321-1-git-send-email-kamal@canonical.com
State New
Headers show

Commit Message

Kamal Mostafa May 30, 2013, 8:34 p.m. UTC 
This is a note to let you know that I have just added a patch titled

    random: fix accounting race condition with lockless irq entropy_count

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

From f0d568d95f929d59bc91b5d3c38beddda81abd51 Mon Sep 17 00:00:00 2001
From: Jiri Kosina <jkosina@suse.cz>
Date: Fri, 24 May 2013 15:55:33 -0700
Subject: random: fix accounting race condition with lockless irq entropy_count
 update

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

--
1.8.1.2
diff mbox

Patch

diff --git a/drivers/char/random.c b/drivers/char/random.c
index ae872f8..b7dc45c 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -865,16 +865,24 @@  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)
 			wakeup_write = 1;
 	}