+ random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update.patch added to -mm tree

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The patch titled
     Subject: random: fix accounting race condition with lockless irq entropy_count update
has been added to the -mm tree.  Its filename is
     random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update.patch

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------------------------------------------------------
From: Jiri Kosina <jkosina@xxxxxxx>
Subject: random: fix accounting race condition with lockless irq entropy_count update

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>
Cc: Greg KH <greg@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 drivers/char/random.c |   22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff -puN drivers/char/random.c~random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update drivers/char/random.c
--- a/drivers/char/random.c~random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update
+++ a/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 -mm which might be from jkosina@xxxxxxx are

linux-next.patch
binfmt_elf-pie-make-pf_randomize-check-comment-more-accurate.patch
i2o-check-copy_from_user-size-parameter.patch
mwave-fix-info-leak-in-mwave_ioctl.patch
random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update.patch
semaphore-give-an-unlikely-for-downs-timeout.patch
semaphore-boolize-semaphore_waiters-up.patch

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