Commit 10b3a32d authored May 24, 2013 by Jiri Kosina Committed by Linus Torvalds May 24, 2013

random: fix accounting race condition with lockless irq entropy_count update



Commit 902c098a ("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 902c098a.

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>

parent 1e7e2e05

drivers/char/random.c

+16 −8

Original line number	Diff line number	Diff line
		@@ -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->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;
		}

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