Messages in this thread Patch in this message | | | Date | Thu, 7 Nov 2013 18:59:32 -0500 | From | Greg Price <> | Subject | [PATCH 10/11] random: pull 'min' check in accounting to inside lockless update |
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Since 902c098a3 ("random: use lockless techniques in the interrupt path") we have protected entropy_count only with cmpxchg, not the lock. In 10b3a32d2 ("random: fix accounting race condition") we put a cmpxchg-retry loop around most of the logic in account(), but the enforcement of the 'min' parameter stayed outside.
Under concurrent accesses to /dev/urandom this can suffer a race that defeats our "catastrophic reseed" measures so that our reseeding isn't effective. If accesses to /dev/urandom and /dev/random are interleaved in the wrong pattern, we could go indefinitely long without a successful catastrophic reseed of /dev/urandom, even while consuming an indefinite amount of entropy in ineffective smaller reseeds.
NB that with or without this bug, if /dev/random is simply accessed constantly, we can go indefinitely long without any reseed of /dev/urandom. So the effect of the bug is not that big.
The race comes when two tasks are in account() on input_pool and access ->entropy_count in the following order:
task A task B if (r->entropy_count / 8 < min + reserved) ACCESS_ONCE(r->entropy_count) cmpxchg ACCESS_ONCE(r->entropy_count) cmpxchg
where B causes r->entropy_count / 8 to fall below A's min + reserved but above reserved. Task A will cheerfully take r->entropy_count / 8 - reserved bytes from the pool, even though this is less than min.
Move the "min" check inside the ACCESS_ONCE/cmpxchg loop to prevent the race.
Cc: Jiri Kosina <jkosina@suse.cz> Cc: "Theodore Ts'o" <tytso@mit.edu> Signed-off-by: Greg Price <price@mit.edu> --- drivers/char/random.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c index 1bf6bf8..87d3728 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -842,18 +842,17 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min, { unsigned long flags; int wakeup_write = 0; + int entropy_count, orig; BUG_ON(r->entropy_count > r->poolinfo->POOLBITS); DEBUG_ENT("trying to extract %zu bits from %s\n", nbytes * 8, r->name); - /* Can we pull enough? */ - if (r->entropy_count / 8 < min + reserved) { +retry: + entropy_count = orig = ACCESS_ONCE(r->entropy_count); + if (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 = min_t(size_t, nbytes, entropy_count/8 - reserved); -- 1.8.3.2
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