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 <jkos...@suse.cz>
Cc: "Theodore Ts'o" <ty...@mit.edu>
Signed-off-by: Greg Price <pr...@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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