drivers/char/random.c: fix a race which can lead to a bogus BUG()
Fix a bug reported by and diagnosed by Aaron Straus.
This is a regression intruduced into 2.6.26 by
commit adc782dae6c4c0f6fb679a48a544cfbcd79ae3dc
Author: Matt Mackall <mpm@selenic.com>
Date: Tue Apr 29 01:03:07 2008 -0700
random: simplify and rename credit_entropy_store
credit_entropy_bits() does:
spin_lock_irqsave(&r->lock, flags);
...
if (r->entropy_count > r->poolinfo->POOLBITS)
r->entropy_count = r->poolinfo->POOLBITS;
so there is a time window in which this BUG_ON():
static size_t account(struct entropy_store *r, size_t nbytes, int min,
int reserved)
{
unsigned long flags;
BUG_ON(r->entropy_count > r->poolinfo->POOLBITS);
/* Hold lock while accounting */
spin_lock_irqsave(&r->lock, flags);
can trigger.
We could fix this by moving the assertion inside the lock, but it seems
safer and saner to revert to the old behaviour wherein
entropy_store.entropy_count at no time exceeds
entropy_store.poolinfo->POOLBITS.
Reported-by: Aaron Straus <aaron@merfinllc.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: <stable@kernel.org> [2.6.26.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 1838aa3..7ce1ac4 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -407,7 +407,7 @@
/* read-write data: */
spinlock_t lock;
unsigned add_ptr;
- int entropy_count;
+ int entropy_count; /* Must at no time exceed ->POOLBITS! */
int input_rotate;
};
@@ -520,6 +520,7 @@
static void credit_entropy_bits(struct entropy_store *r, int nbits)
{
unsigned long flags;
+ int entropy_count;
if (!nbits)
return;
@@ -527,20 +528,20 @@
spin_lock_irqsave(&r->lock, flags);
DEBUG_ENT("added %d entropy credits to %s\n", nbits, r->name);
- r->entropy_count += nbits;
- if (r->entropy_count < 0) {
+ entropy_count = r->entropy_count;
+ entropy_count += nbits;
+ if (entropy_count < 0) {
DEBUG_ENT("negative entropy/overflow\n");
- r->entropy_count = 0;
- } else if (r->entropy_count > r->poolinfo->POOLBITS)
- r->entropy_count = r->poolinfo->POOLBITS;
+ entropy_count = 0;
+ } else if (entropy_count > r->poolinfo->POOLBITS)
+ entropy_count = r->poolinfo->POOLBITS;
+ r->entropy_count = entropy_count;
/* should we wake readers? */
- if (r == &input_pool &&
- r->entropy_count >= random_read_wakeup_thresh) {
+ if (r == &input_pool && entropy_count >= random_read_wakeup_thresh) {
wake_up_interruptible(&random_read_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
}
-
spin_unlock_irqrestore(&r->lock, flags);
}