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drivers/char/random.c: fix a race which can lead to a bogus BUG()

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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>
This commit is contained in:
Andrew Morton 2008-09-02 14:36:14 -07:00 committed by Linus Torvalds
parent 9d35935747
commit 8b76f46a2d
1 changed files with 10 additions and 9 deletions
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19
drivers/char/random.c
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@ -407,7 +407,7 @@ struct entropy_store {
/* 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 mix_pool_bytes(struct entropy_store *r, const void *in, int bytes)
static void credit_entropy_bits(struct entropy_store *r, int nbits)
{
unsigned long flags;
int entropy_count;
if (!nbits)
return;
@ -527,20 +528,20 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
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);
}