From 8b76f46a2db29407fed66cf4aca19d61b3dcb3e1 Mon Sep 17 00:00:00 2001 From: Andrew Morton Date: Tue, 2 Sep 2008 14:36:14 -0700 Subject: [PATCH] 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 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 Cc: Matt Mackall Cc: Theodore Ts'o Cc: [2.6.26.x] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/char/random.c | 19 ++++++++++--------- 1 file changed, 10 insertions(+), 9 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 1838aa3d24fe..7ce1ac4baa6d 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -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); } -- 2.30.2