[PATCH 1/2] random: avoid init'ing twice in credit race

[PATCH 1/2] random: avoid init'ing twice in credit race

[Jason A. Donenfeld]

Since all changes of crng_init now go through credit_init_bits(), we can
fix a long standing race in which two concurrent callers of
credit_init_bits() have the new bit count >= some threshold, but are
doing so with crng_init as a lower threshold, checked outside of a lock,
resulting in crng_reseed() or similar being called twice.

In order to fix this, we can use the original cmpxchg value of the bit
count, and only change crng_init when the bit count transitions from
below a threshold to meeting the threshold.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a1a76487ea35..79409cf27a25 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -823,7 +823,7 @@ static void extract_entropy(void *buf, size_t nbytes)
 
 static void credit_init_bits(size_t nbits)
 {
-	unsigned int init_bits, orig, add;
+	unsigned int new, orig, add;
 	unsigned long flags;
 
 	if (crng_ready() || !nbits)
@@ -833,12 +833,12 @@ static void credit_init_bits(size_t nbits)
 
 	do {
 		orig = READ_ONCE(input_pool.init_bits);
-		init_bits = min_t(unsigned int, POOL_BITS, orig + add);
-	} while (cmpxchg(&input_pool.init_bits, orig, init_bits) != orig);
+		new = min_t(unsigned int, POOL_BITS, orig + add);
+	} while (cmpxchg(&input_pool.init_bits, orig, new) != orig);
 
-	if (!crng_ready() && init_bits >= POOL_READY_BITS)
+	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS)
 		crng_reseed();
-	else if (unlikely(crng_init == CRNG_EMPTY && init_bits >= POOL_EARLY_BITS)) {
+	else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
 		spin_lock_irqsave(&base_crng.lock, flags);
 		if (crng_init == CRNG_EMPTY) {
 			extract_entropy(base_crng.key, sizeof(base_crng.key));
-- 
2.35.1

[PATCH 2/2] random: move initialization out of reseeding hot path

[Jason A. Donenfeld]

Initialization happens once -- by way of credit_init_bits() -- and then
it never happens again. Therefore, it doesn't need to be in
crng_reseed(), which is a hot path that is called multiple times. It
also doesn't make sense to have there, as initialization activity is
better associated with initialization routines.

After the prior commit, crng_reseed() now won't be called by multiple
concurrent callers, which means that we can safely move the
"finialize_init" logic into crng_init_bits() unconditionally.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 43 +++++++++++++++++++------------------------
 1 file changed, 19 insertions(+), 24 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 79409cf27a25..1598bb40376e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -266,7 +266,6 @@ static void crng_reseed(void)
 	unsigned long flags;
 	unsigned long next_gen;
 	u8 key[CHACHA_KEY_SIZE];
-	bool finalize_init = false;
 
 	extract_entropy(key, sizeof(key));
 
@@ -283,28 +282,9 @@ static void crng_reseed(void)
 		++next_gen;
 	WRITE_ONCE(base_crng.generation, next_gen);
 	WRITE_ONCE(base_crng.birth, jiffies);
-	if (!crng_ready()) {
-		crng_init = CRNG_READY;
-		finalize_init = true;
-	}
+	crng_init = CRNG_READY;
 	spin_unlock_irqrestore(&base_crng.lock, flags);
 	memzero_explicit(key, sizeof(key));
-	if (finalize_init) {
-		process_random_ready_list();
-		wake_up_interruptible(&crng_init_wait);
-		kill_fasync(&fasync, SIGIO, POLL_IN);
-		pr_notice("crng init done\n");
-		if (unseeded_warning.missed) {
-			pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
-				  unseeded_warning.missed);
-			unseeded_warning.missed = 0;
-		}
-		if (urandom_warning.missed) {
-			pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
-				  urandom_warning.missed);
-			urandom_warning.missed = 0;
-		}
-	}
 }
 
 /*
@@ -836,10 +816,25 @@ static void credit_init_bits(size_t nbits)
 		new = min_t(unsigned int, POOL_BITS, orig + add);
 	} while (cmpxchg(&input_pool.init_bits, orig, new) != orig);
 
-	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS)
-		crng_reseed();
-	else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
+	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
+		crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
+		process_random_ready_list();
+		wake_up_interruptible(&crng_init_wait);
+		kill_fasync(&fasync, SIGIO, POLL_IN);
+		pr_notice("crng init done\n");
+		if (unseeded_warning.missed) {
+			pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
+				  unseeded_warning.missed);
+			unseeded_warning.missed = 0;
+		}
+		if (urandom_warning.missed) {
+			pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
+				  urandom_warning.missed);
+			urandom_warning.missed = 0;
+		}
+	} else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
 		spin_lock_irqsave(&base_crng.lock, flags);
+		/* Check if crng_init is CRNG_EMPTY, to avoid race with crng_reseed(). */
 		if (crng_init == CRNG_EMPTY) {
 			extract_entropy(base_crng.key, sizeof(base_crng.key));
 			crng_init = CRNG_EARLY;
-- 
2.35.1

Re: [PATCH 1/2] random: avoid init'ing twice in credit race

[Dominik Brodowski]

Am Mon, May 09, 2022 at 02:14:08PM +0200 schrieb Jason A. Donenfeld:
> Since all changes of crng_init now go through credit_init_bits(), we can
> fix a long standing race in which two concurrent callers of
> credit_init_bits() have the new bit count >= some threshold, but are
> doing so with crng_init as a lower threshold, checked outside of a lock,
> resulting in crng_reseed() or similar being called twice.

Sidenote: crng_reseed() did manage quite fine if called twice in short
order.

> In order to fix this, we can use the original cmpxchg value of the bit
> count, and only change crng_init when the bit count transitions from
> below a threshold to meeting the threshold.
> 
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

	Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>

Thanks,
	Dominik

Re: [PATCH 2/2] random: move initialization out of reseeding hot path

[Dominik Brodowski]

Am Mon, May 09, 2022 at 02:14:09PM +0200 schrieb Jason A. Donenfeld:
> Initialization happens once -- by way of credit_init_bits() -- and then
> it never happens again. Therefore, it doesn't need to be in
> crng_reseed(), which is a hot path that is called multiple times. It
> also doesn't make sense to have there, as initialization activity is
> better associated with initialization routines.
> 
> After the prior commit, crng_reseed() now won't be called by multiple
> concurrent callers, which means that we can safely move the
> "finialize_init" logic into crng_init_bits() unconditionally.
> 
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  drivers/char/random.c | 43 +++++++++++++++++++------------------------
>  1 file changed, 19 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 79409cf27a25..1598bb40376e 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -266,7 +266,6 @@ static void crng_reseed(void)
>  	unsigned long flags;
>  	unsigned long next_gen;
>  	u8 key[CHACHA_KEY_SIZE];
> -	bool finalize_init = false;
>  
>  	extract_entropy(key, sizeof(key));
>  
> @@ -283,28 +282,9 @@ static void crng_reseed(void)
>  		++next_gen;
>  	WRITE_ONCE(base_crng.generation, next_gen);
>  	WRITE_ONCE(base_crng.birth, jiffies);
> -	if (!crng_ready()) {
> -		crng_init = CRNG_READY;
> -		finalize_init = true;
> -	}
> +	crng_init = CRNG_READY;

Why unconditionally (you revert that bit in the static branch patch and make
it conditional again; so I see no reason for that here)?

Otherwise, looks good:

	Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>

Thanks,
	Dominik

Re: [PATCH 2/2] random: move initialization out of reseeding hot path

[Jason A. Donenfeld]

Hi Dominik,

On Fri, May 13, 2022 at 08:24:19AM +0200, Dominik Brodowski wrote:
> > -	if (!crng_ready()) {
> > -		crng_init = CRNG_READY;
> > -		finalize_init = true;
> > -	}
> > +	crng_init = CRNG_READY;
> 
> Why unconditionally

To avoid a useless branch.


> (you revert that bit in the static branch patch and make
> it conditional again; so I see no reason for that here)?

With the static branch patch, I can totally remove the branch and the
store all together, so we get the best of both worlds.

Jason

Re: [PATCH 1/2] random: avoid init'ing twice in credit race

[Jason A. Donenfeld]

Hi Dominik,

On Fri, May 13, 2022 at 08:23:40AM +0200, Dominik Brodowski wrote:
> Am Mon, May 09, 2022 at 02:14:08PM +0200 schrieb Jason A. Donenfeld:
> > Since all changes of crng_init now go through credit_init_bits(), we can
> > fix a long standing race in which two concurrent callers of
> > credit_init_bits() have the new bit count >= some threshold, but are
> > doing so with crng_init as a lower threshold, checked outside of a lock,
> > resulting in crng_reseed() or similar being called twice.
> 
> Sidenote: crng_reseed() did manage quite fine if called twice in short
> order.

With regards to crng_finialize, it did, but not with regards to
prematurely emptying patches and all that. IOW, buggy but not that bad.

> 
> > In order to fix this, we can use the original cmpxchg value of the bit
> > count, and only change crng_init when the bit count transitions from
> > below a threshold to meeting the threshold.
> > 
> > Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> 
> 	Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
> 
> Thanks,
> 	Dominik

Jason

RE: [PATCH 2/2] random: move initialization out of reseeding hot path

[David Laight]

From: Jason A. Donenfeld
> Sent: 13 May 2022 11:22
> 
> On Fri, May 13, 2022 at 08:24:19AM +0200, Dominik Brodowski wrote:
> > > -	if (!crng_ready()) {
> > > -		crng_init = CRNG_READY;
> > > -		finalize_init = true;
> > > -	}
> > > +	crng_init = CRNG_READY;
> >
> > Why unconditionally
> 
> To avoid a useless branch.

Are you now dirtying a cache line that would
otherwise be clean?

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH 2/2] random: move initialization out of reseeding hot path

[Jason A. Donenfeld]

Hi David,

On Fri, May 13, 2022 at 1:38 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Jason A. Donenfeld
> > Sent: 13 May 2022 11:22
> >
> > On Fri, May 13, 2022 at 08:24:19AM +0200, Dominik Brodowski wrote:
> > > > - if (!crng_ready()) {
> > > > -         crng_init = CRNG_READY;
> > > > -         finalize_init = true;
> > > > - }
> > > > + crng_init = CRNG_READY;
> > >
> > > Why unconditionally
> >
> > To avoid a useless branch.
>
> Are you now dirtying a cache line that would
> otherwise be clean?

Fair enough. I'll keep the branch.

Jason

[PATCH v2] random: move initialization out of reseeding hot path

[Jason A. Donenfeld]

Initialization happens once -- by way of credit_init_bits() -- and then
it never happens again. Therefore, it doesn't need to be in
crng_reseed(), which is a hot path that is called multiple times. It
also doesn't make sense to have there, as initialization activity is
better associated with initialization routines.

After the prior commit, crng_reseed() now won't be called by multiple
concurrent callers, which means that we can safely move the
"finialize_init" logic into crng_init_bits() unconditionally.

Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Changes v1->v2:
- Keep crng_init = CRNG_READY conditional to avoid dirtying cache line.

 drivers/char/random.c | 42 +++++++++++++++++++-----------------------
 1 file changed, 19 insertions(+), 23 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 3646eac353b7..8cea19024936 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -265,7 +265,6 @@ static void crng_reseed(void)
 	unsigned long flags;
 	unsigned long next_gen;
 	u8 key[CHACHA_KEY_SIZE];
-	bool finalize_init = false;
 
 	extract_entropy(key, sizeof(key));
 
@@ -282,28 +281,10 @@ static void crng_reseed(void)
 		++next_gen;
 	WRITE_ONCE(base_crng.generation, next_gen);
 	WRITE_ONCE(base_crng.birth, jiffies);
-	if (!crng_ready()) {
+	if (!crng_ready())
 		crng_init = CRNG_READY;
-		finalize_init = true;
-	}
 	spin_unlock_irqrestore(&base_crng.lock, flags);
 	memzero_explicit(key, sizeof(key));
-	if (finalize_init) {
-		process_random_ready_list();
-		wake_up_interruptible(&crng_init_wait);
-		kill_fasync(&fasync, SIGIO, POLL_IN);
-		pr_notice("crng init done\n");
-		if (unseeded_warning.missed) {
-			pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
-				  unseeded_warning.missed);
-			unseeded_warning.missed = 0;
-		}
-		if (urandom_warning.missed) {
-			pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
-				  urandom_warning.missed);
-			urandom_warning.missed = 0;
-		}
-	}
 }
 
 /*
@@ -835,10 +816,25 @@ static void credit_init_bits(size_t nbits)
 		new = min_t(unsigned int, POOL_BITS, orig + add);
 	} while (cmpxchg(&input_pool.init_bits, orig, new) != orig);
 
-	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS)
-		crng_reseed();
-	else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
+	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
+		crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
+		process_random_ready_list();
+		wake_up_interruptible(&crng_init_wait);
+		kill_fasync(&fasync, SIGIO, POLL_IN);
+		pr_notice("crng init done\n");
+		if (unseeded_warning.missed) {
+			pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
+				  unseeded_warning.missed);
+			unseeded_warning.missed = 0;
+		}
+		if (urandom_warning.missed) {
+			pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
+				  urandom_warning.missed);
+			urandom_warning.missed = 0;
+		}
+	} else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
 		spin_lock_irqsave(&base_crng.lock, flags);
+		/* Check if crng_init is CRNG_EMPTY, to avoid race with crng_reseed(). */
 		if (crng_init == CRNG_EMPTY) {
 			extract_entropy(base_crng.key, sizeof(base_crng.key));
 			crng_init = CRNG_EARLY;
-- 
2.35.1