Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init

[Sebastian Andrzej Siewior]

On 2017-06-16 14:12:42 [+0200], Jason A. Donenfeld wrote:
> I actually figured that out myself after sending the initial email, so
> then I wrote a follow-up patch which I attached to this thread. You
> should have received it. Can you take a look?

replied to the patch.

Sebastian

Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init

[Jason A. Donenfeld]

On Fri, Jun 16, 2017 at 10:31 AM, Sebastian Andrzej Siewior
 wrote:
> am talking about. Again:

I actually figured that out myself after sending the initial email, so
then I wrote a follow-up patch which I attached to this thread. You
should have received it. Can you take a look?

https://lkml.org/lkml/2017/6/14/942

Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init

[Sebastian Andrzej Siewior]

On 2017-06-15 00:33:12 [+0200], Jason A. Donenfeld wrote:
> There's a potential race that I fixed in my v5 of that patch set, but
> Ted only took v4, and for whatever reason has been to busy to submit
> the additional patch I already posted showing the diff between v4
> Hopefully he actually gets around to it and sends this for the next
> rc. Here it is:
> 
> https://patchwork.kernel.org/patch/9774563/

So you replace "crng_init < 2" with use_lock instead. That is not what I
am talking about. Again:
add_interrupt_randomness()
->   crng_fast_load()   
spin_trylock_irqsave(_crng.lock, )
->invalidate_batched_entropy()  
write_lock_irqsave(_entropy_reset_lock, );

in that order while the code path
get_random_uXX()
read_lock_irqsave(_entropy_reset_lock, );
->   extract_crng()
->_extract_crng()spin_lock_irqsave(>lock, );

which allocates the same lock in opposite order.
That means
  T1T2
  crng_fast_load()  get_random_u64()
extract_crng()
  *dead lock*
invalidate_batched_entropy()
_extract_crng()

So T1 waits for batched_entropy_reset_lock holding primary_crng.lock and
T2 waits for primary_crng.lock holding batched_entropy_reset_lock.

Sebastian

Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init

[Jason A. Donenfeld]

There's a potential race that I fixed in my v5 of that patch set, but
Ted only took v4, and for whatever reason has been to busy to submit
the additional patch I already posted showing the diff between v4
Hopefully he actually gets around to it and sends this for the next
rc. Here it is:

https://patchwork.kernel.org/patch/9774563/

Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init

[Sebastian Andrzej Siewior]

On 2017-06-08 01:25:55 [+0200], Jason A. Donenfeld wrote:
> It's possible that get_random_{u32,u64} is used before the crng has
> initialized, in which case, its output might not be cryptographically
> secure. For this problem, directly, this patch set is introducing the
> *_wait variety of functions, but even with that, there's a subtle issue:
> what happens to our batched entropy that was generated before
> initialization. Prior to this commit, it'd stick around, supplying bad
> numbers. After this commit, we force the entropy to be re-extracted
> after each phase of the crng has initialized.
> 
> In order to avoid a race condition with the position counter, we
> introduce a simple rwlock for this invalidation. Since it's only during
> this awkward transition period, after things are all set up, we stop
> using it, so that it doesn't have an impact on performance.
> 
> This should probably be backported to 4.11.
> 
> (Also: adding my copyright to the top. With the patch series from
> January, this patch, and then the ones that come after, I think there's
> a relevant amount of code in here to add my name to the top.)
> 
> Signed-off-by: Jason A. Donenfeld 
> Cc: Greg Kroah-Hartman 

I don't understand why lockdep notices this possible deadlock only in
RT:

| the existing dependency chain (in reverse order) is:
|
| -> #1 (primary_crng.lock){+.+...}:
|lock_acquire+0xb5/0x2b0
|rt_spin_lock+0x46/0x50
|_extract_crng+0x39/0xa0
|extract_crng+0x3a/0x40
|get_random_u64+0x17a/0x200
|cache_random_seq_create+0x51/0x100
|init_cache_random_seq+0x35/0x90
|__kmem_cache_create+0xd3/0x560
|create_boot_cache+0x8c/0xb2
|create_kmalloc_cache+0x54/0x9f
|create_kmalloc_caches+0xe3/0xfd
|kmem_cache_init+0x14f/0x1f0
|start_kernel+0x1e7/0x3b3
|x86_64_start_reservations+0x2a/0x2c
|x86_64_start_kernel+0x13d/0x14c
|verify_cpu+0x0/0xfc
|
| -> #0 (batched_entropy_reset_lock){+.+...}:
|__lock_acquire+0x11b4/0x1320
|lock_acquire+0xb5/0x2b0
|rt_write_lock+0x26/0x40
|rt_write_lock_irqsave+0x9/0x10
|invalidate_batched_entropy+0x28/0xb0
|crng_fast_load+0xb5/0xe0
|add_interrupt_randomness+0x16c/0x1a0
|irq_thread+0x15c/0x1e0
|kthread+0x112/0x150
|ret_from_fork+0x31/0x40

We have the path add_interrupt_randomness() -> crng_fast_load() which
take
  primary_crng.lock -> batched_entropy_reset_lock
and we have get_random_uXX() -> extract_crng() which take the locks in
opposite order:
  batched_entropy_reset_lock -> crng->lock

however batched_entropy_reset_lock() is only taken if "crng_init < 2" so
it is possible RT hits this constraint more reliably.

> ---
>  drivers/char/random.c | 37 +
>  1 file changed, 37 insertions(+)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index a561f0c2f428..d35da1603e12 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1,6 +1,9 @@
>  /*
>   * random.c -- A strong random number generator
>   *
> + * Copyright (C) 2017 Jason A. Donenfeld . All
> + * Rights Reserved.
> + *
>   * Copyright Matt Mackall , 2003, 2004, 2005
>   *
>   * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999.  All
> @@ -762,6 +765,8 @@ static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
>  static struct crng_state **crng_node_pool __read_mostly;
>  #endif
>  
> +static void invalidate_batched_entropy(void);
> +
>  static void crng_initialize(struct crng_state *crng)
>  {
>   int i;
> @@ -799,6 +804,7 @@ static int crng_fast_load(const char *cp, size_t len)
>   cp++; crng_init_cnt++; len--;
>   }
>   if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
> + invalidate_batched_entropy();
>   crng_init = 1;
>   wake_up_interruptible(_init_wait);
>   pr_notice("random: fast init done\n");
> @@ -836,6 +842,7 @@ static void crng_reseed(struct crng_state *crng, struct 
> entropy_store *r)
>   memzero_explicit(, sizeof(buf));
>   crng->init_time = jiffies;
>   if (crng == _crng && crng_init < 2) {
> + invalidate_batched_entropy();
>   crng_init = 2;
>   process_random_ready_list();
>   wake_up_interruptible(_init_wait);
> @@ -2023,6 +2030,7 @@ struct batched_entropy {
>   };
>   unsigned int position;
>  };
> +static rwlock_t batched_entropy_reset_lock = 
> __RW_LOCK_UNLOCKED(batched_entropy_reset_lock);
>  
>  /*
>   * Get a random word for internal kernel use only. The quality of the random
> @@ -2033,6 +2041,8 @@ static DEFINE_PER_CPU(struct batched_entropy, 
> batched_entropy_u64);
>  u64 get_random_u64(void)
>  {
>   u64 ret;
> + const bool use_lock = READ_ONCE(crng_init) < 2;
> + unsigned long flags = 0;
>   struct 

[PATCH v5 01/13] random: invalidate batched entropy after crng init

[Jason A. Donenfeld]

It's possible that get_random_{u32,u64} is used before the crng has
initialized, in which case, its output might not be cryptographically
secure. For this problem, directly, this patch set is introducing the
*_wait variety of functions, but even with that, there's a subtle issue:
what happens to our batched entropy that was generated before
initialization. Prior to this commit, it'd stick around, supplying bad
numbers. After this commit, we force the entropy to be re-extracted
after each phase of the crng has initialized.

In order to avoid a race condition with the position counter, we
introduce a simple rwlock for this invalidation. Since it's only during
this awkward transition period, after things are all set up, we stop
using it, so that it doesn't have an impact on performance.

This should probably be backported to 4.11.

(Also: adding my copyright to the top. With the patch series from
January, this patch, and then the ones that come after, I think there's
a relevant amount of code in here to add my name to the top.)

Signed-off-by: Jason A. Donenfeld 
Cc: Greg Kroah-Hartman 
---
 drivers/char/random.c | 37 +
 1 file changed, 37 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a561f0c2f428..d35da1603e12 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1,6 +1,9 @@
 /*
  * random.c -- A strong random number generator
  *
+ * Copyright (C) 2017 Jason A. Donenfeld . All
+ * Rights Reserved.
+ *
  * Copyright Matt Mackall , 2003, 2004, 2005
  *
  * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999.  All
@@ -762,6 +765,8 @@ static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 static struct crng_state **crng_node_pool __read_mostly;
 #endif
 
+static void invalidate_batched_entropy(void);
+
 static void crng_initialize(struct crng_state *crng)
 {
int i;
@@ -799,6 +804,7 @@ static int crng_fast_load(const char *cp, size_t len)
cp++; crng_init_cnt++; len--;
}
if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
+   invalidate_batched_entropy();
crng_init = 1;
wake_up_interruptible(_init_wait);
pr_notice("random: fast init done\n");
@@ -836,6 +842,7 @@ static void crng_reseed(struct crng_state *crng, struct 
entropy_store *r)
memzero_explicit(, sizeof(buf));
crng->init_time = jiffies;
if (crng == _crng && crng_init < 2) {
+   invalidate_batched_entropy();
crng_init = 2;
process_random_ready_list();
wake_up_interruptible(_init_wait);
@@ -2023,6 +2030,7 @@ struct batched_entropy {
};
unsigned int position;
 };
+static rwlock_t batched_entropy_reset_lock = 
__RW_LOCK_UNLOCKED(batched_entropy_reset_lock);
 
 /*
  * Get a random word for internal kernel use only. The quality of the random
@@ -2033,6 +2041,8 @@ static DEFINE_PER_CPU(struct batched_entropy, 
batched_entropy_u64);
 u64 get_random_u64(void)
 {
u64 ret;
+   const bool use_lock = READ_ONCE(crng_init) < 2;
+   unsigned long flags = 0;
struct batched_entropy *batch;
 
 #if BITS_PER_LONG == 64
@@ -2045,11 +2055,15 @@ u64 get_random_u64(void)
 #endif
 
batch = _cpu_var(batched_entropy_u64);
+   if (use_lock)
+   read_lock_irqsave(_entropy_reset_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
extract_crng((u8 *)batch->entropy_u64);
batch->position = 0;
}
ret = batch->entropy_u64[batch->position++];
+   if (use_lock)
+   read_unlock_irqrestore(_entropy_reset_lock, flags);
put_cpu_var(batched_entropy_u64);
return ret;
 }
@@ -2059,22 +2073,45 @@ static DEFINE_PER_CPU(struct batched_entropy, 
batched_entropy_u32);
 u32 get_random_u32(void)
 {
u32 ret;
+   const bool use_lock = READ_ONCE(crng_init) < 2;
+   unsigned long flags = 0;
struct batched_entropy *batch;
 
if (arch_get_random_int())
return ret;
 
batch = _cpu_var(batched_entropy_u32);
+   if (use_lock)
+   read_lock_irqsave(_entropy_reset_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
extract_crng((u8 *)batch->entropy_u32);
batch->position = 0;
}
ret = batch->entropy_u32[batch->position++];
+   if (use_lock)
+   read_unlock_irqrestore(_entropy_reset_lock, flags);
put_cpu_var(batched_entropy_u32);
return ret;
 }
 EXPORT_SYMBOL(get_random_u32);
 
+/* It's important to invalidate all potential batched entropy that might
+ * be stored before the crng is initialized, which we can do lazily by
+ * simply resetting the counter to zero so that it's re-extracted on the
+ * next usage. */
+static void 

[PATCH v5 01/13] random: invalidate batched entropy after crng init

[Jason A. Donenfeld]

It's possible that get_random_{u32,u64} is used before the crng has
initialized, in which case, its output might not be cryptographically
secure. For this problem, directly, this patch set is introducing the
*_wait variety of functions, but even with that, there's a subtle issue:
what happens to our batched entropy that was generated before
initialization. Prior to this commit, it'd stick around, supplying bad
numbers. After this commit, we force the entropy to be re-extracted
after each phase of the crng has initialized.

In order to avoid a race condition with the position counter, we
introduce a simple rwlock for this invalidation. Since it's only during
this awkward transition period, after things are all set up, we stop
using it, so that it doesn't have an impact on performance.

This should probably be backported to 4.11.

(Also: adding my copyright to the top. With the patch series from
January, this patch, and then the ones that come after, I think there's
a relevant amount of code in here to add my name to the top.)

Signed-off-by: Jason A. Donenfeld 
Cc: Greg Kroah-Hartman 
---
 drivers/char/random.c | 37 +
 1 file changed, 37 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a561f0c2f428..d35da1603e12 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1,6 +1,9 @@
 /*
  * random.c -- A strong random number generator
  *
+ * Copyright (C) 2017 Jason A. Donenfeld . All
+ * Rights Reserved.
+ *
  * Copyright Matt Mackall , 2003, 2004, 2005
  *
  * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999.  All
@@ -762,6 +765,8 @@ static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 static struct crng_state **crng_node_pool __read_mostly;
 #endif
 
+static void invalidate_batched_entropy(void);
+
 static void crng_initialize(struct crng_state *crng)
 {
int i;
@@ -799,6 +804,7 @@ static int crng_fast_load(const char *cp, size_t len)
cp++; crng_init_cnt++; len--;
}
if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
+   invalidate_batched_entropy();
crng_init = 1;
wake_up_interruptible(_init_wait);
pr_notice("random: fast init done\n");
@@ -836,6 +842,7 @@ static void crng_reseed(struct crng_state *crng, struct 
entropy_store *r)
memzero_explicit(, sizeof(buf));
crng->init_time = jiffies;
if (crng == _crng && crng_init < 2) {
+   invalidate_batched_entropy();
crng_init = 2;
process_random_ready_list();
wake_up_interruptible(_init_wait);
@@ -2023,6 +2030,7 @@ struct batched_entropy {
};
unsigned int position;
 };
+static rwlock_t batched_entropy_reset_lock = 
__RW_LOCK_UNLOCKED(batched_entropy_reset_lock);
 
 /*
  * Get a random word for internal kernel use only. The quality of the random
@@ -2033,6 +2041,8 @@ static DEFINE_PER_CPU(struct batched_entropy, 
batched_entropy_u64);
 u64 get_random_u64(void)
 {
u64 ret;
+   const bool use_lock = READ_ONCE(crng_init) < 2;
+   unsigned long flags = 0;
struct batched_entropy *batch;
 
 #if BITS_PER_LONG == 64
@@ -2045,11 +2055,15 @@ u64 get_random_u64(void)
 #endif
 
batch = _cpu_var(batched_entropy_u64);
+   if (use_lock)
+   read_lock_irqsave(_entropy_reset_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
extract_crng((u8 *)batch->entropy_u64);
batch->position = 0;
}
ret = batch->entropy_u64[batch->position++];
+   if (use_lock)
+   read_unlock_irqrestore(_entropy_reset_lock, flags);
put_cpu_var(batched_entropy_u64);
return ret;
 }
@@ -2059,22 +2073,45 @@ static DEFINE_PER_CPU(struct batched_entropy, 
batched_entropy_u32);
 u32 get_random_u32(void)
 {
u32 ret;
+   const bool use_lock = READ_ONCE(crng_init) < 2;
+   unsigned long flags = 0;
struct batched_entropy *batch;
 
if (arch_get_random_int())
return ret;
 
batch = _cpu_var(batched_entropy_u32);
+   if (use_lock)
+   read_lock_irqsave(_entropy_reset_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
extract_crng((u8 *)batch->entropy_u32);
batch->position = 0;
}
ret = batch->entropy_u32[batch->position++];
+   if (use_lock)
+   read_unlock_irqrestore(_entropy_reset_lock, flags);
put_cpu_var(batched_entropy_u32);
return ret;
 }
 EXPORT_SYMBOL(get_random_u32);
 
+/* It's important to invalidate all potential batched entropy that might
+ * be stored before the crng is initialized, which we can do lazily by
+ * simply resetting the counter to zero so that it's re-extracted on the
+ * next usage. */
+static void