[v2 PATCH] crypto: api - Do not load modules if called by async probing

[v2 PATCH] crypto: api - Do not load modules if called by async probing

[Herbert Xu]

On Mon, May 20, 2024 at 11:49:56AM -0400, Nícolas F. R. A. Prado wrote:
>
> Unfortunately this patch didn't work either. The warning is still there
> unchanged.

OK perhaps we can do it by calling current_is_async ourselves.
But this is really a nasty hack because it basically defeats
the whole point of loading optional algorithm by module.

Linus/Tejun, is it time perhaps to remove the warning introduced
by commit 0fdff3ec6d87856cdcc99e69cf42143fdd6c56b4 since it's
been ten years since the warning caused a real problem?

For the Crypto API, if it is called by some random driver via the
async context, this warning stops us from loading any modules
without printing a nasty warning that isn't relevant as the Crypto
API never calls async_synchronize_full.

---8<---
Do not call request_module if this is the case or a warning will
be printed.

Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reported-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
---
 crypto/api.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/crypto/api.c b/crypto/api.c
index 22556907b3bc..7c4b9f86c1ad 100644
--- a/crypto/api.c
+++ b/crypto/api.c
@@ -10,6 +10,7 @@
  * and Nettle, by Niels Möller.
  */
 
+#include <linux/async.h>
 #include <linux/err.h>
 #include <linux/errno.h>
 #include <linux/jump_label.h>
@@ -280,7 +281,8 @@ static struct crypto_alg *crypto_larval_lookup(const char *name, u32 type,
 	mask &= ~(CRYPTO_ALG_LARVAL | CRYPTO_ALG_DEAD);
 
 	alg = crypto_alg_lookup(name, type, mask);
-	if (!alg && !(mask & CRYPTO_NOLOAD)) {
+	if (!alg && !(mask & CRYPTO_NOLOAD) &&
+	    (!IS_BUILTIN(CONFIG_CRYPTO) || !current_is_async())) {
 		request_module("crypto-%s", name);
 
 		if (!((type ^ CRYPTO_ALG_NEED_FALLBACK) & mask &
-- 
2.39.2

-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [v2 PATCH] crypto: api - Do not load modules if called by async probing

[Nícolas F. R. A. Prado]

On Tue, May 21, 2024 at 10:53:18AM +0800, Herbert Xu wrote:
> On Mon, May 20, 2024 at 11:49:56AM -0400, Nícolas F. R. A. Prado wrote:
> >
> > Unfortunately this patch didn't work either. The warning is still there
> > unchanged.
> 
> OK perhaps we can do it by calling current_is_async ourselves.
> But this is really a nasty hack because it basically defeats
> the whole point of loading optional algorithm by module.
> 
> Linus/Tejun, is it time perhaps to remove the warning introduced
> by commit 0fdff3ec6d87856cdcc99e69cf42143fdd6c56b4 since it's
> been ten years since the warning caused a real problem?
> 
> For the Crypto API, if it is called by some random driver via the
> async context, this warning stops us from loading any modules
> without printing a nasty warning that isn't relevant as the Crypto
> API never calls async_synchronize_full.
> 
> ---8<---
> Do not call request_module if this is the case or a warning will
> be printed.
> 
> Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> Reported-by: Eric Biggers <ebiggers@kernel.org>
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
> ---
>  crypto/api.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/crypto/api.c b/crypto/api.c
> index 22556907b3bc..7c4b9f86c1ad 100644
> --- a/crypto/api.c
> +++ b/crypto/api.c
> @@ -10,6 +10,7 @@
>   * and Nettle, by Niels Möller.
>   */
>  
> +#include <linux/async.h>
>  #include <linux/err.h>
>  #include <linux/errno.h>
>  #include <linux/jump_label.h>
> @@ -280,7 +281,8 @@ static struct crypto_alg *crypto_larval_lookup(const char *name, u32 type,
>  	mask &= ~(CRYPTO_ALG_LARVAL | CRYPTO_ALG_DEAD);
>  
>  	alg = crypto_alg_lookup(name, type, mask);
> -	if (!alg && !(mask & CRYPTO_NOLOAD)) {
> +	if (!alg && !(mask & CRYPTO_NOLOAD) &&
> +	    (!IS_BUILTIN(CONFIG_CRYPTO) || !current_is_async())) {
>  		request_module("crypto-%s", name);
>  
>  		if (!((type ^ CRYPTO_ALG_NEED_FALLBACK) & mask &
> -- 
> 2.39.2

FWIW this patch fixes the warning. So feel free to add

Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>

if you choose to apply this patch (I'm happy to help test other patches too). In
any case, please also add the following trailers so the regression gets closed
automatically in regzbot:

Fixes: 1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")
Link: https://lore.kernel.org/r/119dc5ed-f159-41be-9dda-1a056f29888d@notapiano/

Thanks,
Nícolas

[v3 PATCH] hwrng: core - Remove add_early_randomness

[Herbert Xu]

On Tue, May 21, 2024 at 03:37:16PM -0400, Nícolas F. R. A. Prado wrote:
>
> FWIW this patch fixes the warning. So feel free to add
> 
> Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>

Could you please test this patch instead?

---8<---
A potential deadlock was reported with the config file at

https://web.archive.org/web/20240522052129/https://0x0.st/XPN_.txt

In this particular configuration, the deadlock doesn't exist because
the warning triggered at a point before modules were even available.
However, the deadlock can be real because any module loaded would
invoke async_synchronize_full.

The issue is spurious for software crypto algorithms which aren't
themselves involved in async probing.  However, it would be hard to
avoid for a PCI crypto driver using async probing.

In this particular call trace, the problem is easily avoided because
the only reason the module is being requested during probing is the
add_early_randomness call in the hwrng core.  This feature is
vestigial since there is now a kernel thread dedicated to doing
exactly this.

So remove add_early_randomness as it is no longer needed.

Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reported-by: Eric Biggers <ebiggers@kernel.org>
Fixes: 1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")
Link: https://lore.kernel.org/r/119dc5ed-f159-41be-9dda-1a056f29888d@notapiano/
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index f5c71a617a99..4084df65c9fa 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -64,19 +64,6 @@ static size_t rng_buffer_size(void)
 	return RNG_BUFFER_SIZE;
 }
 
-static void add_early_randomness(struct hwrng *rng)
-{
-	int bytes_read;
-
-	mutex_lock(&reading_mutex);
-	bytes_read = rng_get_data(rng, rng_fillbuf, 32, 0);
-	mutex_unlock(&reading_mutex);
-	if (bytes_read > 0) {
-		size_t entropy = bytes_read * 8 * rng->quality / 1024;
-		add_hwgenerator_randomness(rng_fillbuf, bytes_read, entropy, false);
-	}
-}
-
 static inline void cleanup_rng(struct kref *kref)
 {
 	struct hwrng *rng = container_of(kref, struct hwrng, ref);
@@ -340,13 +327,12 @@ static ssize_t rng_current_store(struct device *dev,
 				 const char *buf, size_t len)
 {
 	int err;
-	struct hwrng *rng, *old_rng, *new_rng;
+	struct hwrng *rng, *new_rng;
 
 	err = mutex_lock_interruptible(&rng_mutex);
 	if (err)
 		return -ERESTARTSYS;
 
-	old_rng = current_rng;
 	if (sysfs_streq(buf, "")) {
 		err = enable_best_rng();
 	} else {
@@ -362,11 +348,8 @@ static ssize_t rng_current_store(struct device *dev,
 	new_rng = get_current_rng_nolock();
 	mutex_unlock(&rng_mutex);
 
-	if (new_rng) {
-		if (new_rng != old_rng)
-			add_early_randomness(new_rng);
+	if (new_rng)
 		put_rng(new_rng);
-	}
 
 	return err ? : len;
 }
@@ -544,7 +527,6 @@ int hwrng_register(struct hwrng *rng)
 {
 	int err = -EINVAL;
 	struct hwrng *tmp;
-	bool is_new_current = false;
 
 	if (!rng->name || (!rng->data_read && !rng->read))
 		goto out;
@@ -573,25 +555,8 @@ int hwrng_register(struct hwrng *rng)
 		err = set_current_rng(rng);
 		if (err)
 			goto out_unlock;
-		/* to use current_rng in add_early_randomness() we need
-		 * to take a ref
-		 */
-		is_new_current = true;
-		kref_get(&rng->ref);
 	}
 	mutex_unlock(&rng_mutex);
-	if (is_new_current || !rng->init) {
-		/*
-		 * Use a new device's input to add some randomness to
-		 * the system.  If this rng device isn't going to be
-		 * used right away, its init function hasn't been
-		 * called yet by set_current_rng(); so only use the
-		 * randomness from devices that don't need an init callback
-		 */
-		add_early_randomness(rng);
-	}
-	if (is_new_current)
-		put_rng(rng);
 	return 0;
 out_unlock:
 	mutex_unlock(&rng_mutex);
@@ -602,12 +567,11 @@ EXPORT_SYMBOL_GPL(hwrng_register);
 
 void hwrng_unregister(struct hwrng *rng)
 {
-	struct hwrng *old_rng, *new_rng;
+	struct hwrng *new_rng;
 	int err;
 
 	mutex_lock(&rng_mutex);
 
-	old_rng = current_rng;
 	list_del(&rng->list);
 	complete_all(&rng->dying);
 	if (current_rng == rng) {
@@ -626,11 +590,8 @@ void hwrng_unregister(struct hwrng *rng)
 	} else
 		mutex_unlock(&rng_mutex);
 
-	if (new_rng) {
-		if (old_rng != new_rng)
-			add_early_randomness(new_rng);
+	if (new_rng)
 		put_rng(new_rng);
-	}
 
 	wait_for_completion(&rng->cleanup_done);
 }
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Jarkko Sakkinen]

On Wed May 22, 2024 at 8:37 AM EEST, Herbert Xu wrote:
> On Tue, May 21, 2024 at 03:37:16PM -0400, Nícolas F. R. A. Prado wrote:
> >
> > FWIW this patch fixes the warning. So feel free to add
> > 
> > Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
>
> Could you please test this patch instead?
>
> ---8<---
> A potential deadlock was reported with the config file at
>
> https://web.archive.org/web/20240522052129/https://0x0.st/XPN_.txt
>
> In this particular configuration, the deadlock doesn't exist because
> the warning triggered at a point before modules were even available.
> However, the deadlock can be real because any module loaded would
> invoke async_synchronize_full.
>
> The issue is spurious for software crypto algorithms which aren't
> themselves involved in async probing.  However, it would be hard to
> avoid for a PCI crypto driver using async probing.
>
> In this particular call trace, the problem is easily avoided because
> the only reason the module is being requested during probing is the
> add_early_randomness call in the hwrng core.  This feature is
> vestigial since there is now a kernel thread dedicated to doing
> exactly this.
>
> So remove add_early_randomness as it is no longer needed.

"vestigial" did not know that word before ;-) Something learned.

What is the kthread doing this currently?

>
> Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> Reported-by: Eric Biggers <ebiggers@kernel.org>
> Fixes: 1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")
> Link: https://lore.kernel.org/r/119dc5ed-f159-41be-9dda-1a056f29888d@notapiano/
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index f5c71a617a99..4084df65c9fa 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -64,19 +64,6 @@ static size_t rng_buffer_size(void)
>  	return RNG_BUFFER_SIZE;
>  }
>  
> -static void add_early_randomness(struct hwrng *rng)
> -{
> -	int bytes_read;
> -
> -	mutex_lock(&reading_mutex);
> -	bytes_read = rng_get_data(rng, rng_fillbuf, 32, 0);
> -	mutex_unlock(&reading_mutex);
> -	if (bytes_read > 0) {
> -		size_t entropy = bytes_read * 8 * rng->quality / 1024;
> -		add_hwgenerator_randomness(rng_fillbuf, bytes_read, entropy, false);
> -	}
> -}
> -
>  static inline void cleanup_rng(struct kref *kref)
>  {
>  	struct hwrng *rng = container_of(kref, struct hwrng, ref);
> @@ -340,13 +327,12 @@ static ssize_t rng_current_store(struct device *dev,
>  				 const char *buf, size_t len)
>  {
>  	int err;
> -	struct hwrng *rng, *old_rng, *new_rng;
> +	struct hwrng *rng, *new_rng;
>  
>  	err = mutex_lock_interruptible(&rng_mutex);
>  	if (err)
>  		return -ERESTARTSYS;
>  
> -	old_rng = current_rng;
>  	if (sysfs_streq(buf, "")) {
>  		err = enable_best_rng();
>  	} else {
> @@ -362,11 +348,8 @@ static ssize_t rng_current_store(struct device *dev,
>  	new_rng = get_current_rng_nolock();
>  	mutex_unlock(&rng_mutex);
>  
> -	if (new_rng) {
> -		if (new_rng != old_rng)
> -			add_early_randomness(new_rng);
> +	if (new_rng)
>  		put_rng(new_rng);
> -	}
>  
>  	return err ? : len;
>  }
> @@ -544,7 +527,6 @@ int hwrng_register(struct hwrng *rng)
>  {
>  	int err = -EINVAL;
>  	struct hwrng *tmp;
> -	bool is_new_current = false;
>  
>  	if (!rng->name || (!rng->data_read && !rng->read))
>  		goto out;
> @@ -573,25 +555,8 @@ int hwrng_register(struct hwrng *rng)
>  		err = set_current_rng(rng);
>  		if (err)
>  			goto out_unlock;
> -		/* to use current_rng in add_early_randomness() we need
> -		 * to take a ref
> -		 */
> -		is_new_current = true;
> -		kref_get(&rng->ref);
>  	}
>  	mutex_unlock(&rng_mutex);
> -	if (is_new_current || !rng->init) {
> -		/*
> -		 * Use a new device's input to add some randomness to
> -		 * the system.  If this rng device isn't going to be
> -		 * used right away, its init function hasn't been
> -		 * called yet by set_current_rng(); so only use the
> -		 * randomness from devices that don't need an init callback
> -		 */
> -		add_early_randomness(rng);
> -	}
> -	if (is_new_current)
> -		put_rng(rng);
>  	return 0;
>  out_unlock:
>  	mutex_unlock(&rng_mutex);
> @@ -602,12 +567,11 @@ EXPORT_SYMBOL_GPL(hwrng_register);
>  
>  void hwrng_unregister(struct hwrng *rng)
>  {
> -	struct hwrng *old_rng, *new_rng;
> +	struct hwrng *new_rng;
>  	int err;
>  
>  	mutex_lock(&rng_mutex);
>  
> -	old_rng = current_rng;
>  	list_del(&rng->list);
>  	complete_all(&rng->dying);
>  	if (current_rng == rng) {
> @@ -626,11 +590,8 @@ void hwrng_unregister(struct hwrng *rng)
>  	} else
>  		mutex_unlock(&rng_mutex);
>  
> -	if (new_rng) {
> -		if (old_rng != new_rng)
> -			add_early_randomness(new_rng);
> +	if (new_rng)
>  		put_rng(new_rng);
> -	}
>  
>  	wait_for_completion(&rng->cleanup_done);
>  }

I have no doubts that such thread would not exist, so:

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Herbert Xu]

On Wed, May 22, 2024 at 02:51:36PM +0300, Jarkko Sakkinen wrote:
>
> What is the kthread doing this currently?

It's hwrng_fillfn.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Nícolas F. R. A. Prado]

On Wed, May 22, 2024 at 01:37:54PM +0800, Herbert Xu wrote:
> On Tue, May 21, 2024 at 03:37:16PM -0400, Nícolas F. R. A. Prado wrote:
> >
> > FWIW this patch fixes the warning. So feel free to add
> > 
> > Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> 
> Could you please test this patch instead?
> 
> ---8<---
> A potential deadlock was reported with the config file at
> 
> https://web.archive.org/web/20240522052129/https://0x0.st/XPN_.txt
> 
> In this particular configuration, the deadlock doesn't exist because
> the warning triggered at a point before modules were even available.
> However, the deadlock can be real because any module loaded would
> invoke async_synchronize_full.
> 
> The issue is spurious for software crypto algorithms which aren't
> themselves involved in async probing.  However, it would be hard to
> avoid for a PCI crypto driver using async probing.
> 
> In this particular call trace, the problem is easily avoided because
> the only reason the module is being requested during probing is the
> add_early_randomness call in the hwrng core.  This feature is
> vestigial since there is now a kernel thread dedicated to doing
> exactly this.
> 
> So remove add_early_randomness as it is no longer needed.
> 
> Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> Reported-by: Eric Biggers <ebiggers@kernel.org>
> Fixes: 1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")
> Link: https://lore.kernel.org/r/119dc5ed-f159-41be-9dda-1a056f29888d@notapiano/
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

This patch also fixes the warning.

Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>

Thanks,
Nícolas

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Linus Torvalds]

On Tue, 21 May 2024 at 22:38, Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> In this particular configuration, the deadlock doesn't exist because
> the warning triggered at a point before modules were even available.
> However, the deadlock can be real because any module loaded would
> invoke async_synchronize_full.

I think this crapectomy is good regardless of any deadlock - the
"register this driver" should not just blindly call back into the
driver.

That said, looking at the code in question, there are other oddities
going on. Even the "we found a favorite new rng" case looks rather
strange. The thread we use - nice and asynchronous - seems to sleep
only if the randomness source is emptied.

What if you have a really good source of hw randomness? That looks
like a busy loop to me, but hopefully I'm missing something obvious.

So I think this hw_random code has other serious issues, and I get the
feeling there might be more code that needs looking at..

              Linus

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Herbert Xu]

On Wed, May 22, 2024 at 03:53:23PM -0700, Linus Torvalds wrote:
> 
> That said, looking at the code in question, there are other oddities
> going on. Even the "we found a favorite new rng" case looks rather
> strange. The thread we use - nice and asynchronous - seems to sleep
> only if the randomness source is emptied.
> 
> What if you have a really good source of hw randomness? That looks
> like a busy loop to me, but hopefully I'm missing something obvious.

Yes that does look strange.  So I dug up the original patch at

	https://lore.kernel.org/all/20140317165012.GC1763@lst.de/

and therein lies the answer.  It's relying on random.c to push back
when the amount of new entropy exceeds what it needs.  IOW we will
sleep via add_hwgenerator_randomness when random.c decides that
enough is enough.  In fact the rate is much less now compared to
when the patch was first applied.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Jarkko Sakkinen]

On Thu May 23, 2024 at 7:49 AM EEST, Herbert Xu wrote:
> On Wed, May 22, 2024 at 03:53:23PM -0700, Linus Torvalds wrote:
> > 
> > That said, looking at the code in question, there are other oddities
> > going on. Even the "we found a favorite new rng" case looks rather
> > strange. The thread we use - nice and asynchronous - seems to sleep
> > only if the randomness source is emptied.
> > 
> > What if you have a really good source of hw randomness? That looks
> > like a busy loop to me, but hopefully I'm missing something obvious.
>
> Yes that does look strange.  So I dug up the original patch at
>
> 	https://lore.kernel.org/all/20140317165012.GC1763@lst.de/
>
> and therein lies the answer.  It's relying on random.c to push back
> when the amount of new entropy exceeds what it needs.  IOW we will
> sleep via add_hwgenerator_randomness when random.c decides that
> enough is enough.  In fact the rate is much less now compared to
> when the patch was first applied.

Just throwing something because came to mind, not a serious suggestion.

In crypto_larval_lookup I see statements like this:

	request_module("crypto-%s", name);

You could potentially bake up a section/table to vmlinux which would
have entries like:

	"module name", 1/0

'1' would mean built-in. Then for early randomness use only stuff
that is built-in.

Came to mind from arch/x86/realmode for which I baked in a table
for relocation (this was a collaborative work with H. Peter Anvin
in 2012 to make trampoline code relocatable but is still a legit
example to do such shenanigans in a subystem).

BR, Jarkko

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Herbert Xu]

On Thu, May 23, 2024 at 12:53:04PM +0300, Jarkko Sakkinen wrote:
>
> Just throwing something because came to mind, not a serious suggestion.
> 
> In crypto_larval_lookup I see statements like this:
> 
> 	request_module("crypto-%s", name);
> 
> You could potentially bake up a section/table to vmlinux which would
> have entries like:
> 
> 	"module name", 1/0
> 
> '1' would mean built-in. Then for early randomness use only stuff
> that is built-in.

This early random stuff is obsolete not just because we have a
kernel thread doing the same thing, but moreover random.c itself
has been modified so that it is no longer starved of entropy on
startup.  There is no reason to feed any early randomness.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Jarkko Sakkinen]

On Thu May 23, 2024 at 12:58 PM EEST, Herbert Xu wrote:
> On Thu, May 23, 2024 at 12:53:04PM +0300, Jarkko Sakkinen wrote:
> >
> > Just throwing something because came to mind, not a serious suggestion.
> > 
> > In crypto_larval_lookup I see statements like this:
> > 
> > 	request_module("crypto-%s", name);
> > 
> > You could potentially bake up a section/table to vmlinux which would
> > have entries like:
> > 
> > 	"module name", 1/0
> > 
> > '1' would mean built-in. Then for early randomness use only stuff
> > that is built-in.
>
> This early random stuff is obsolete not just because we have a
> kernel thread doing the same thing, but moreover random.c itself
> has been modified so that it is no longer starved of entropy on
> startup.  There is no reason to feed any early randomness.

As a feature that would still sometimes nice to have. I've
sometimes wished there was "lsmod -b" or similar to display
built-in stuff ;-)

So overally I think it was good to have at least documented
here... 

BR, Jarkko

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Jarkko Sakkinen]

On Thu May 23, 2024 at 12:53 PM EEST, Jarkko Sakkinen wrote:
> On Thu May 23, 2024 at 7:49 AM EEST, Herbert Xu wrote:
> > On Wed, May 22, 2024 at 03:53:23PM -0700, Linus Torvalds wrote:
> > > 
> > > That said, looking at the code in question, there are other oddities
> > > going on. Even the "we found a favorite new rng" case looks rather
> > > strange. The thread we use - nice and asynchronous - seems to sleep
> > > only if the randomness source is emptied.
> > > 
> > > What if you have a really good source of hw randomness? That looks
> > > like a busy loop to me, but hopefully I'm missing something obvious.
> >
> > Yes that does look strange.  So I dug up the original patch at
> >
> > 	https://lore.kernel.org/all/20140317165012.GC1763@lst.de/
> >
> > and therein lies the answer.  It's relying on random.c to push back
> > when the amount of new entropy exceeds what it needs.  IOW we will
> > sleep via add_hwgenerator_randomness when random.c decides that
> > enough is enough.  In fact the rate is much less now compared to
> > when the patch was first applied.
>
> Just throwing something because came to mind, not a serious suggestion.
>
> In crypto_larval_lookup I see statements like this:
>
> 	request_module("crypto-%s", name);
>
> You could potentially bake up a section/table to vmlinux which would
> have entries like:
>
> 	"module name", 1/0
>
> '1' would mean built-in. Then for early randomness use only stuff
> that is built-in.
>
> Came to mind from arch/x86/realmode for which I baked in a table
> for relocation (this was a collaborative work with H. Peter Anvin
> in 2012 to make trampoline code relocatable but is still a legit
> example to do such shenanigans in a subystem).

This could be even parameter in __request_module() itself e.g.

int __request_module(bool wait, bool builtin_only, const char *fmt, ...);

And would not matter which initcall layer we are running at.

BR, Jarkko

Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

[Torsten Duwe]

On Thu, 23 May 2024 12:49:50 +0800
Herbert Xu <herbert@gondor.apana.org.au> wrote:

> On Wed, May 22, 2024 at 03:53:23PM -0700, Linus Torvalds wrote:
> > 
> > That said, looking at the code in question, there are other oddities
> > going on. Even the "we found a favorite new rng" case looks rather
> > strange. The thread we use - nice and asynchronous - seems to sleep
> > only if the randomness source is emptied.
> > 
> > What if you have a really good source of hw randomness? That looks
> > like a busy loop to me, but hopefully I'm missing something obvious.
> 
> Yes that does look strange.  So I dug up the original patch at
> 
> 	https://lore.kernel.org/all/20140317165012.GC1763@lst.de/
> 
> and therein lies the answer.  It's relying on random.c to push back
> when the amount of new entropy exceeds what it needs.  IOW we will
> sleep via add_hwgenerator_randomness when random.c decides that
> enough is enough.

Yes, I thought that this was the obvious choice, the lesser evil. If it
just discarded the excess and returned immediately I had expected some
kernel threads to spin constantly.

>  In fact the rate is much less now compared to
> when the patch was first applied.

You mean the rate of required entropy? Doesn't that make things worse?
Maybe redesign the API to use a pull scheme? RNGs register at the
randomness facility with a callback that can be used when there is a
need for fresh entropy?

	Torsten