linux-kernel - Re: x86/random: Speculation to the rescue

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Date:   Tue, 1 Oct 2019 15:51:08 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>,
        Nicholas Mc Guire <hofrat@...ntech.at>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: x86/random: Speculation to the rescue

On Mon, Sep 30, 2019 at 09:06:36AM -0700, Linus Torvalds wrote:
> Obviously, that can be a problem if you then need sshd in order to get
> into a headless box, so my patch fixes things for you too, but at
> least your box doesn't show the problem that Ahmed had, and the boot
> completing presumably means that you got more entropy from other disk
> IO being done by the rest of the boot.

Right, another observation I did was that when it would wait for
entropy, if I press random keys, it would get done faster because
apparently it would collect entropy from the key presses too.

> If you want to test my hacky "do /dev/urandom too", it was this one-liner:
> 
>   --- a/drivers/char/random.c
>   +++ b/drivers/char/random.c
>   @@ -2027,6 +2027,7 @@ urandom_read(struct file *file, char __user
> *buf, size_t nbytes, loff_t *ppos)
>         static int maxwarn = 10;
>         int ret;
> 
>   +     if (!crng_ready()) try_to_generate_entropy();
>         if (!crng_ready() && maxwarn > 0) {
>                 maxwarn--;
>                 if (__ratelimit(&urandom_warning))
> 
> and that should get rid of the warnings.

So when I add this by hand and do git diff, it adds a second hunk:

---
diff --git a/drivers/char/random.c b/drivers/char/random.c
index c2f7de9dc543..93bad17bef98 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2027,6 +2027,7 @@ urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 	static int maxwarn = 10;
 	int ret;
 
+	if (!crng_ready()) try_to_generate_entropy();
 	if (!crng_ready() && maxwarn > 0) {
 		maxwarn--;
 		if (__ratelimit(&urandom_warning))
@@ -2520,4 +2521,4 @@ void add_bootloader_randomness(const void *buf, unsigned int size)
 	else
 		add_device_randomness(buf, size);
 }
-EXPORT_SYMBOL_GPL(add_bootloader_randomness);
\ No newline at end of file
+EXPORT_SYMBOL_GPL(add_bootloader_randomness);
---

and I kinda get what it is trying to tell me but this is new. And when I
do

$ xxd drivers/char/random.c
..

000125e0: 646f 6d6e 6573 7329 3b0a                 domness);.

there's a 0xa at the end so what's git really trying to tell me?

Anyway, that does get rid of the warns too.

> Doing something like the above to /dev/urandom is likely the right
> thing to do eventually, but I didn't want to mix up "we can perhaps
> improve the urandom situation too" with the basic "let's fix the boot
> problem". The urandom behavior change would be a separate thing.

So make it a separate patch and let's hammer on it during the next weeks
and see what happens?

> Also, talking about "future changes". Right now
> "try_to_generate_entropy()" is actually uninterruptible once it gets
> started. I think we should add a test for signal_pending() too, but it

Wouldn't that even increase its entropy, which would be a good thing?

> should generally complete really fairly quickly so I left it without
> one just to see if anybody even notices.

Right.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette