Subject: Re: [PATCH v4 2/6] treewide: use prandom_u32_max() when possible

From: "Jason A. Donenfeld" <[email-masked]>
Date: Fri, 7 Oct 2022 20:21:28 -0600

On Fri, Oct 07, 2022 at 03:47:44PM -0700, Kees Cook wrote:
> On Fri, Oct 07, 2022 at 12:01:03PM -0600, Jason A. Donenfeld wrote:
> > Rather than incurring a division or requesting too many random bytes for
> > the given range, use the prandom_u32_max() function, which only takes
> > the minimum required bytes from the RNG and avoids divisions.
> 
> I actually meant splitting the by-hand stuff by subsystem, but nearly
> all of these can be done mechanically too, so it shouldn't be bad. Notes
> below...

Oh, cool, more coccinelle. You're basically giving me a class on these
recipes. Much appreciated.

> > [...]
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 92bcc1768f0b..87203429f802 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -595,7 +595,7 @@ unsigned long __get_wchan(struct task_struct *p)
> >  unsigned long arch_align_stack(unsigned long sp)
> >  {
> >  	if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
> > -		sp -= get_random_int() & ~PAGE_MASK;
> > +		sp -= prandom_u32_max(PAGE_SIZE);
> >  	return sp & ~0xf;
> >  }
> >  
> 
> @mask@
> expression MASK;
> @@
> 
> - (get_random_int() & ~(MASK))
> + prandom_u32_max(MASK)

Not quite! PAGE_MASK != PAGE_SIZE. In this case, things get a litttttle
more complicated where you can do:

get_random_int() & MASK == prandom_u32_max(MASK + 1)
*only if all the top bits of MASK are set* That is, if MASK one less
than a power of two. Or if MASK & (MASK + 1) == 0.

(If those top bits aren't set, you can technically do
prandom_u32_max(MASK >> n + 1) << n. That'd be a nice thing to work out.
But yeesh, maybe a bit much for the time being and probably a bit beyond
coccinelle.)

This case here, though, is a bit more special, where we can just rely on
an obvious given kernel identity. Namely, PAGE_MASK == ~(PAGE_SIZE - 1).
So ~PAGE_MASK == PAGE_SIZE - 1.
So get_random_int() & ~PAGE_MASK == prandom_u32_max(PAGE_SIZE - 1 + 1).
So get_random_int() & ~PAGE_MASK == prandom_u32_max(PAGE_SIZE).

And most importantly, this makes the code more readable, since everybody
knows what bounding by PAGE_SIZE means, where as what on earth is
happening with the &~PAGE_MASK thing. So it's a good change. I'll try to
teach coccinelle about that special case.

> > diff --git a/arch/loongarch/kernel/vdso.c b/arch/loongarch/kernel/vdso.c
> > index f32c38abd791..8c9826062652 100644
> > --- a/arch/loongarch/kernel/vdso.c
> > +++ b/arch/loongarch/kernel/vdso.c
> > @@ -78,7 +78,7 @@ static unsigned long vdso_base(void)
> >  	unsigned long base = STACK_TOP;
> >  
> >  	if (current->flags & PF_RANDOMIZE) {
> > -		base += get_random_int() & (VDSO_RANDOMIZE_SIZE - 1);
> > +		base += prandom_u32_max(VDSO_RANDOMIZE_SIZE);
> >  		base = PAGE_ALIGN(base);
> >  	}
> >  
> 
> @minus_one@
> expression FULL;
> @@
> 
> - (get_random_int() & ((FULL) - 1)
> + prandom_u32_max(FULL)

Ahh, well, okay, this is the example I mentioned above. Only works if
FULL is saturated. Any clever way to get coccinelle to prove that? Can
it look at the value of constants?

> 
> > diff --git a/arch/parisc/kernel/vdso.c b/arch/parisc/kernel/vdso.c
> > index 63dc44c4c246..47e5960a2f96 100644
> > --- a/arch/parisc/kernel/vdso.c
> > +++ b/arch/parisc/kernel/vdso.c
> > @@ -75,7 +75,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm,
> >  
> >  	map_base = mm->mmap_base;
> >  	if (current->flags & PF_RANDOMIZE)
> > -		map_base -= (get_random_int() & 0x1f) * PAGE_SIZE;
> > +		map_base -= prandom_u32_max(0x20) * PAGE_SIZE;
> >  
> >  	vdso_text_start = get_unmapped_area(NULL, map_base, vdso_text_len, 0, 0);
> >  
> 
> These are more fun, but Coccinelle can still do them with a little
> Pythonic help:
> 
> // Find a potential literal
> @literal_mask@
> expression LITERAL;
> identifier randfunc =~ "get_random_int|prandom_u32|get_random_u32";
> position p;
> @@
> 
>         (randfunc()@p & (LITERAL))
> 
> // Add one to the literal.
> @script:python add_one@
> literal << literal_mask.LITERAL;
> RESULT;
> @@
> 
> if literal.startswith('0x'):
>         value = int(literal, 16) + 1
>         coccinelle.RESULT = cocci.make_expr("0x%x" % (value))
> elif literal[0] in '123456789':
>         value = int(literal, 10) + 1
>         coccinelle.RESULT = cocci.make_expr("%d" % (value))
> else:
>         print("I don't know how to handle: %s" % (literal))
> 
> // Replace the literal mask with the calculated result.
> @plus_one@
> expression literal_mask.LITERAL;
> position literal_mask.p;
> expression add_one.RESULT;
> identifier FUNC;
> @@
> 
> -       (FUNC()@p & (LITERAL))
> +       prandom_u32_max(RESULT)

Oh that's pretty cool. I can do the saturation check in python, since
`value` holds the parsed result. Neat.

> > diff --git a/fs/ext2/ialloc.c b/fs/ext2/ialloc.c
> > index 998dd2ac8008..f4944c4dee60 100644
> > --- a/fs/ext2/ialloc.c
> > +++ b/fs/ext2/ialloc.c
> > @@ -277,8 +277,7 @@ static int find_group_orlov(struct super_block *sb, struct inode *parent)
> >  		int best_ndir = inodes_per_group;
> >  		int best_group = -1;
> >  
> > -		group = prandom_u32();
> > -		parent_group = (unsigned)group % ngroups;
> > +		parent_group = prandom_u32_max(ngroups);
> >  		for (i = 0; i < ngroups; i++) {
> >  			group = (parent_group + i) % ngroups;
> >  			desc = ext2_get_group_desc (sb, group, NULL);
> 
> Okay, that one is too much for me -- checking that group is never used
> after the assignment removal is likely possible, but beyond my cocci
> know-how. :)

Yea this is a tricky one, which I initially didn't do by hand, but Jan
seemed fine with it, and it's clear if you look at it. Trixy cocci
indeed.

> > diff --git a/lib/test_hexdump.c b/lib/test_hexdump.c
> > index 0927f44cd478..41a0321f641a 100644
> > --- a/lib/test_hexdump.c
> > +++ b/lib/test_hexdump.c
> > @@ -208,7 +208,7 @@ static void __init test_hexdump_overflow(size_t buflen, size_t len,
> >  static void __init test_hexdump_overflow_set(size_t buflen, bool ascii)
> >  {
> >  	unsigned int i = 0;
> > -	int rs = (prandom_u32_max(2) + 1) * 16;
> > +	int rs = prandom_u32_max(2) + 1 * 16;
> >  
> >  	do {
> >  		int gs = 1 << i;
> 
> This looks wrong. Cocci says:
> 
> -       int rs = (get_random_int() % 2 + 1) * 16;
> +       int rs = (prandom_u32_max(2) + 1) * 16;

!! Nice catch.

Alright, I'll give this a try with more cocci. The big difficulty at the
moment is the power of 2 constant checking thing. If you have any
pointers on that, would be nice.

Thanks a bunch for the guidance.

Jason