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

Kees Cook
2022-10-07T22:48:02Z

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...

>
> [...]
> 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)

> 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)

> 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)

> diff --git a/drivers/mtd/tests/stresstest.c b/drivers/mtd/tests/stresstest.c
> index cb29c8c1b370..d2faaca7f19d 100644
> --- a/drivers/mtd/tests/stresstest.c
> +++ b/drivers/mtd/tests/stresstest.c
> @@ -45,9 +45,8 @@ static int rand_eb(void)
> unsigned int eb;
>
> again:
> - eb = prandom_u32();
> /* Read or write up 2 eraseblocks at a time - hence 'ebcnt - 1' */
> - eb %= (ebcnt - 1);
> + eb = prandom_u32_max(ebcnt - 1);
> if (bbt[eb])
> goto again;
> return eb;

This can also be done mechanically:

@multi_line@
identifier randfunc =~ "get_random_int|prandom_u32|get_random_u32";
identifier RAND;
expression E;
@@

- RAND = randfunc();
... when != RAND
- RAND %= (E);
+ RAND = prandom_u32_max(E);

@collapse_ret@
type TYPE;
identifier VAR;
expression E;
@@

{
- TYPE VAR;
- VAR = (E);
- return VAR;
+ return E;
}

@drop_var@
type TYPE;
identifier VAR;
@@

{
- TYPE VAR;
... when != VAR
}

> 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. :)

> diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> index f73e5eb43eae..36d5bc595cc2 100644
> --- a/fs/ext4/ialloc.c
> +++ b/fs/ext4/ialloc.c
> @@ -463,10 +463,9 @@ static int find_group_orlov(struct super_block *sb, struct inode *parent,
> hinfo.hash_version = DX_HASH_HALF_MD4;
> hinfo.seed = sbi->s_hash_seed;
> ext4fs_dirhash(parent, qstr->name, qstr->len, &hinfo);
> - grp = hinfo.hash;
> + parent_group = hinfo.hash % ngroups;
> } else
> - grp = prandom_u32();
> - parent_group = (unsigned)grp % ngroups;
> + parent_group = prandom_u32_max(ngroups);
> for (i = 0; i < ngroups; i++) {
> g = (parent_group + i) % ngroups;
> get_orlov_stats(sb, g, flex_size, &stats);

Much less easy mechanically. :)

> 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;

> diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c
> index 4f2f2d1bac56..56ffaa8dd3f6 100644
> --- a/lib/test_vmalloc.c
> +++ b/lib/test_vmalloc.c
> @@ -151,9 +151,7 @@ static int random_size_alloc_test(void)
> int i;
>
> for (i = 0; i < test_loop_count; i++) {
> - n = prandom_u32();
> - n = (n % 100) + 1;
> -
> + n = prandom_u32_max(n % 100) + 1;
> p = vmalloc(n * PAGE_SIZE);
>
> if (!p)

This looks wrong. Cocci says:

- n = prandom_u32();
- n = (n % 100) + 1;
+ n = prandom_u32_max(100) + 1;

> @@ -293,16 +291,12 @@ pcpu_alloc_test(void)
> return -1;
>
> for (i = 0; i < 35000; i++) {
> - unsigned int r;
> -
> - r = prandom_u32();
> - size = (r % (PAGE_SIZE / 4)) + 1;
> + size = prandom_u32_max(PAGE_SIZE / 4) + 1;
>
> /*
> * Maximum PAGE_SIZE
> */
> - r = prandom_u32();
> - align = 1 << ((r % 11) + 1);
> + align = 1 << (prandom_u32_max(11) + 1);
>
> pcpu[i] = __alloc_percpu(size, align);
> if (!pcpu[i])
> @@ -393,14 +387,11 @@ static struct test_driver {
>
> static void shuffle_array(int *arr, int n)
> {
> - unsigned int rnd;
> int i, j;
>
> for (i = n - 1; i > 0; i--) {
> - rnd = prandom_u32();
> -
> /* Cut the range. */
> - j = rnd % i;
> + j = prandom_u32_max(i);
>
> /* Swap indexes. */
> swap(arr[i], arr[j]);

Yup, agrees with Cocci on these.

--
Kees Cook