From: Jason A. Donenfeld <Jason@xxxxxxxxx>
commit 7f576b2593a978451416424e75f69ad1e3ae4efe upstream.
Now that we have get_random_u32_below(), it's nearly trivial to make
inline helpers to compute get_random_u32_above() and
get_random_u32_inclusive(), which will help clean up open coded loops
and manual computations throughout the tree.
One snag is that in order to make get_random_u32_inclusive() operate on
closed intervals, we have to do some (unlikely) special case handling if
get_random_u32_inclusive(0, U32_MAX) is called. The least expensive way
of doing this is actually to adjust the slowpath of
get_random_u32_below() to have its undefined 0 result just return the
output of get_random_u32(). We can make this basically free by calling
get_random_u32() before the branch, so that the branch latency gets
interleaved.
Cc: stable@xxxxxxxxxxxxxxx # to ease future backports that use this api
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/char/random.c | 18 +++++++++++++++++-
include/linux/random.h | 25 +++++++++++++++++++++++++
2 files changed, 42 insertions(+), 1 deletion(-)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -161,6 +161,8 @@ EXPORT_SYMBOL(wait_for_random_bytes);
* u16 get_random_u16()
* u32 get_random_u32()
* u32 get_random_u32_below(u32 ceil)
+ * u32 get_random_u32_above(u32 floor)
+ * u32 get_random_u32_inclusive(u32 floor, u32 ceil)
* u64 get_random_u64()
* unsigned long get_random_long()
*
@@ -522,7 +524,21 @@ u32 __get_random_u32_below(u32 ceil)
* of `-ceil % ceil` is analogous to `2^32 % ceil`, but is computable
* in 32-bits.
*/
- u64 mult = (u64)ceil * get_random_u32();
+ u32 rand = get_random_u32();
+ u64 mult;
+
+ /*
+ * This function is technically undefined for ceil == 0, and in fact
+ * for the non-underscored constant version in the header, we build bug
+ * on that. But for the non-constant case, it's convenient to have that
+ * evaluate to being a straight call to get_random_u32(), so that
+ * get_random_u32_inclusive() can work over its whole range without
+ * undefined behavior.
+ */
+ if (unlikely(!ceil))
+ return rand;
+
+ mult = (u64)ceil * rand;
if (unlikely((u32)mult < ceil)) {
u32 bound = -ceil % ceil;
while (unlikely((u32)mult < bound))
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -92,6 +92,31 @@ static inline u32 get_random_u32_below(u
}
/*
+ * Returns a random integer in the interval (floor, U32_MAX], with uniform
+ * distribution, suitable for all uses. Fastest when floor is a constant, but
+ * still fast for variable floor as well.
+ */
+static inline u32 get_random_u32_above(u32 floor)
+{
+ BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && floor == U32_MAX,
+ "get_random_u32_above() must take floor < U32_MAX");
+ return floor + 1 + get_random_u32_below(U32_MAX - floor);
+}
+
+/*
+ * Returns a random integer in the interval [floor, ceil], with uniform
+ * distribution, suitable for all uses. Fastest when floor and ceil are
+ * constant, but still fast for variable floor and ceil as well.
+ */
+static inline u32 get_random_u32_inclusive(u32 floor, u32 ceil)
+{
+ BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && __builtin_constant_p(ceil) &&
+ (floor > ceil || ceil - floor == U32_MAX),
+ "get_random_u32_inclusive() must take floor <= ceil");
+ return floor + get_random_u32_below(ceil - floor + 1);
+}
+
+/*
* On 64-bit architectures, protect against non-terminated C string overflows
* by zeroing out the first byte of the canary; this leaves 56 bits of entropy.
*/
