crypto: rng - Fix double percpu offset in local_unlock() calls

local_unlock() is supposed to take a percpu pointer, which it then offsets
for the current CPU. The problem is that the per-CPU DRBG code passes
local_unlock() a pointer that has *already been offset for the current
CPU*, so the percpu offset gets added twice. `pcri` is the percpu pointer
and `cri` is the offset `pcri` pointer for the current CPU; passing `cri`
to local_unlock() results in the current CPU's offset getting added again
and thus the resulting pointer is garbage.

This went unnoticed at runtime because local_unlock() only dereferences
that pointer on PREEMPT_RT or CONFIG_DEBUG_LOCK_ALLOC=y kernels. The
pointer is never actually used otherwise.

This also went unnoticed at compile time because, for x86_64, __percpu only
expands to something on GCC 14+; __percpu is otherwise an empty macro. So
there's no pointer type mismatch detected at compile time on older GCC
versions. On GCC 14+, the following compile error occurs:

  ../crypto/rng.c: In function 'lock_default_rng':
  ../include/linux/percpu-defs.h:221:45: error: initialization from pointer to non-enclosed address space

Fix it by using __local_unlock() instead, which eliminates the duplicate
per-CPU offset addition. While using local_unlock() with `pcri` also works,
using `cri` with __local_unlock() is leaner because it doesn't need to redo
the offset addition, saving an instruction on kernels that actually use the
pointer.

Signed-off-by: Sultan Alsawaf <sultan@ciq.com>

Author: kerneltoast

crypto/rng.c

@@ -338,7 +338,7 @@ lock_default_rng(struct crypto_rng **rng) __acquires(&cri->lock)
 		 *
 		 * Note that this task may be migrated to a different CPU now!
 		 */
-		local_unlock(&cri->lock);
+		__local_unlock(&cri->lock);
 		rt_mutex_lock(&cri->mlock);
 		if (!cri->rng) {
 			struct crypto_rng *new_rng = NULL;
@@ -411,7 +411,7 @@ do {						\
 	if (reseed)				\
 		rt_mutex_unlock(&(cri)->mlock);	\
 	else					\
-		local_unlock(&(cri)->lock);	\
+		__local_unlock(&(cri)->lock);	\
 } while (0)
 
 static __always_inline void