kfence: fix is_kfence_address() for addresses below KFENCE_POOL_SIZE

Originally the addr != NULL check was meant to take care of the case
where __kfence_pool == NULL (KFENCE is disabled).  However, this does
not work for addresses where addr > 0 && addr < KFENCE_POOL_SIZE.

This can be the case on NULL-deref where addr > 0 && addr < PAGE_SIZE or
any other faulting access with addr < KFENCE_POOL_SIZE.  While the
kernel would likely crash, the stack traces and report might be
confusing due to double faults upon KFENCE's attempt to unprotect such
an address.

Fix it by just checking that __kfence_pool != NULL instead.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0ce20dd ("mm: add Kernel Electric-Fence infrastructure")
Signed-off-by: Marco Elver <[email protected]>
Reported-by: Kuan-Ying Lee <[email protected]>
Acked-by: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: <[email protected]>    [5.12+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: melver

include/linux/kfence.h

@@ -51,10 +51,11 @@ extern atomic_t kfence_allocation_gate;
 static __always_inline bool is_kfence_address(const void *addr)
 {
 	/*
-	 * The non-NULL check is required in case the __kfence_pool pointer was
-	 * never initialized; keep it in the slow-path after the range-check.
+	 * The __kfence_pool != NULL check is required to deal with the case
+	 * where __kfence_pool == NULL && addr < KFENCE_POOL_SIZE. Keep it in
+	 * the slow-path after the range-check!
 	 */
-	return unlikely((unsigned long)((char *)addr - __kfence_pool) < KFENCE_POOL_SIZE && addr);
+	return unlikely((unsigned long)((char *)addr - __kfence_pool) < KFENCE_POOL_SIZE && __kfence_pool);
 }
 
 /**