block: add allocation size check in blkdev_pr_read_keys()

blkdev_pr_read_keys() takes num_keys from userspace and uses it to
calculate the allocation size for keys_info via struct_size(). While
there is a check for SIZE_MAX (integer overflow), there is no upper
bound validation on the allocation size itself.

A malicious or buggy userspace can pass a large num_keys value that
doesn't trigger overflow but still results in an excessive allocation
attempt, causing a warning in the page allocator when the order exceeds
MAX_PAGE_ORDER.

Fix this by introducing PR_KEYS_MAX to limit the number of keys to
a sane value. This makes the SIZE_MAX check redundant, so remove it.
Also switch to kvzalloc/kvfree to handle larger allocations gracefully.

Fixes: 22a1ffea5f80 ("block: add IOC_PR_READ_KEYS ioctl")
Tested-by: [e-mail]
Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=660d079d90f8a1baf54d
Link: https://lore.kernel.org/all/[e-mail]/T/ [v1]
Signed-off-by: Deepanshu Kartikey <[e-mail]>
Reviewed-by: Martin K. Petersen <[e-mail]>
Reviewed-by: Stefan Hajnoczi <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/ioctl.c

···
 	if (copy_from_user(&read_keys, arg, sizeof(read_keys)))
 		return -EFAULT;
 
-	keys_info_len = struct_size(keys_info, keys, read_keys.num_keys);
-	if (keys_info_len == SIZE_MAX)
+	if (read_keys.num_keys > PR_KEYS_MAX)
 		return -EINVAL;
 
-	keys_info = kzalloc(keys_info_len, GFP_KERNEL);
+	keys_info_len = struct_size(keys_info, keys, read_keys.num_keys);
+
+	keys_info = kvzalloc(keys_info_len, GFP_KERNEL);
 	if (!keys_info)
 		return -ENOMEM;
 
···
 	if (copy_to_user(arg, &read_keys, sizeof(read_keys)))
 		ret = -EFAULT;
 out:
-	kfree(keys_info);
+	kvfree(keys_info);
 	return ret;
 }
 

include/uapi/linux/pr.h

···
 #define IOC_PR_READ_KEYS	_IOWR('p', 206, struct pr_read_keys)
 #define IOC_PR_READ_RESERVATION	_IOR('p', 207, struct pr_read_reservation)
 
+#define PR_KEYS_MAX		(1u << 16)
+
 #endif /* _UAPI_PR_H */