misc: fastrpc: fix list iterator in fastrpc_req_mem_unmap_impl

This is another instance of incorrect use of list iterator and
checking it for NULL.

The list iterator value 'map' will *always* be set and non-NULL
by list_for_each_entry(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty (in this case, the
check 'if (!map) {' will always be false and never exit as expected).

To fix the bug, use a new variable 'iter' as the list iterator,
while use the original variable 'map' as a dedicated pointer to
point to the found element.

Without this patch, Kernel crashes with below trace:

Unable to handle kernel access to user memory outside uaccess routines
 at virtual address 0000ffff7fb03750
...
Call trace:
 fastrpc_map_create+0x70/0x290 [fastrpc]
 fastrpc_req_mem_map+0xf0/0x2dc [fastrpc]
 fastrpc_device_ioctl+0x138/0xc60 [fastrpc]
 __arm64_sys_ioctl+0xa8/0xec
 invoke_syscall+0x48/0x114
 el0_svc_common.constprop.0+0xd4/0xfc
 do_el0_svc+0x28/0x90
 el0_svc+0x3c/0x130
 el0t_64_sync_handler+0xa4/0x130
 el0t_64_sync+0x18c/0x190
Code: 14000016 f94000a5 eb05029f 54000260 (b94018a6)
---[ end trace 0000000000000000 ]---

Fixes: 5c1b97c7 ("misc: fastrpc: add support for FASTRPC_IOCTL_MEM_MAP/UNMAP")
Cc: stable@vger.kernel.org
Reported-by: Jan Jablonsky <jjablonsky@snapchat.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220518152353.13058-1-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -1748,17 +1748,18 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
 static int fastrpc_req_mem_unmap_impl(struct fastrpc_user *fl, struct fastrpc_mem_unmap *req)
 {
 	struct fastrpc_invoke_args args[1] = { [0] = { 0 } };
-	struct fastrpc_map *map = NULL, *m;
+	struct fastrpc_map *map = NULL, *iter, *m;
 	struct fastrpc_mem_unmap_req_msg req_msg = { 0 };
 	int err = 0;
 	u32 sc;
 	struct device *dev = fl->sctx->dev;
 
 	spin_lock(&fl->lock);
-	list_for_each_entry_safe(map, m, &fl->maps, node) {
-		if ((req->fd < 0 || map->fd == req->fd) && (map->raddr == req->vaddr))
+	list_for_each_entry_safe(iter, m, &fl->maps, node) {
+		if ((req->fd < 0 || iter->fd == req->fd) && (iter->raddr == req->vaddr)) {
+			map = iter;
 			break;
-		map = NULL;
+		}
 	}
 
 	spin_unlock(&fl->lock);