uml: fix request->sector update

It is theoretically possible for a request to finish and be freed between
writing it to the I/O thread and updating the sector count.  In this case, the
update will dereference a freed pointer.

To avoid this, I delay the update until processing the next sg segment, when
the request pointer is known to be good.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index 2e09f162c42f..0947f2e20045 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -1083,7 +1083,7 @@ static void do_ubd_request(request_queue_t *q)
 {
         struct io_thread_req *io_req;
         struct request *req;
-        int n;
+        int n, last_sectors;
 
         while(1){
                 struct ubd *dev = q->queuedata;
@@ -1099,9 +1099,11 @@ static void do_ubd_request(request_queue_t *q)
                 }
 
                 req = dev->request;
+                last_sectors = 0;
                 while(dev->start_sg < dev->end_sg){
                         struct scatterlist *sg = &dev->sg[dev->start_sg];
 
+                        req->sector += last_sectors;
                         io_req = kmalloc(sizeof(struct io_thread_req),
                                          GFP_ATOMIC);
                         if(io_req == NULL){
@@ -1113,6 +1115,7 @@ static void do_ubd_request(request_queue_t *q)
                                         (unsigned long long) req->sector << 9,
                                         sg->offset, sg->length, sg->page);
 
+                        last_sectors = sg->length >> 9;
                         n = os_write_file(thread_fd, &io_req,
                                           sizeof(struct io_thread_req *));
                         if(n != sizeof(struct io_thread_req *)){
@@ -1124,7 +1127,6 @@ static void do_ubd_request(request_queue_t *q)
                                 return;
                         }
 
-                        req->sector += sg->length >> 9;
                         dev->start_sg++;
                 }
                 dev->end_sg = 0;