3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Julius Werner <jwerner@chromium.org>

commit 68e5254adb88bede68285f11fb442a4d34fb550c upstream.

xhci_alloc_segments_for_ring() builds a list of xhci_segments and links
the tail to head at the end (forming a ring). When it bails out for OOM
reasons half-way through, it tries to destroy its half-built list with
xhci_free_segments_for_ring(), even though it is not a ring yet. This
causes a null-pointer dereference upon hitting the last element.

Furthermore, one of its callers (xhci_ring_alloc()) mistakenly believes
the output parameters to be valid upon this kind of OOM failure, and
calls xhci_ring_free() on them. Since the (incomplete) list/ring should
already be destroyed in that case, this would lead to a use after free.

This patch fixes those issues by having xhci_alloc_segments_for_ring()
destroy its half-built, non-circular list manually and destroying the
invalid struct xhci_ring in xhci_ring_alloc() with a plain kfree().

This patch should be backported to kernels as old as 2.6.31, that
contains the commit 0ebbab37422315a5d0cb29792271085bafdf38c0 "USB: xhci:
Ring allocation and initialization."

A separate patch will need to be developed for kernels older than 3.4,
since the ring allocation code was refactored in that kernel.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Sarah Sharp <redacted>
[bwh: Backported to 3.2:
 - Adjust context
 - Since segment allocation is done directly in xhci_ring_alloc(), walk
   the list starting from ring->first_seg when freeing]
Signed-off-by: Ben Hutchings <redacted>
---

--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c

@@ -178,8 +178,15 @@ static struct xhci_ring *xhci_ring_alloc
 		struct xhci_segment	*next;
 
 		next = xhci_segment_alloc(xhci, flags);
-		if (!next)
+		if (!next) {
+			prev = ring->first_seg;
+			while (prev) {
+				next = prev->next;
+				xhci_segment_free(xhci, prev);
+				prev = next;
+			}
 			goto fail;
+		}
 		xhci_link_segments(xhci, prev, next, link_trbs, isoc);
 
 		prev = next;

@@ -199,7 +206,7 @@ static struct xhci_ring *xhci_ring_alloc
 	return ring;
 
 fail:
-	xhci_ring_free(xhci, ring);
+	kfree(ring);
 	return NULL;
 }