[ 066/123] xhci: fix null-pointer dereference when destroying half-built segment rings

[Greg Kroah-Hartman]

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

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

From: Julius Werner <jwerner@xxxxxxxxxxxx>

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@xxxxxxxxxxxx>
Signed-off-by: Sarah Sharp <sarah.a.sharp@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/usb/host/xhci-mem.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -205,7 +205,12 @@ static int xhci_alloc_segments_for_ring(
 
 		next = xhci_segment_alloc(xhci, cycle_state, flags);
 		if (!next) {
-			xhci_free_segments_for_ring(xhci, *first);
+			prev = *first;
+			while (prev) {
+				next = prev->next;
+				xhci_segment_free(xhci, prev);
+				prev = next;
+			}
 			return -ENOMEM;
 		}
 		xhci_link_segments(xhci, prev, next, type);
@@ -258,7 +263,7 @@ static struct xhci_ring *xhci_ring_alloc
 	return ring;
 
 fail:
-	xhci_ring_free(xhci, ring);
+	kfree(ring);
 	return NULL;
 }
 


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