[ 11/16] xhci: fix null-pointer dereference when destroying half-built segment rings

Greg Kroah-Hartman
2013-01-19T01:22:16Z

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 <sarah.a.sharp@linux.intel.com>
Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
[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 <ben@decadent.org.uk>
Signed-off-by: CAI Qian <caiqian@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/usb/host/xhci-mem.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -180,8 +180,15 @@ 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;
@@ -201,7 +208,7 @@ static struct xhci_ring *xhci_ring_alloc
return ring;

fail:
- xhci_ring_free(xhci, ring);
+ kfree(ring);
return NULL;
}