[PATCH 3.0.x] xhci: fix null-pointer dereference when destroying half-built segment rings

[CAI Qian]

Hi Sarah,

This is based on Ben's v3.2.y stable backport,
https://lkml.org/lkml/2012/12/28/126

Please provide an ACK/NAK here if you can. :)

Thanks,
CAI Qian

>From 68e5254adb88bede68285f11fb442a4d34fb550c Mon Sep 17 00:00:00 2001
From: Julius Werner <jwer...@chromium.org>
Date: Thu, 1 Nov 2012 12:47:59 -0700
Subject: [PATCH] xhci: fix null-pointer dereference when destroying half-built
 segment rings

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 <jwer...@chromium.org>
Signed-off-by: Sarah Sharp <sarah.a.sh...@linux.intel.com>
Signed-off-by: CAI Qian <caiq...@redhat.com>

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index a44f2d4..fb0d029 100644
--- 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_hcd 
*xhci,
                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(struct xhci_hcd 
*xhci,
        return ring;
 
 fail:
-       xhci_ring_free(xhci, ring);
+       kfree(ring);
        return NULL;
 }
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