[PATCH] usb: xhci: Fix NULL pointer dereference as part of completion

From: Sriharsha Allenki <sallenki@codeaurora.org>
To: mathias.nyman@intel.com, gregkh@linuxfoundation.org
Cc: linux-usb@vger.kernel.org, mgautam@codeaurora.org,
	jackp@codeaurora.org, Sriharsha Allenki <sallenki@codeaurora.org>,
	stable@vger.kernel.org
Subject: [PATCH] usb: xhci: Fix NULL pointer dereference as part of completion
Date: Tue,  5 May 2020 11:44:46 +0530	[thread overview]
Message-ID: <20200505061446.21850-1-sallenki@codeaurora.org> (raw)

On platforms with IOMMU enabled, multiple SGs can be
coalesced into one by the IOMMU driver. In that case
the SG list processing as part of the completion of
a urb on a bulk endpoint can result into a NULL pointer
dereference with the below stack dump.

<6> Unable to handle kernel NULL pointer dereference at virtual address 0000000c
<6> pgd = c0004000
<6> [0000000c] *pgd=00000000
<6> Internal error: Oops: 5 [#1] PREEMPT SMP ARM
<2> PC is at xhci_queue_bulk_tx+0x454/0x80c
<2> LR is at xhci_queue_bulk_tx+0x44c/0x80c
<2> pc : [<c08907c4>]    lr : [<c08907bc>]    psr: 000000d3
<2> sp : ca337c80  ip : 00000000  fp : ffffffff
<2> r10: 00000000  r9 : 50037000  r8 : 00004000
<2> r7 : 00000000  r6 : 00004000  r5 : 00000000  r4 : 00000000
<2> r3 : 00000000  r2 : 00000082  r1 : c2c1a200  r0 : 00000000
<2> Flags: nzcv  IRQs off  FIQs off  Mode SVC_32  ISA ARM  Segment none
<2> Control: 10c0383d  Table: b412c06a  DAC: 00000051
<6> Process usb-storage (pid: 5961, stack limit = 0xca336210)
<snip>
<2> [<c08907c4>] (xhci_queue_bulk_tx)
<2> [<c0881b3c>] (xhci_urb_enqueue)
<2> [<c0831068>] (usb_hcd_submit_urb)
<2> [<c08350b4>] (usb_sg_wait)
<2> [<c089f384>] (usb_stor_bulk_transfer_sglist)
<2> [<c089f2c0>] (usb_stor_bulk_srb)
<2> [<c089fe38>] (usb_stor_Bulk_transport)
<2> [<c089f468>] (usb_stor_invoke_transport)
<2> [<c08a11b4>] (usb_stor_control_thread)
<2> [<c014a534>] (kthread)

The above NULL pointer dereference is the result of block_len and
the sent_len set to zero after the first SG of the list when IOMMU
driver is enabled. Because of this the loop of processing the SGs
has run more than num_sgs which resulted in a sg_next on the
last SG of the list which has SG_END set.

Fix this by check for the sg before any attributes of the sg are
accessed.

Fixes: f9c589e142d04 ("xhci: TD-fragment, align the unsplittable case with a bounce buffer")
Cc: stable@vger.kernel.org
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>
---
 drivers/usb/host/xhci-ring.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index a78787bb5133..18141b38f7bf 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -3399,8 +3399,8 @@ int xhci_queue_bulk_tx(struct xhci_hcd *xhci, gfp_t mem_flags,
 			/* New sg entry */
 			--num_sgs;
 			sent_len -= block_len;
-			if (num_sgs != 0) {
-				sg = sg_next(sg);
+			sg = sg_next(sg);
+			if (num_sgs != 0 && sg) {
 				block_len = sg_dma_len(sg);
 				addr = (u64) sg_dma_address(sg);
 				addr += sent_len;
-- 
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