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.

[modified reason for null pointer dereference in commit message subject -Mathias]
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>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20200514110432.25564-2-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>