[Bug 91481] [Patch?] DisplayPort MST (Multistream Transport) hotplug kernel memory fault

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https://bugs.freedesktop.org/show_bug.cgi?id=91481

            Bug ID: 91481
           Summary: [Patch?] DisplayPort MST (Multistream Transport)
                    hotplug kernel memory fault
           Product: DRI
           Version: XOrg git
          Hardware: x86 (IA32)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/other
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: adam_richter2004 at yahoo.com

Created attachment 117412
  --> https://bugs.freedesktop.org/attachment.cgi?id=117412&action=edit
Path to make drm_dp_send_link_address set link_address_sent before hotplug
handler changes the pointer to it

When I "hot plug" insert a DisplayPort multistream transport ("MST") hub into a
computer that is running a Linux 4.2-rc4 kernel or any recent version, I get a
kernel memory fault in drm_dp_add_port.  In earlier kernels, I traced it to the
line after drm_dp_add_port's call to drm_dp_send_link_address:

             drm_dp_send_link_address(mstb->mgr, port->mstb);
            port->mstb->link_address_sent = true;

I believe that the problem is that drm_dp_send_link_address can call a hotplug
handler, which can change port->mstb so that it cannot be dereferenced (I think
it is set to NULL, but I have forgotten).

Since drm_dp_send_link_address is a static function in this file, its address
is never taken, and there are only two call sites, both of which set
link_address_sent to true, the patch that I have attached changes
drm_dp_send_link_address so that it sets that flag, reducing duplication of
code, and also allowing drm_dp_send_link_address to skip setting the flag if
sending the link address fails.

An intentional behavior change introduced by this patch is that the
mstb->link_address_sent is not set if sending the link address was aborted due
to the memory allocation in drm_dp_send_link_address failing.

Perhaps it should also not be set if the action receives a NAK reply, but I
haven't studied the code and DisplayPort documentation enough to guess. 
Whoever examines this patch should feel free to revise it as they see fit.

I should also warn that after I made this change in linux-4.2-rc4, the
resulting kernel eventually got another kernel memory fault in i2c_transfer
(called by drm_do_probe_ddc_edid), but I believe that that is a separate issue.

I am happy to answer questions about this patch.  If nobody sees any problems
with it, I would ask that whoever takes this bug report submit the patch
upstream.

Thanks for considering this bug report and proposed patch.

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