Re: [PATCH 1/1] Drivers: input: serio: New driver to support Hyper-Vsynthetic keyboard

[Dmitry Torokhov]

On Mon, Sep 16, 2013 at 06:29:45PM +0000, KY Srinivasan wrote:
> 
> 
> > -----Original Message-----
> > From: Dmitry Torokhov [mailto:dmitry.torokhov@xxxxxxxxx]
> > Sent: Monday, September 16, 2013 10:10 AM
> > To: KY Srinivasan
> > Cc: Dan Carpenter; olaf@xxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx;
> > jasowang@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; vojtech@xxxxxxx; linux-
> > input@xxxxxxxxxxxxxxx; apw@xxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx
> > Subject: Re: [PATCH 1/1] Drivers: input: serio: New driver to support Hyper-V
> > synthetic keyboard
> > 
> > On Mon, Sep 16, 2013 at 04:56:03PM +0000, KY Srinivasan wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Dan Carpenter [mailto:dan.carpenter@xxxxxxxxxx]
> > > > Sent: Monday, September 16, 2013 8:06 AM
> > > > To: KY Srinivasan
> > > > Cc: olaf@xxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; jasowang@xxxxxxxxxx;
> > > > dmitry.torokhov@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > vojtech@xxxxxxx;
> > > > linux-input@xxxxxxxxxxxxxxx; apw@xxxxxxxxxxxxx;
> > devel@xxxxxxxxxxxxxxxxxxxxxx
> > > > Subject: Re: [PATCH 1/1] Drivers: input: serio: New driver to support Hyper-V
> > > > synthetic keyboard
> > > >
> > > > On Mon, Sep 16, 2013 at 02:46:24PM +0000, KY Srinivasan wrote:
> > > > > > > +			case VM_PKT_DATA_INBAND:
> > > > > > > +				hv_kbd_on_receive(device, desc);
> > > > > >
> > > > > > This is the error handling I mentioned at the top.  hv_kbd_on_receive()
> > > > > > doesn't take into consideration the amount of data we recieved, it
> > > > > > trusts the offset we recieved from the user.  There is an out of bounds
> > > > > > read.
> > > > >
> > > > > What user are you referring to. The message is sent by the host - the user
> > > > keystroke
> > > > > is normalized into a fixed size packet by the host and sent to the  guest. We
> > will
> > > > parse this
> > > > > packet, based on the host specified layout here.
> > > > >
> > > >
> > > > The user means the hypervisor, yes.
> > > >
> > > > I don't want the hypervisor accessing outside of the buffer.  It is
> > > > robustness issue.  Just check the offset against "bytes_recvd".  It's
> > > > not complicated.
> > >
> > > At the outset, let me apologize for not understanding your concern.
> > > You say: " I don't want the hypervisor accessing outside of the buffer"
> > > Where did you see the hypervisor accessing anything outside the buffer?
> > > The buffer is allocated by this driver and a packet from vmbus is read into this
> > > buffer - this is the call to vmbus_recvpacket(). If the specified buffer is smaller
> > > than the packet that needs to be read, then nothing will be read. Once the read
> > > completes, we can be sure we have read a valid packet and can proceed to
> > parse it in
> > > this driver.
> > 
> > The concern is that number of bytes received and contents of a packet
> > are not in sync. Imagine if we were told that 16 butes was received but
> > in the packet offset is 78. Then we'll try reading well past the buffer
> > boundary that we allocated for the packets.
> 
> I am not sure how this would be the case. Following are the semantics of the function
> vmbus_recvpacket_raw():
> 
> If the packet to be read is larger than the buffer specified; nothing will be read and 
> appropriate error is returned. If a  packet is read, the complete packet is read and 
> so we can safely peek into this packet based on the information in the header.

No, you can not safely use contents of the packet because it has not
been vetted. The semantics you are talking about is provided by
vmbus_recvpacket(). That function does indeed look inside the packet end
ensures that offset specified in the packet is sane and would not exceed
the buffer. The vmbus_recvpacket_raw() does not do such validation.

Thanks.

-- 
Dmitry
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