Re: [PATCH XENSTORE v1 06/10] xenstored: handle port reads correctly

[Jürgen Groß <jgross@xxxxxxxx>]

On 02.03.21 08:48, Norbert Manthey wrote:
> On 3/2/21 6:15 AM, Jürgen Groß wrote:
> > On 26.02.21 16:36, Andrew Cooper wrote:
> > > On 26/02/2021 14:41, Norbert Manthey wrote:
> > > > The read value could be larger than a signed 32bit integer. As -1 is
> > > > used as error value, we should not rely on using the full 32 bits.
> > > > Hence, when reading the port number, we should make sure we only return
> > > > valid values.
> > > >
> > > > This change sanity checks the input.
> > > > The issue is that the value for the port is
> > > >    1. transmitted as a string, with a fixed amount of digits.
> > > >    2. Next, this string is parsed by a function that can deal with
> > > > strings
> > > >       representing 64bit integers
> > > >    3. A 64bit integer is returned, and will be truncated to it's lower
> > > >       32bits, resulting in a wrong port number (in case the sender of
> > > > the
> > > >       string decides to craft a suitable 64bit value).
> > > >
> > > > The value is typically provided by the kernel, which has this value
> > > > hard
> > > > coded in the proper range. As we use the function strtoul, non-digit
> > > > character are considered as end of the input, and hence do not require
> > > > checking. Therefore, this change only covers the corner case to make
> > > > sure we stay in the 32 bit range.
> > > >
> > > > This bug was discovered and resolved using Coverity Static Analysis
> > > > Security Testing (SAST) by Synopsys, Inc.
> > > >
> > > > Signed-off-by: Norbert Manthey <nmanthey@xxxxxxxxx>
> > > > Reviewed-by: Thomas Friebel <friebelt@xxxxxxxxx>
> > > > Reviewed-by: Julien Grall <jgrall@xxxxxxxxxxxx>
> > > Port numbers are currently limited at 2^17, with easy extension to 2^29
> > > (iirc), but the entire event channel infrastructure would have to
> > > undergo another redesign to get beyond that.
> > >
> > > I think we can reasonably make an ABI statement saying that a port
> > > number will never exceed 2^31.  This is already pseudo-encoded in the
> > > evtchn_port_or_error_t mouthful.
> > I agree. There is no need for this patch.
> I understand, and am fine with dropping this patch.
>
> Out of curiosity, if the actual limit is lower than what the patch
> currently enforces, would it make sense to adapt the bound check to that
> number?
No, I don't think so. Especially as the boundary to check against isn't
known by Xenstore (the boundary value depends on 2-level or fifo events
being used, and this information is not exported to user land).

The value is coming from the kernel, and it is used with another kernel
interface. So if the kernel wants to play dirty tricks with Xenstore, it
doesn't need to deliver a wrong event channel number, it can just play
those games in the event channel driver.


Juergen

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