Re: [PATCH 0/3] Unexport kallsyms_lookup_name() and kallsyms_on_each_symbol()

From: Masami Hiramatsu
Date: Fri Feb 21 2020 - 18:44:46 EST

On Fri, 21 Feb 2020 14:48:54 +0000
Will Deacon <will@xxxxxxxxxx> wrote:

> Hi Masami,
> On Fri, Feb 21, 2020 at 11:27:46PM +0900, Masami Hiramatsu wrote:
> > On Fri, 21 Feb 2020 11:44:01 +0000
> > Will Deacon <will@xxxxxxxxxx> wrote:
> > > Despite having just a single modular in-tree user that I could spot,
> > > kallsyms_lookup_name() is exported to modules and provides a mechanism
> > > for out-of-tree modules to access and invoke arbitrary, non-exported
> > > kernel symbols when kallsyms is enabled.
> > >
> > > This patch series fixes up that one user and unexports the symbol along
> > > with kallsyms_on_each_symbol(), since that could also be abused in a
> > > similar manner.
> >
> > What kind of issue would you like to fix with this?
> I would like to avoid out-of-tree modules being easily able to call
> functions that are not exported. kallsyms_lookup_name() makes this
> trivial to the point that there is very little incentive to rework these
> modules to either use upstream interfaces correctly or propose functionality
> which may be otherwise missing upstream. Both of these latter solutions
> would be pre-requisites to upstreaming these modules, and the current state
> of things actively discourages that approach.
> The background here is that we are aiming for Android devices to be able
> to use a generic binary kernel image closely following upstream, with
> any vendor extensions coming in as kernel modules. In this case, we
> (Google) end up maintaining the binary module ABI within the scope of a
> single LTS kernel. Monitoring and managing the ABI surface is not feasible
> if it effectively includes all data and functions via kallsyms_lookup_name().
> Of course, we could just carry this patch in the Android kernel tree,
> but we're aiming to carry as little as possible (ideally nothing) and
> I think it's a sensible change in its own right. I'm surprised you object
> to it, in all honesty.
> Now, you could turn around and say "that's not upstream's problem", but
> it still seems highly undesirable to me to have an upstream bypass for
> exported symbols that isn't even used by upstream modules. It's ripe for
> abuse and encourages people to work outside of the upstream tree. The
> usual rule is that we don't export symbols without a user in the tree
> and that seems especially relevant in this case.

So this is to officially states our policy that if out-of-tree driver
developers need some symbol exposed, they should work with upstream to
find better solution. Not for fixing some kind of security hole.

> > There are many ways to find (estimate) symbol address, especially, if
> > the programmer already has the symbol map, it is *very* easy to find
> > the target symbol address even from one exported symbol (the distance
> > of 2 symbols doesn't change.) If not, they can use kprobes to find
> > their required symbol address. If they have a time, they can use
> > snprintf("%pF") to search symbol.
> I would say that both of these are inconvenient enough that the developer
> would think twice before considering to use them in production.

Fair enough.

> > So, for me, this series just make it hard for casual developers (but
> > maybe they will find the answer on any technical Q&A site soon).
> Which casual developers? I don't understand who you're referring to here.
> Do you have a specific example in mind?

No, I don't. :)

> > Hmm, are there other good way to detect such bad-manner out-of-tree
> > module and reject them? What about decoding them and monitor their
> > all call instructions?
> That sounds like using a sledge hammer to crack a nut to me.

Agreed. Just for discouraging abuse of unexposed symbols, I think this is

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

for thise series.

Thank you,

Masami Hiramatsu <mhiramat@xxxxxxxxxx>