On Mon, Nov 23, 2020 at 10:53:24AM +0100, Petr Mladek wrote:
> On Fri 2020-11-20 13:42:42, Steven Rostedt wrote:
> > On Fri, 20 Nov 2020 11:44:12 -0500
> > Alan Stern <[email protected]> wrote:
> >
> > > To the VSPRINTF maintainers:
> > >
> > > Documentation/core-api/printk-formats.rst lists a large number of format
> > > specifiers for pointers of various sorts. Yet as far as I can see,
> > > there is no specifier meant for use with __user pointers.
> > >
> > > The security implications of printing the true, unmangled value of a
> > > __user pointer are minimal, since doing so does not leak any kernel
> > > information. So %px would work, but tools like checkpatch.pl don't like
> > > it.
>
> Just to be sure as I am not a security expert. Is there really that
> big difference in the risk? The following scenarios come to my mind:
>
> 1. The address would show a well defined location in the userspace
> application? Could it be used to attack the application?
Yes -- this is the primary risk in my view. Exposing addresses of any
kind can be a risk. While an unprivileged user may not have direct
access to dmesg, there tend to be many indirect ways to see its
contents. As such, exposing a userspace address (when not then also
terminating the process, as seen with the segv reporting) poses a
potential exposure risk. I admit it's not a LARGE risk, but modern
attacks use these kind of building blocks to construct all the steps to
reaching their target.
> 2. The address shows a location that is being accessed by kernel.
> Could not it be used to pass a value that might be used to attack
> kernel?
This is also a risk: it provides feedback about where something may be
as a target within a confused-deputy style attack. (i.e. set up one
process to confuse the kernel, and exploit it from another).
> > > Should a new specifier be added? If not, should we simply use %px?
> >
> > There's currently no user of '%pu' (although there is a '%pus'. Perhaps we
> > should have a '%pux'?
> >
> > I would even state that if it is used, that if makes sure that the value is
> > indeed a user space pointer (goes through the same checks as accessing user
> > space), before its printed, otherwise it shows "(fault)" or something.
>
> I have mixed feelings about this.
>
> One one hand, it might make sense to mark locations where userspace
> address is printed. We could easily decide how to print them (hash or
> value) and we could check that it is really from a userspace one.
>
> But I have few concerns:
>
> 1. The existing "%pus" has a kind of opposite meaning. It says what
> address space should be used when the kernel and userspace address
> space is overlapping.
>
> 2. There is the history with "%pk". It did not work because people did
> not use it.
>
> 3. I am not sure about the output when the address is not from
> userspace. Printing ("fault") is not much helpful. Printing
> hashed value might be confusing. Well, I am still not sure
> that it is really safe to print real userspace addresses
> by default.
I think this should just be %px. Or better yet, not printed at all. See
Linus's prior comments:
https://www.kernel.org/doc/html/latest/process/deprecated.html#p-format-specifier
--
Kees Cook
