Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-09-05, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> > A common pattern for syscall extensions is increasing the size of a
> > struct passed from userspace, such that the zero-value of the new fields
> > result in the old kernel behaviour (allowing for a mix of userspace and
> > kernel vintages to operate on one another in most cases). This is done
> > in both directions -- hence two helpers -- though it's more common to
> > have to copy user space structs into kernel space.
> > 
> > Previously there was no common lib/ function that implemented
> > the necessary extension-checking semantics (and different syscalls
> > implemented them slightly differently or incompletely[1]). A future
> > patch replaces all of the common uses of this pattern to use the new
> > copy_struct_{to,from}_user() helpers.
> > 
> > [1]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
> >      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
> >      always rejects differently-sized struct arguments.
> > 
> > Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

[...]

> > +	if (unlikely(!access_ok(src, usize)))
> > +		return -EFAULT;
> > +
> > +	/* Deal with trailing bytes. */
> > +	if (usize < ksize)
> > +		memset(dst + size, 0, rest);
[...]
> That's a change in behavior for clone3() and sched at least, no? Unless
> - which I guess you might have done - you have moved the "error out when
> the struct is too small" part before the call to copy_struct_from_user()
> for them.

Yes, I've put the minimum size check to the callers in all of the
cases (in the case of clone3() I've #define'd a CLONE_ARGS_SIZE_VER0 to
match the others -- see patch 2 of the series).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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