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

Aleksa Sarai
2019-09-11T10:46:01Z

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On 2019-09-05, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Sep 05, 2019 at 11:43:05AM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote:
> > > On 2019-09-05, Peter Zijlstra <peterz@infradead.org> wrote:
> > > > On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> > > > > +/**
> > > > > + * copy_struct_to_user: copy a struct to user space
> > > > > + * @dst: Destination address, in user space.
> > > > > + * @usize: Size of @dst struct.
> > > > > + * @src: Source address, in kernel space.
> > > > > + * @ksize: Size of @src struct.
> > > > > + *
> > > > > + * Copies a struct from kernel space to user space, in a way that guarantees
> > > > > + * backwards-compatibility for struct syscall arguments (as long as future
> > > > > + * struct extensions are made such that all new fields are *appended* to the
> > > > > + * old struct, and zeroed-out new fields have the same meaning as the old
> > > > > + * struct).
> > > > > + *
> > > > > + * @ksize is just sizeof(*dst), and @usize should've been passed by user space.
> > > > > + * The recommended usage is something like the following:
> > > > > + *
> > > > > + * SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, usize)
> > > > > + * {
> > > > > + * int err;
> > > > > + * struct foo karg = {};
> > > > > + *
> > > > > + * // do something with karg
> > > > > + *
> > > > > + * err = copy_struct_to_user(uarg, usize, &karg, sizeof(karg));
> > > > > + * if (err)
> > > > > + * return err;
> > > > > + *
> > > > > + * // ...
> > > > > + * }
> > > > > + *
> > > > > + * There are three cases to consider:
> > > > > + * * If @usize == @ksize, then it's copied verbatim.
> > > > > + * * If @usize < @ksize, then kernel space is "returning" a newer struct to an
> > > > > + * older user space. In order to avoid user space getting incomplete
> > > > > + * information (new fields might be important), all trailing bytes in @src
> > > > > + * (@ksize - @usize) must be zerored
> > > >
> > > > s/zerored/zero/, right?
> > >
> > > It should've been "zeroed".
> >
> > That reads wrong to me; that way it reads like this function must take
> > that action and zero out the 'rest'; which is just wrong.
> >
> > This function must verify those bytes are zero, not make them zero.
> >
> > > > > , otherwise -EFBIG is returned.
> > > >
> > > > 'Funny' that, copy_struct_from_user() below seems to use E2BIG.
> > >
> > > This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG for
> > > a "too big" struct passed to the kernel, and EFBIG for a "too big"
> > > struct passed to user-space. I would personally have preferred EMSGSIZE
> > > instead of EFBIG, but felt using the existing error codes would be less
> > > confusing.
> >
> > Sadly a recent commit:
> >
> > 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify sched_read_attr() ABI logic and code")
> >
> > Made the situation even 'worse'.
>
> And thinking more about things; I'm not convinced the above patch is
> actually right.
>
> Do we really want to simply truncate all the attributes of the task?
>
> And should we not at least set sched_flags when there are non-default
> clamp values applied?
>
> See; that is I think the primary bug that had chrt failing; we tried to
> publish the default clamp values as !0.

I just saw this patch in -rc8 -- should I even attempt to port
sched_getattr(2) to copy_struct_to_user()? I agree that publishing a
default non-zero value is a mistake -- once you do that, old user space
will either get confused or lose information.

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

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