netdev - Re: [PATCH bpf-next 1/5] uaccess: Add non-pagefault user-space write function

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Date:   Sat, 26 Oct 2019 00:15:30 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/5] uaccess: Add non-pagefault user-space write
 function

On Fri, Oct 25, 2019 at 02:53:07PM -0700, Andrii Nakryiko wrote:
> On Fri, Oct 25, 2019 at 1:44 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> >
> > Commit 3d7081822f7f ("uaccess: Add non-pagefault user-space read functions")
> > missed to add probe write function, therefore factor out a probe_write_common()
> > helper with most logic of probe_kernel_write() except setting KERNEL_DS, and
> > add a new probe_user_write() helper so it can be used from BPF side.
> >
> > Again, on some archs, the user address space and kernel address space can
> > co-exist and be overlapping, so in such case, setting KERNEL_DS would mean
> > that the given address is treated as being in kernel address space.
> >
> > Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> 
> LGTM. See an EFAULT comment below, though.
> 
> Acked-by: Andrii Nakryiko <andriin@...com>
> 
> [...]
> 
> > +/**
> > + * probe_user_write(): safely attempt to write to a user-space location
> > + * @dst: address to write to
> > + * @src: pointer to the data that shall be written
> > + * @size: size of the data chunk
> > + *
> > + * Safely write to address @dst from the buffer at @src.  If a kernel fault
> > + * happens, handle that and return -EFAULT.
> > + */
> > +
> > +long __weak probe_user_write(void __user *dst, const void *src, size_t size)
> > +    __attribute__((alias("__probe_user_write")));
> 
> curious, why is there this dance of probe_user_write alias to
> __probe_user_write (and for other pairs of functions as well)?

Seems done by convention to allow archs to override the __weak marked
functions in order to add additional checks and being able to then call
into the __ prefixed variant.

> > +long __probe_user_write(void __user *dst, const void *src, size_t size)
> > +{
> > +       long ret = -EFAULT;
> 
> This initialization is not necessary, is it? Similarly in
> __probe_user_read higher in this file.

Not entirely sure what you mean. In both there's access_ok() check before
invoking the common helper.

> > +       mm_segment_t old_fs = get_fs();
> > +
> > +       set_fs(USER_DS);
> > +       if (access_ok(dst, size))
> > +               ret = probe_write_common(dst, src, size);
> > +       set_fs(old_fs);
> > +
> > +       return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(probe_user_write);
> >
> >  /**
> >   * strncpy_from_unsafe: - Copy a NUL terminated string from unsafe address.
> > --
> > 2.21.0
> >