[Toybox] [landley/toybox] cp -R --preserve timing (#66)

[enh]

On Wed, May 3, 2017 at 8:06 PM, Rob Landley <rob at landley.net> wrote:

> On 05/03/2017 04:48 AM, Pierre-Hugues HUSSON wrote:
> > cp -R --preserve copies the content of a folder, before copying
> > owner/permissions.
>
> You can't create things with arbitrary ownership, and writing to
> something with an suid bit removes the suid bit.
>
> > This can be problematic if the copy is canceled.
> >
> > For instance, if we have
> > /a 1000 1000 0755
> > /a/b 2000 2000 0755
> >
> > We'll have a point in time where during cp -R --preserve as root, we
> have:
> > /a 0 0 0775
> > /a/b 2000 2000 0755
> >
> > The "cp" might command might be behind a daemon-wall, which leads to an
> > unrecoverable state.
>
> You can run cp as root but can't run cleanup as root?
>

yeah, i'm not sure how you'd actually hit this with vold. here's a fragment
of the relevant calling code:

    // Step 2: clean up any stale data
    if (execRm(toPath, 10, 10) != OK) {
        goto fail;
    }

    // Step 3: perform actual copy
    if (execCp(fromPath, toPath, 20, 60) != OK) {
        goto copy_fail;
    }

if you have repro steps, [file a bug](
https://source.android.com/source/report-bugs).

> For instance, in Android, vold uses cp -R --preserve to move data from
> > internal to external storage and back.
> > vold runs as root, this means that if the copy is stopped during the
> > migration, /data/media will contain a root-owned directory. This means
> > this folder won't ever be removable or writable ever, unless the user
> > does a factory reset.
>
> This sounds like a bug in vold. (What's a vold?)
>
>   https://www.slideshare.net/wiliwe/android-storage-vold
>
> Android volume daemon. Ok.
>
> Part of the reason I did that was to minimize the likelihood of symlink
> attacks where you cp -a and somebody drops a symlink to a new file it
> wants you to overwrite. If they can't write to the directory until
> you're done with it, they can't do that. So it creates directories
> belonging to the user and non-world-writeable, copies the contents into
> them, and then does the chown/chmod to the final ownership and
> permissions on the way out. (This also avoids the problem of writing
> files into a read-only directory.)
>
> I could instead open the files O_EXCL but cp historically doesn't do
> that, and has defined behavior for "cp file /dev/ttyS0" and so on where
> it does O_TRUNC and then writes, so if you have a removable disk it
> images it and if you have hardlinks it updates all copies.
>
> I'm fine with changing how it works, just explaining why I did it in the
> first place.
>
> Elliott: any opinions on the right place to fix this?
>
> Rob
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>



-- 
Elliott Hughes - http://who/enh - http://jessies.org/~enh/
Android native code/tools questions? Mail me/drop by/add me as a reviewer.
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