Re: [PATCH] fs: clear file set[ug]id when writing via mmap

[Willy Tarreau <w@1wt.eu>]

On Thu, Nov 19, 2015 at 05:03:15PM -0800, Kees Cook wrote:
> On Thu, Nov 19, 2015 at 5:00 PM, Willy Tarreau <w@1wt.eu> wrote:
> > Hi Kees,
> >
> > On Thu, Nov 19, 2015 at 04:10:43PM -0800, Kees Cook wrote:
> >> Normally, when a user can modify a file that has setuid or setgid bits,
> >> those bits are cleared when they are not the file owner or a member of the
> >> group. This is enforced when using write() directly but not when writing
> >> to a shared mmap on the file. This could allow the file writer to gain
> >> privileges by changing the binary without losing the setuid/setgid bits.
> >>
> >> Signed-off-by: Kees Cook <keescook@chromium.org>
> >> Cc: stable@vger.kernel.org
> >> ---
> >>  mm/memory.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index deb679c31f2a..4c970a4e0057 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -2036,6 +2036,7 @@ static inline int wp_page_reuse(struct mm_struct *mm,
> >>
> >>               if (!page_mkwrite)
> >>                       file_update_time(vma->vm_file);
> >> +             file_remove_privs(vma->vm_file);
> >
> > I thought you said in one of the early mails of this thread that it
> > didn't work. Or maybe I misunderstood.
> 
> I had a think-o in my earlier attempts. I understood the meaning of
> page_mkwrite incorrectly.
>
> > Also, don't you think we should move that into the if (!page_mkwrite)
> > just like for the time update ?
> 
> Nope, page_mkwrite indicates if there was a vmops call to
> page_mkwrite. In this case, it means "I will update the file time if
> the filesystem driver didn't take care of it like it should". For
> file_remove_privs, we want to always do it, since we should not depend
> on filesystems to do it.

Ah OK, thanks for the explanation, I didn't understand it like this
at all last time I read it.

Cheers,
Willy

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