linux-kernel - Re: [PATCH] exec/binfmt_script: trip zero bytes from the buffer

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Message-ID: <CANaxB-zVMxxvt8c1XNKfy6-hAUoodxp=ChJpP_Rn5cTD=26p9w@mail.gmail.com>
Date:   Tue, 15 Jun 2021 15:35:14 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] exec/binfmt_script: trip zero bytes from the buffer

On Tue, Jun 15, 2021 at 12:33 PM Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> Andrei Vagin <avagin@...il.com> writes:
>
> > Without this fix, if we try to run a script that contains only the
> > interpreter line, the interpreter is executed with one extra empty
> > argument.
> >
> > The code is written so that i_end has to be set to the end of valuable
> > data in the buffer.
>
> Out of curiosity how did you spot this change in behavior?

gVisor tests started failing with this change:
https://github.com/google/gvisor/blob/5e05950c1c520724e2e03963850868befb95efeb/test/syscalls/linux/exec.cc#L307

We run these tests on Ubuntu 20.04 and this is the reason why we
caught this issue just a few days ago.

>
> > Fixes: ccbb18b67323 ("exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC")
> > Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> > Signed-off-by: Andrei Vagin <avagin@...il.com>
> > ---
> >  fs/binfmt_script.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
> > index 1b6625e95958..e242680f96e1 100644
> > --- a/fs/binfmt_script.c
> > +++ b/fs/binfmt_script.c
> > @@ -68,6 +68,9 @@ static int load_script(struct linux_binprm *bprm)
> >               if (!next_terminator(i_end, buf_end))
> >                       return -ENOEXEC;
> >               i_end = buf_end;
> > +             /* Trim zero bytes from i_end */
> > +             while (i_end[-1] == 0)
> > +                     i_end--;
> >       }
> >       /* Trim any trailing spaces/tabs from i_end */
> >       while (spacetab(i_end[-1]))