linux-kernel - Re: [PATCH] exec: Weaken dumpability for secureexec

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Message-ID: <20180103070622.GA6950@mail.hallyn.com>
Date:   Wed, 3 Jan 2018 01:06:22 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Horsley <horsley1953@...il.com>,
        Laura Abbott <labbott@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <james.l.morris@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exec: Weaken dumpability for secureexec

On Tue, Jan 02, 2018 at 03:21:33PM -0800, Kees Cook wrote:
> This is a logical revert of:
> 
>     commit e37fdb785a5f ("exec: Use secureexec for setting dumpability")
> 
> This weakens dumpability back to checking only for uid/gid changes in
> current (which is useless), but userspace depends on dumpability not
> being tied to secureexec.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1528633
> 
> Reported-by: Tom Horsley <horsley1953@...il.com>
> Fixes: e37fdb785a5f ("exec: Use secureexec for setting dumpability")
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  fs/exec.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 5688b5e1b937..7eb8d21bcab9 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1349,9 +1349,14 @@ void setup_new_exec(struct linux_binprm * bprm)
>  
>  	current->sas_ss_sp = current->sas_ss_size = 0;
>  
> -	/* Figure out dumpability. */
> +	/*
> +	 * Figure out dumpability. Note that this checking only of current
> +	 * is wrong, but userspace depends on it. This should be testing
> +	 * bprm->secureexec instead.
> +	 */
>  	if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP ||
> -	    bprm->secureexec)
> +	    !(uid_eq(current_euid(), current_uid()) &&
> +	      gid_eq(current_egid(), current_gid())))

So what about the pdeath_signal?  Is that going to be another subtle
time-bomb?

>  		set_dumpable(current->mm, suid_dumpable);
>  	else
>  		set_dumpable(current->mm, SUID_DUMP_USER);
> -- 
> 2.7.4
> 
> 
> -- 
> Kees Cook
> Pixel Security