linux-security-module.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Kees Cook <keescook@chromium.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
	Greg Ungerer <gerg@linux-m68k.org>, Rob Landley <rob@landley.net>,
	Bernd Edlinger <bernd.edlinger@hotmail.de>,
	linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Casey Schaufler <casey@schaufler-ca.com>,
	linux-security-module@vger.kernel.org,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC
Date: Tue, 19 May 2020 12:08:25 -0700	[thread overview]
Message-ID: <202005191144.E3112135@keescook> (raw)
In-Reply-To: <874ksczru6.fsf_-_@x220.int.ebiederm.org>

On Mon, May 18, 2020 at 07:33:21PM -0500, Eric W. Biederman wrote:
> 
> The return code -ENOEXEC serves to tell search_binary_handler that it
> should continue searching for the binfmt to handle a given file.  This
> makes return -ENOEXEC with a bprm->buf that is needed to continue the
> search problematic.
> 
> The current binfmt_script manages to escape problems as it closes and
> clears bprm->file before return -ENOEXEC with bprm->buf modified.
> This prevents search_binary_handler from looping as it explicitly
> handles a NULL bprm->file.
> 
> I plan on moving all of the bprm->file managment into fs/exec.c and out
> of the binary handlers so this will become a problem.
> 
> Move closing bprm->file and the test for BINPRM_PATH_INACCESSIBLE
> down below the last return of -ENOEXEC.
> 
> Introduce i_sep and i_end to track the end of the first argument and
> the end of the parameters respectively.  Using those, constification
> of all char * pointers, and the helpers next_terminator and
> next_non_spacetab guarantee the parameter parsing will not modify
> bprm->buf.

I'm quite pleased this could be implemented using the existing helpers!
It seems Linus and I were on the right track with these. :)

> 
> Only modify bprm->buf to terminate the strings i_arg and i_name with
> '\0' for passing to copy_strings_kernel.
> 
> When replacing loops with next_non_spacetab and next_terminator care
> has been take that the logic of the parsing code (short of replacing
> characters by '\0') remains the same.

Ah, interesting. As in, bprm->buf must not be modified unless the binfmt
handler is going to succeed. I think this requirement should be
documented in the binfmt struct header file.

> [...]
> diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
> index 8d718d8fd0fe..85e0ef86eb11 100644
> --- a/fs/binfmt_script.c
> +++ b/fs/binfmt_script.c
> @@ -71,39 +56,48 @@ static int load_script(struct linux_binprm *bprm)
>  	 * parse them on its own.
>  	 */
>  	buf_end = bprm->buf + sizeof(bprm->buf) - 1;
> -	cp = strnchr(bprm->buf, sizeof(bprm->buf), '\n');
> -	if (!cp) {
> -		cp = next_non_spacetab(bprm->buf + 2, buf_end);
> -		if (!cp)
> +	i_end = strnchr(bprm->buf, sizeof(bprm->buf), '\n');
> +	if (!i_end) {
> +		i_end = next_non_spacetab(bprm->buf + 2, buf_end);
> +		if (!i_end)
>  			return -ENOEXEC; /* Entire buf is spaces/tabs */
>  		/*
>  		 * If there is no later space/tab/NUL we must assume the
>  		 * interpreter path is truncated.
>  		 */
> -		if (!next_terminator(cp, buf_end))
> +		if (!next_terminator(i_end, buf_end))
>  			return -ENOEXEC;
> -		cp = buf_end;
> +		i_end = buf_end;
>  	}
> -	/* NUL-terminate the buffer and any trailing spaces/tabs. */
> -	*cp = '\0';
> -	while (cp > bprm->buf) {
> -		cp--;
> -		if ((*cp == ' ') || (*cp == '\t'))
> -			*cp = '\0';
> -		else
> -			break;
> -	}
> -	for (cp = bprm->buf+2; (*cp == ' ') || (*cp == '\t'); cp++);
> -	if (*cp == '\0')
> +	/* Trim any trailing spaces/tabs from i_end */
> +	while (spacetab(i_end[-1]))
> +		i_end--;
> +
> +	/* Skip over leading spaces/tabs */
> +	i_name = next_non_spacetab(bprm->buf+2, i_end);
> +	if (!i_name || (i_name == i_end))
>  		return -ENOEXEC; /* No interpreter name found */
> -	i_name = cp;
> +
> +	/* Is there an optional argument? */
>  	i_arg = NULL;
> -	for ( ; *cp && (*cp != ' ') && (*cp != '\t'); cp++)
> -		/* nothing */ ;
> -	while ((*cp == ' ') || (*cp == '\t'))
> -		*cp++ = '\0';
> -	if (*cp)
> -		i_arg = cp;
> +	i_sep = next_terminator(i_name, i_end);
> +	if (i_sep && (*i_sep != '\0'))
> +		i_arg = next_non_spacetab(i_sep, i_end);
> +
> +	/*
> +	 * If the script filename will be inaccessible after exec, typically
> +	 * because it is a "/dev/fd/<fd>/.." path against an O_CLOEXEC fd, give
> +	 * up now (on the assumption that the interpreter will want to load
> +	 * this file).
> +	 */
> +	if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
> +		return -ENOENT;
> +
> +	/* Release since we are not mapping a binary into memory. */
> +	allow_write_access(bprm->file);
> +	fput(bprm->file);
> +	bprm->file = NULL;
> +
>  	/*
>  	 * OK, we've parsed out the interpreter name and
>  	 * (optional) argument.
> @@ -121,7 +115,9 @@ static int load_script(struct linux_binprm *bprm)
>  	if (retval < 0)
>  		return retval;
>  	bprm->argc++;
> +	*((char *)i_end) = '\0';
>  	if (i_arg) {
> +		*((char *)i_sep) = '\0';
>  		retval = copy_strings_kernel(1, &i_arg, bprm);
>  		if (retval < 0)
>  			return retval;

I think this is all correct, though I'm always suspicious of my visual
inspection of string parsers. ;)

I had a worry the \n was not handled correctly in some case. I.e. before
any \n was converted into \0, and so next_terminator() didn't need to
consider \n separately. (next_non_spacetab() doesn't care since \n and \0
are both not ' ' nor '\t'.) For next_terminator(), though, I was worried
there was a case where *i_end == '\n', and next_terminator()
will return NULL instead of "last" due to *last being '\n' instead of
'\0', causing a problem, but you're using the adjusted i_end so I think
it's correct. And you've handled i_name == i_end.

I will see if I can find my testing scripts I used when commit
b5372fe5dc84 originally landed to double-check... until then:

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

  reply	other threads:[~2020-05-19 19:08 UTC|newest]

Thread overview: 108+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <87h7wujhmz.fsf@x220.int.ebiederm.org>
     [not found] ` <87sgga6ze4.fsf@x220.int.ebiederm.org>
2020-05-09 19:40   ` [PATCH 0/5] exec: Control flow simplifications Eric W. Biederman
2020-05-09 19:40     ` [PATCH 1/5] exec: Call cap_bprm_set_creds directly from prepare_binprm Eric W. Biederman
2020-05-09 20:04       ` Linus Torvalds
2020-05-09 19:41     ` [PATCH 2/5] exec: Directly call security_bprm_set_creds from __do_execve_file Eric W. Biederman
2020-05-09 20:07       ` Linus Torvalds
2020-05-09 20:12         ` Eric W. Biederman
2020-05-09 20:19           ` Linus Torvalds
2020-05-11  3:15       ` Kees Cook
2020-05-11 16:52         ` Eric W. Biederman
2020-05-11 21:18           ` Kees Cook
2020-05-09 19:41     ` [PATCH 3/5] exec: Remove recursion from search_binary_handler Eric W. Biederman
2020-05-09 20:16       ` Linus Torvalds
2020-05-10  4:22       ` Tetsuo Handa
2020-05-10 19:38         ` Linus Torvalds
2020-05-11 14:33           ` Eric W. Biederman
2020-05-11 19:10             ` Rob Landley
2020-05-13 21:59               ` Eric W. Biederman
2020-05-14 18:46                 ` Rob Landley
2020-05-11 21:55             ` Kees Cook
2020-05-12 18:42               ` Eric W. Biederman
2020-05-12 19:25                 ` Kees Cook
2020-05-12 20:31                   ` Eric W. Biederman
2020-05-12 23:08                     ` Kees Cook
2020-05-12 23:47                       ` Kees Cook
2020-05-12 23:51                         ` Kees Cook
2020-05-14 14:56                           ` Eric W. Biederman
2020-05-14 16:56                             ` Casey Schaufler
2020-05-14 17:02                               ` Eric W. Biederman
2020-05-13  0:20                 ` Linus Torvalds
2020-05-13  2:39                   ` Rob Landley
2020-05-13 19:51                     ` Linus Torvalds
2020-05-14 16:49                   ` Eric W. Biederman
2020-05-09 19:42     ` [PATCH 4/5] exec: Allow load_misc_binary to call prepare_binfmt unconditionally Eric W. Biederman
2020-05-11 22:09       ` Kees Cook
2020-05-09 19:42     ` [PATCH 5/5] exec: Move the call of prepare_binprm into search_binary_handler Eric W. Biederman
2020-05-11 22:24       ` Kees Cook
2020-05-19  0:29     ` [PATCH v2 0/8] exec: Control flow simplifications Eric W. Biederman
2020-05-19  0:29       ` [PATCH v2 1/8] exec: Teach prepare_exec_creds how exec treats uids & gids Eric W. Biederman
2020-05-19 18:03         ` Kees Cook
2020-05-19 18:28           ` Linus Torvalds
2020-05-19 18:57             ` Eric W. Biederman
2020-05-19  0:30       ` [PATCH v2 2/8] exec: Factor security_bprm_creds_for_exec out of security_bprm_set_creds Eric W. Biederman
2020-05-19 15:34         ` Casey Schaufler
2020-05-19 18:10         ` Kees Cook
2020-05-19 21:28           ` James Morris
2020-05-19  0:31       ` [PATCH v2 3/8] exec: Convert security_bprm_set_creds into security_bprm_repopulate_creds Eric W. Biederman
2020-05-19 18:21         ` Kees Cook
2020-05-19 19:03           ` Eric W. Biederman
2020-05-19 19:14             ` Kees Cook
2020-05-20 20:22               ` Eric W. Biederman
2020-05-20 20:53                 ` Kees Cook
2020-05-19 21:52         ` James Morris
2020-05-20 12:40           ` Eric W. Biederman
2020-05-19  0:31       ` [PATCH v2 4/8] exec: Allow load_misc_binary to call prepare_binfmt unconditionally Eric W. Biederman
2020-05-19 18:27         ` Kees Cook
2020-05-19 19:08           ` Eric W. Biederman
2020-05-19 19:17             ` Kees Cook
2020-05-19  0:32       ` [PATCH v2 5/8] exec: Move the call of prepare_binprm into search_binary_handler Eric W. Biederman
2020-05-19 18:27         ` Kees Cook
2020-05-19 21:30         ` James Morris
2020-05-19  0:33       ` [PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC Eric W. Biederman
2020-05-19 19:08         ` Kees Cook [this message]
2020-05-19 19:19           ` Eric W. Biederman
2020-05-19  0:33       ` [PATCH v2 7/8] exec: Generic execfd support Eric W. Biederman
2020-05-19 19:46         ` Kees Cook
2020-05-19 19:54           ` Linus Torvalds
2020-05-19 20:20             ` Eric W. Biederman
2020-05-19 21:59         ` Rob Landley
2020-05-20 16:05           ` Eric W. Biederman
2020-05-21 22:50             ` Rob Landley
2020-05-22  3:28               ` Eric W. Biederman
2020-05-22  4:51                 ` Rob Landley
2020-05-22 13:35                   ` Eric W. Biederman
2020-05-19  0:34       ` [PATCH v2 8/8] exec: Remove recursion from search_binary_handler Eric W. Biederman
2020-05-19 20:37         ` Kees Cook
2020-05-19  1:25       ` [PATCH v2 0/8] exec: Control flow simplifications Linus Torvalds
2020-05-19 21:55       ` Kees Cook
2020-05-20 13:02         ` Eric W. Biederman
2020-05-20 22:12       ` Eric W. Biederman
2020-05-20 23:43         ` Kees Cook
2020-05-21 11:53           ` Eric W. Biederman
2020-05-28 15:38       ` [PATCH 0/11] exec: cred calculation simplifications Eric W. Biederman
2020-05-28 15:41         ` [PATCH 01/11] exec: Reduce bprm->per_clear to a single bit Eric W. Biederman
2020-05-28 19:04           ` Linus Torvalds
2020-05-28 19:17             ` Eric W. Biederman
2020-05-28 15:42         ` [PATCH 02/11] exec: Introduce active_per_clear the per file version of per_clear Eric W. Biederman
2020-05-28 19:05           ` Linus Torvalds
2020-05-28 15:42         ` [PATCH 03/11] exec: Compute file based creds only once Eric W. Biederman
2020-05-28 15:43         ` [PATCH 04/11] exec: Move uid/gid handling from creds_from_file into bprm_fill_uid Eric W. Biederman
2020-05-28 15:44         ` Eric W. Biederman
2020-05-28 15:44         ` [PATCH 05/11] exec: In bprm_fill_uid use CAP_SETGID to see if a gid change is safe Eric W. Biederman
2020-05-28 15:48         ` [PATCH 06/11] exec: Don't set secureexec when the uid or gid changes are abandoned Eric W. Biederman
2020-05-28 15:48         ` [PATCH 07/11] exec: Set saved, fs, and effective ids together in bprm_fill_uid Eric W. Biederman
2020-05-28 15:49         ` [PATCH 08/11] exec: In bprm_fill_uid remove unnecessary no new privs check Eric W. Biederman
2020-05-28 15:49         ` [PATCH 09/11] exec: In bprm_fill_uid only set per_clear when honoring suid or sgid Eric W. Biederman
2020-05-28 19:08           ` Linus Torvalds
2020-05-28 19:21             ` Eric W. Biederman
2020-05-28 15:50         ` [PATCH 10/11] exec: In bprm_fill_uid set secureexec at same time as per_clear Eric W. Biederman
2020-05-28 15:50         ` [PATCH 11/11] exec: Remove the label after_setid from bprm_fill_uid Eric W. Biederman
2020-05-29 16:45         ` [PATCH 0/2] exec: Remove the computation of bprm->cred Eric W. Biederman
2020-05-29 16:46           ` [PATCH 1/2] exec: Add a per bprm->file version of per_clear Eric W. Biederman
2020-05-29 21:06             ` Kees Cook
2020-05-30  3:23               ` Eric W. Biederman
2020-05-30  5:14                 ` Kees Cook
2020-05-29 16:47           ` [PATCH 2/2] exec: Compute file based creds only once Eric W. Biederman
2020-05-29 21:24             ` Kees Cook
2020-05-30  3:28               ` Eric W. Biederman
2020-05-30  5:18                 ` Kees Cook

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=202005191144.E3112135@keescook \
    --to=keescook@chromium.org \
    --cc=adobriyan@gmail.com \
    --cc=akpm@linux-foundation.org \
    --cc=bernd.edlinger@hotmail.de \
    --cc=casey@schaufler-ca.com \
    --cc=ebiederm@xmission.com \
    --cc=gerg@linux-m68k.org \
    --cc=jannh@google.com \
    --cc=jmorris@namei.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-security-module@vger.kernel.org \
    --cc=luto@amacapital.net \
    --cc=oleg@redhat.com \
    --cc=rob@landley.net \
    --cc=serge@hallyn.com \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@ZenIV.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).