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From: ebiederm@xmission.com (Eric W. Biederman)
To: Kees Cook <keescook@chromium.org>
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 14:19:56 -0500
Message-ID: <87zha3n34z.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <202005191144.E3112135@keescook> (Kees Cook's message of "Tue, 19 May 2020 12:08:25 -0700")

Kees Cook <keescook@chromium.org> writes:

> On Mon, May 18, 2020 at 07:33:21PM -0500, Eric W. Biederman wrote:
>> 
>> 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.

I think the best way to document this is to modify bprm->buf to be
"const char buf[BINPRM_BUF_SIZE]" or something like that and not
allow any modifications by anything except for the code that
initially reads in contets of the file.

That unfortunately requires copy_strings_kernel which has become
copy_string_kernel to take a length.  Then I don't need to modify the
buffer at all here.

I believe binfmt_scripts is a bit unique in wanting to modify the buffer
because it is parsing strings.

The requirement is that a binfmt should not modify bprm unless it will
succeed or fail with an error that is not -ENOEXEC.  The fundamental
issue is that search_binary_handler will reuse bprm if -ENOEXEC is
returned.

Until the next patch there is an escape hatch by clearing and closing
bprm->file but that goes away.  Which is why I need this patch.

I guess I can see adding a comment about the general case of not
changing bprm unless you are doing something other than returning
-ENOEXEC and letting the search continue.

Eric


>> [...]
>> 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>

  reply index

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
2020-05-19 19:19           ` Eric W. Biederman [this message]
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

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