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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 2/5] exec: Directly call security_bprm_set_creds from __do_execve_file
Date: Mon, 11 May 2020 14:18:22 -0700
Message-ID: <202005111245.6E390B46@keescook> (raw)
In-Reply-To: <87y2pytnvq.fsf@x220.int.ebiederm.org>

On Mon, May 11, 2020 at 11:52:41AM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > On Sat, May 09, 2020 at 02:41:17PM -0500, Eric W. Biederman wrote:
> >> 
> >> Now that security_bprm_set_creds is no longer responsible for calling
> >> cap_bprm_set_creds, security_bprm_set_creds only does something for
> >> the primary file that is being executed (not any interpreters it may
> >> have).  Therefore call security_bprm_set_creds from __do_execve_file,
> >> instead of from prepare_binprm so that it is only called once, and
> >> remove the now unnecessary called_set_creds field of struct binprm.
> >> 
> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >> ---
> >>  fs/exec.c                  | 11 +++++------
> >>  include/linux/binfmts.h    |  6 ------
> >>  security/apparmor/domain.c |  3 ---
> >>  security/selinux/hooks.c   |  2 --
> >>  security/smack/smack_lsm.c |  3 ---
> >>  security/tomoyo/tomoyo.c   |  6 ------
> >>  6 files changed, 5 insertions(+), 26 deletions(-)
> >> 
> >> diff --git a/fs/exec.c b/fs/exec.c
> >> index 765bfd51a546..635b5085050c 100644
> >> --- a/fs/exec.c
> >> +++ b/fs/exec.c
> >> @@ -1635,12 +1635,6 @@ int prepare_binprm(struct linux_binprm *bprm)
> >>  
> >>  	bprm_fill_uid(bprm);
> >>  
> >> -	/* fill in binprm security blob */
> >> -	retval = security_bprm_set_creds(bprm);
> >> -	if (retval)
> >> -		return retval;
> >> -	bprm->called_set_creds = 1;
> >> -
> >>  	retval = cap_bprm_set_creds(bprm);
> >>  	if (retval)
> >>  		return retval;
> >> @@ -1858,6 +1852,11 @@ static int __do_execve_file(int fd, struct filename *filename,
> >>  	if (retval < 0)
> >>  		goto out;
> >>  
> >> +	/* fill in binprm security blob */
> >> +	retval = security_bprm_set_creds(bprm);
> >> +	if (retval)
> >> +		goto out;
> >> +
> >>  	retval = prepare_binprm(bprm);
> >>  	if (retval < 0)
> >>  		goto out;
> >> 
> >
> > Here I go with a Sunday night review, so hopefully I'm thinking better
> > than Friday night's review, but I *think* this patch is broken from
> > the LSM sense of the world in that security_bprm_set_creds() is getting
> > called _before_ the creds actually get fully set (in prepare_binprm()
> > by the calls to bprm_fill_uid(), cap_bprm_set_creds(), and
> > check_unsafe_exec()).
> >
> > As a specific example, see the setting of LSM_UNSAFE_NO_NEW_PRIVS in
> > bprm->unsafe during check_unsafe_exec(), which must happen after
> > bprm_fill_uid(bprm) and cap_bprm_set_creds(bprm), to have a "true" view
> > of the execution privileges. Apparmor checks for this flag in its
> > security_bprm_set_creds() hook. Similarly do selinux, smack, etc...
> 
> I think you are getting prepare_binprm confused with prepare_bprm_creds.
> Understandable given the similarity of their names.

I fixated on a bad example, having confused myself about when
check_unsafe_exec() happens. My original concern (with the bad example)
was that the LSM is having security_bprm_set_creds() called before the
new cred in bprm->cred has been initialized with all the correct uid/gid,
caps, and associated flags.

But anything associated with capabilities should be confined to the
commoncap LSM, though there is "leakage" into the uid/gid states and some
bprm state (more on this later). That said, as you also found, I can't
find any LSM that examines those fields of the cred (I had stopped this
research last night when I saw check_unsafe_exec() and confused myself);
they're all looking at other bprm state not associated with caps and uid
changes (file, unsafe_exec, security field of new cred, etc). So that's
very good! That means we've actually kept a bright line between things
here -- whew.

> > The security_bprm_set_creds() boundary for LSM is to see the "final"
> > state of the process privileges, and that needs to happen after
> > bprm_fill_uid(), cap_bprm_set_creds(), and check_unsafe_exec() have all
> > finished.
> >
> > So, as it stands, I don't think this will work, but perhaps it can still
> > be rearranged to avoid the called_set_creds silliness. I'll look more
> > this week...
> 
> If you look at the flow of the code in __do_execve_file before this
> change it is:
> 
> 	prepare_bprm_creds()
>         check_unsafe_exec()
> 
> 	...
> 
>         prepare_binprm()
>         	bprm_file_uid()

(bprm_fill_uid(), but yes)

>                 	bprm->cred->euid = current_euid()
>                         bprm->cred->egid = current_egid()
> 		security_bprm_set_creds()
>                 	for_each_lsm()
>                         	lsm->bprm_set_creds()
>                                 	if (called_set_creds)
>                                         	return;
>                                         ...
> 		bprm->called_set_creds = 1;
> 	...
> 
> 	exec_binprm()
>         	search_binary_handler()
>                 	security_bprm_check()
>                         	tomoyo_bprm_check_security()
>                                 ima_bprm_check()
>    			load_script()
>                         	prepare_binprm()
>                                 	/* called_set_creds already == 1 */
>                                 	bprm_file_uid()
>                                         security_bprm_set_creds()
> 			                	for_each_lsm()
> 			                        	lsm->bprm_set_creds()
> 		                                	if (called_set_creds)
>                 		                        	return;
>                                 		        ...
>                                 search_binary_handler()
>                                 	security_bprm_check_security()
>                                         load_elf_binary()
>                                         	...
>                                                 setup_new_exec
>                                                 ...
> 
> 
> Assuming you are executing a shell script.
> 
> Now bprm_file_uid is written with the assumption that it will be called
> multiple times and it reinitializes all of it's variables each time.

Right -- and the same is true for cap_bprm_set_creds() (in that
it needs to be run multiple times and depends on the work done in
bprm_fill_uid()). If we encounter a future use-case for having other
LSMs call out here multiple time, we can introduce a new LSM hook.

> As you can see in above the implementations of bprm_set_creds() only
> really execute before called_set_creds is set, aka the first time.
> They in no way see the final state.
> 
> Further when I looked as those hooks they were not looking at the values
> set by bprm_file_uid at all.  There were busy with the values their
> they needed to set in that hook for their particular lsm.

Agreed (though I'd love some other LSM eyes on this conclusion).

> So while in theory I can see the danger of moving above bprm_file_uid
> I don't see anything in practice that would be a problem.
> 
> Further by moving the call of security_bprm_set_creds out of
> prepare_binprm int __do_execve_file just before the call of
> prepare_binprm I am just moving the call above binprm_fill_uid
> and nothing else.
> 
> So I think you just confused prepare_bprm_creds with prepare_binprm.
> As most of your criticisms appear valid in that case.  Can you take a
> second look?

So, in earlier attempts to clean up code near all this, I removed the
LSM's bprm_secureexec hook, which only commoncap was using to impart
details about privilege elevation. I switched the semantics to having LSMs
set bprm->secureexec to true (but never to zero). Since commoncap's idea
of "was I elevated?" might repeatedly change, I had to store its results
"privately" in the bprm, which got us cap_elevated (in 46d98eb4e1d2):

c425e189ffd7 ("binfmt: Introduce secureexec flag")
993b3ab0642e ("apparmor: Refactor to remove bprm_secureexec hook")
62874c3adf70 ("selinux: Refactor to remove bprm_secureexec hook")
46d98eb4e1d2 ("commoncap: Refactor to remove bprm_secureexec hook")
ee67ae7ef6ff ("commoncap: Move cap_elevated calculation into bprm_set_creds")
2af622802696 ("LSM: drop bprm_secureexec hook")

So, given the special-case nature of capabilities here, this does seem
to be the right choice (assuming we're not missing something in the
other LSMs). As such, I think the comment for cap_elevated needs to be
updated to reflect the change to function call flow, and to specify it
cannot be used by the other LSMs. Maybe something like:

               /*
                * True if most recent call to cap_bprm_set_creds()
                * (due to multiple prepare_binprm() calls from the
                * binfmt_script/misc handlers) resulted in elevated
                * privileges. This is used internally by fs/exec.c
		* to set bprm->secureexec.
                */
               cap_elevated:1,

And that brings us to naming. Whee. I think we should make the following
name changes:

bprm_fill_uid      ->	bprm_establish_privileges
cap_bprm_set_creds ->	cap_establish_privileges

Finally, I think we should update the comment on bprm_set_creds (which,
actually, I think is the correct name now) to something like:

 * @bprm_set_creds:
 *	Save security information in the @bprm->cred->security field,
 *	typically based on information about the bprm->file, for later
 *	use during the @bprm_committing_creds hook. Specifically
 *	the credentials themselves (uid, gid, etc), are not finalized
 *	yet and must not be examined until the @bprm_committing_creds
 *	hook.
 *      This hook is called once, after the creds structure has been
 *	allocated.
 *      The hook must set @bprm->secureexec to 1 if a "secure exec"
 *	has happened as a result of this hook call. The flag is used to
 *      indicate the need for a sanitized execution environment, and is
 *      also passed in the ELF auxiliary table on the initial stack to
 *      indicate whether libc should enable secure mode.
 *	This hook may also optionally check LSM-specific permissions
 *	(e.g. for transitions between security domains).
 *      @bprm contains the linux_binprm structure.
 *      Return 0 if the hook is successful and permission is granted.

-Kees

-- 
Kees Cook

  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 [this message]
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
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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