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 [thread overview]
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
next prev parent reply other threads:[~2020-05-11 21:18 UTC|newest]
Thread overview: 149+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-05 19:39 exec: Promised cleanups after introducing exec_update_mutex Eric W. Biederman
2020-05-05 19:41 ` [PATCH 1/7] binfmt: Move install_exec_creds after setup_new_exec to match binfmt_elf Eric W. Biederman
2020-05-05 20:45 ` Kees Cook
2020-05-06 12:42 ` Greg Ungerer
2020-05-06 12:56 ` Eric W. Biederman
2020-05-05 19:41 ` [PATCH 2/7] exec: Make unlocking exec_update_mutex explict Eric W. Biederman
2020-05-05 20:46 ` Kees Cook
2020-05-05 19:42 ` [PATCH 3/7] exec: Rename the flag called_exec_mmap point_of_no_return Eric W. Biederman
2020-05-05 20:49 ` Kees Cook
2020-05-05 19:43 ` [PATCH 4/7] exec: Merge install_exec_creds into setup_new_exec Eric W. Biederman
2020-05-05 20:50 ` Kees Cook
2020-05-05 19:44 ` [PATCH 5/7] exec: In setup_new_exec cache current in the local variable me Eric W. Biederman
2020-05-05 20:51 ` Kees Cook
2020-05-05 19:45 ` [PATCH 6/7] exec: Move most of setup_new_exec into flush_old_exec Eric W. Biederman
2020-05-05 21:29 ` Kees Cook
2020-05-06 14:57 ` Eric W. Biederman
2020-05-06 15:30 ` Kees Cook
2020-05-07 19:51 ` Eric W. Biederman
2020-05-07 21:51 ` Eric W. Biederman
2020-05-08 5:50 ` Kees Cook
2020-05-05 19:46 ` [PATCH 7/7] exec: Rename flush_old_exec begin_new_exec Eric W. Biederman
2020-05-05 21:30 ` Kees Cook
2020-05-06 12:41 ` exec: Promised cleanups after introducing exec_update_mutex Greg Ungerer
2020-05-08 18:43 ` [PATCH 0/6] exec: Trivial cleanups for exec Eric W. Biederman
2020-05-08 18:44 ` [PATCH 1/6] exec: Move the comment from above de_thread to above unshare_sighand Eric W. Biederman
2020-05-09 5:02 ` Kees Cook
2020-05-08 18:44 ` [PATCH 2/6] exec: Fix spelling of search_binary_handler in a comment Eric W. Biederman
2020-05-09 5:03 ` Kees Cook
2020-05-08 18:45 ` [PATCH 3/6] exec: Stop open coding mutex_lock_killable of cred_guard_mutex Eric W. Biederman
2020-05-09 5:08 ` Kees Cook
2020-05-09 19:18 ` Linus Torvalds
2020-05-09 19:57 ` Eric W. Biederman
2020-05-10 20:33 ` Kees Cook
2020-05-08 18:45 ` [PATCH 4/6] exec: Run sync_mm_rss before taking exec_update_mutex Eric W. Biederman
2020-05-09 5:15 ` Kees Cook
2020-05-09 14:17 ` Eric W. Biederman
2020-05-08 18:47 ` [PATCH 5/6] exec: Move handling of the point of no return to the top level Eric W. Biederman
2020-05-09 5:31 ` Kees Cook
2020-05-09 13:39 ` Eric W. Biederman
2020-05-08 18:48 ` [PATCH 6/6] exec: Set the point of no return sooner Eric W. Biederman
2020-05-09 5:33 ` Kees Cook
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
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=202005111245.6E390B46@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).