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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id a23sm9923384pfo.145.2020.05.11.14.18.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 May 2020 14:18:23 -0700 (PDT) Date: Mon, 11 May 2020 14:18:22 -0700 From: Kees Cook To: "Eric W. Biederman" Cc: linux-kernel@vger.kernel.org, Linus Torvalds , Oleg Nesterov , Jann Horn , Greg Ungerer , Rob Landley , Bernd Edlinger , linux-fsdevel@vger.kernel.org, Al Viro , Alexey Dobriyan , Andrew Morton , Casey Schaufler , linux-security-module@vger.kernel.org, James Morris , "Serge E. Hallyn" , Andy Lutomirski Subject: Re: [PATCH 2/5] exec: Directly call security_bprm_set_creds from __do_execve_file Message-ID: <202005111245.6E390B46@keescook> References: <87h7wujhmz.fsf@x220.int.ebiederm.org> <87sgga6ze4.fsf@x220.int.ebiederm.org> <87v9l4zyla.fsf_-_@x220.int.ebiederm.org> <87k11kzyjm.fsf_-_@x220.int.ebiederm.org> <202005101929.A4374D0F56@keescook> <87y2pytnvq.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87y2pytnvq.fsf@x220.int.ebiederm.org> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, May 11, 2020 at 11:52:41AM -0500, Eric W. Biederman wrote: > Kees Cook 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" > >> --- > >> 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