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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id w75sm7834318pfc.156.2020.05.10.20.15.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 10 May 2020 20:15:35 -0700 (PDT) Date: Sun, 10 May 2020 20:15:34 -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: <202005101929.A4374D0F56@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87k11kzyjm.fsf_-_@x220.int.ebiederm.org> Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: 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... 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... -Kees -- Kees Cook