linux-kernel - [PATCH v2 1/8] exec: Correct comments about "point of no return"

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Date:   Mon, 10 Jul 2017 00:57:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        John Johansen <john.johansen@...onical.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Michal Hocko <mhocko@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Paul Moore <paul@...l-moore.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov
Subject: [PATCH v2 1/8] exec: Correct comments about "point of no return"

In commit 221af7f87b97 ("Split 'flush_old_exec' into two functions"),
the comment about the point of no return should have stayed in
flush_old_exec() since it refers to "bprm->mm = NULL;" line, but prior
changes in commits c89681ed7d0e ("remove steal_locks()"), and
fd8328be874f ("sanitize handling of shared descriptor tables in failing
execve()") made it look like it meant the current->sas_ss_sp line instead.

The comment is referring to the fact that once bprm->mm is NULL, all
failures from a binfmt load_binary hook (e.g. load_elf_binary), will
get SEGV raised against current. Move this comment and expand the
explanation a bit, putting it above the assignment this time.

This also removes an erroneous commet about when credentials are being
installed. That has its own dedicated function, install_exec_creds(),
which carries a similar (and correct) comment, so remove the bogus comment
where installation is not actually happening.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/exec.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 904199086490..7842ae661e34 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1285,7 +1285,14 @@ int flush_old_exec(struct linux_binprm * bprm)
 	if (retval)
 		goto out;
 
-	bprm->mm = NULL;		/* We're using it now */
+	/*
+	 * After clearing bprm->mm (to mark that current is using the
+	 * prepared mm now), we are at the point of no return. If
+	 * anything from here on returns an error, the check in
+	 * search_binary_handler() will kill current (since the mm has
+	 * been replaced).
+	 */
+	bprm->mm = NULL;
 
 	set_fs(USER_DS);
 	current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
@@ -1332,7 +1339,6 @@ void setup_new_exec(struct linux_binprm * bprm)
 {
 	arch_pick_mmap_layout(current->mm);
 
-	/* This is the point of no return */
 	current->sas_ss_sp = current->sas_ss_size = 0;
 
 	if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid()))
@@ -1350,7 +1356,6 @@ void setup_new_exec(struct linux_binprm * bprm)
 	 */
 	current->mm->task_size = TASK_SIZE;
 
-	/* install the new credentials */
 	if (!uid_eq(bprm->cred->uid, current_euid()) ||
 	    !gid_eq(bprm->cred->gid, current_egid())) {
 		current->pdeath_signal = 0;
-- 
2.7.4