linux-kernel - [PATCH RFC 4/4] exec: only set the suid flag if the current proc isn't root

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Message-Id: <20211227223436.317091-5-wander@redhat.com>
Date:   Mon, 27 Dec 2021 19:34:35 -0300
From:   Wander Lairson Costa <wander@...hat.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        YunQiang Su <ysu@...ecomp.com>,
        Laurent Vivier <laurent@...ier.eu>,
        Wander Lairson Costa <wander@...hat.com>,
        Helge Deller <deller@....de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jens Axboe <axboe@...nel.dk>,
        Alexey Gladkov <legion@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rafael Aquini <aquini@...hat.com>,
        Phil Auld <pauld@...hat.com>, Rolf Eike Beer <eb@...ix.com>,
        Muchun Song <songmuchun@...edance.com>,
        linux-fsdevel@...r.kernel.org (open list:FILESYSTEMS (VFS and
        infrastructure)), linux-kernel@...r.kernel.org (open list)
Subject: [PATCH RFC 4/4] exec: only set the suid flag if the current proc isn't root

The goal of PF_SUID flag is to check if it is safe to coredump the
process. If the current process is already privileged, there is no
point in performing security checks because the name image is a
set-uid process.

Because of that, we don't set the suid flag if the forked process
already runs as root.

Signed-off-by: Wander Lairson Costa <wander@...hat.com>
---
 fs/exec.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index b4bd157a5282..d73b21b6298c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1312,7 +1312,11 @@ int begin_new_exec(struct linux_binprm * bprm)
 	me->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
 					PF_NOFREEZE | PF_NO_SETAFFINITY);
 
-	if (bprm->suid_bin)
+	/*
+	 * We set the PF_SUID flags for security reasons. There is no
+	 * point in setting it if the parent is root.
+	 */
+	if (bprm->suid_bin && !capable(CAP_SYS_ADMIN))
 		me->flags |= PF_SUID;
 
 	flush_thread();
-- 
2.27.0