linux-kernel - [PATCH 09/11] exec: In bprm_fill_uid only set per

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Date:   Thu, 28 May 2020 10:49:44 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Rob Landley <rob@...dley.net>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        <linux-fsdevel@...r.kernel.org>, Al Viro <viro@...IV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        linux-security-module@...r.kernel.org,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: [PATCH 09/11] exec: In bprm_fill_uid only set per_clear when honoring suid or sgid


It makes no sense to set active_per_clear when the kernel decides not
to honor the executables setuid or or setgid bits.  Instead set
active_per_clear when the kernel actually decides to honor the suid or
sgid permission bits of an executable.

As far as I can tell this was the intended behavior but with the
ptrace logic hiding out in security/commcap.c:cap_bprm_apply_creds I
believe it was just overlooked that the setuid or setgid operation
could be cancelled.

History Tree: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Fixes: 1bb0fa189c6a ("[PATCH] NX: clean up legacy binary support")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/exec.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index af108ecf9632..347dade4bc54 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1634,15 +1634,16 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
 	need_cap = bprm->unsafe & LSM_UNSAFE_SHARE ||
 		!ptracer_capable(current, new->user_ns);
 
-	if (mode & S_ISUID) {
+	if ((mode & S_ISUID) &&
+	    (!need_cap || ns_capable(new->user_ns, CAP_SETUID))) {
 		bprm->per_clear = 1;
-		if (!need_cap || ns_capable(new->user_ns, CAP_SETUID))
-			new->suid = new->fsuid = new->euid = uid;
+		new->suid = new->fsuid = new->euid = uid;
 	}
-	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
+
+	if (((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) &&
+	    (!need_cap || ns_capable(new->user_ns, CAP_SETGID))) {
 		bprm->per_clear = 1;
-		if (!need_cap || ns_capable(new->user_ns, CAP_SETGID))
-			new->sgid = new->fsgid = new->egid = gid;
+		new->sgid = new->fsgid = new->egid = gid;
 	}
 
 after_setid:
-- 
2.25.0