linux-kernel - [PATCH] exec: Make suid_dumpable apply to SUID/SGID binaries irrespective of invoking users

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Date:   Mon, 20 Dec 2021 21:17:44 -0500
From:   Waiman Long <longman@...hat.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Laurent Vivier <laurent@...ier.eu>,
        YunQiang Su <ysu@...ecomp.com>, Helge Deller <deller@....de>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] exec: Make suid_dumpable apply to SUID/SGID binaries irrespective of invoking users

The begin_new_exec() function checks for SUID or SGID binaries by
comparing effective uid and gid against real uid and gid and using
the suid_dumpable sysctl parameter setting only if either one of them
differs.

In the special case that the uid and/or gid of the SUID/SGID binaries
matches the id's of the user invoking it, the suid_dumpable is not
used and SUID_DUMP_USER will be used instead. The documentation for the
suid_dumpable sysctl parameter does not include that exception and so
this will be an undocumented behavior.

Eliminate this undocumented behavior by adding a flag in the linux_binprm
structure to designate a SUID/SGID binary and use it for determining
if the suid_dumpable setting should be applied or not.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 fs/exec.c               | 6 +++---
 include/linux/binfmts.h | 5 ++++-
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 537d92c41105..60e02e678fb6 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1344,9 +1344,7 @@ int begin_new_exec(struct linux_binprm * bprm)
 	 * is wrong, but userspace depends on it. This should be testing
 	 * bprm->secureexec instead.
 	 */
-	if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP ||
-	    !(uid_eq(current_euid(), current_uid()) &&
-	      gid_eq(current_egid(), current_gid())))
+	if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP || bprm->is_sugid)
 		set_dumpable(current->mm, suid_dumpable);
 	else
 		set_dumpable(current->mm, SUID_DUMP_USER);
@@ -1619,11 +1617,13 @@ static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
 	if (mode & S_ISUID) {
 		bprm->per_clear |= PER_CLEAR_ON_SETID;
 		bprm->cred->euid = uid;
+		bprm->is_sugid = 1;
 	}
 
 	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
 		bprm->per_clear |= PER_CLEAR_ON_SETID;
 		bprm->cred->egid = gid;
+		bprm->is_sugid = 1;
 	}
 }
 
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 049cf9421d83..6d9893c59085 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -41,7 +41,10 @@ struct linux_binprm {
 		 * Set when errors can no longer be returned to the
 		 * original userspace.
 		 */
-		point_of_no_return:1;
+		point_of_no_return:1,
+
+		/* Is a SUID or SGID binary? */
+		is_sugid:1;
 #ifdef __alpha__
 	unsigned int taso:1;
 #endif
-- 
2.27.0