linux-kernel - [PATCH 3.13.y-ckt 20/90] fs: take i_mutex during prepare

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Date:	Tue,  5 May 2015 13:53:52 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Jann Horn <jann@...jh.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Luis Henriques <luis.henriques@...onical.com>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.13.y-ckt 20/90] fs: take i_mutex during prepare_binprm for set[ug]id executables

3.13.11-ckt20 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jann@...jh.net>

commit 8b01fc86b9f425899f8a3a8fc1c47d73c2c20543 upstream.

This prevents a race between chown() and execve(), where chowning a
setuid-user binary to root would momentarily make the binary setuid
root.

This patch was mostly written by Linus Torvalds.

Signed-off-by: Jann Horn <jann@...jh.net>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[ luis: backported to 3.16:
  - relaced task_no_new_privs() by current->no_new_privs
  - replaced READ_ONCE() by ACCESS_ONCE() ]
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 fs/exec.c | 76 ++++++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 48 insertions(+), 28 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 5743b8f..f1a8244 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1267,6 +1267,53 @@ static int check_unsafe_exec(struct linux_binprm *bprm)
 	return res;
 }
 
+static void bprm_fill_uid(struct linux_binprm *bprm)
+{
+	struct inode *inode;
+	unsigned int mode;
+	kuid_t uid;
+	kgid_t gid;
+
+	/* clear any previous set[ug]id data from a previous binary */
+	bprm->cred->euid = current_euid();
+	bprm->cred->egid = current_egid();
+
+	if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
+		return;
+
+	if (current->no_new_privs)
+		return;
+
+	inode = file_inode(bprm->file);
+	mode = ACCESS_ONCE(inode->i_mode);
+	if (!(mode & (S_ISUID|S_ISGID)))
+		return;
+
+	/* Be careful if suid/sgid is set */
+	mutex_lock(&inode->i_mutex);
+
+	/* reload atomically mode/uid/gid now that lock held */
+	mode = inode->i_mode;
+	uid = inode->i_uid;
+	gid = inode->i_gid;
+	mutex_unlock(&inode->i_mutex);
+
+	/* We ignore suid/sgid if there are no mappings for them in the ns */
+	if (!kuid_has_mapping(bprm->cred->user_ns, uid) ||
+		 !kgid_has_mapping(bprm->cred->user_ns, gid))
+		return;
+
+	if (mode & S_ISUID) {
+		bprm->per_clear |= PER_CLEAR_ON_SETID;
+		bprm->cred->euid = uid;
+	}
+
+	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
+		bprm->per_clear |= PER_CLEAR_ON_SETID;
+		bprm->cred->egid = gid;
+	}
+}
+
 /* 
  * Fill the binprm structure from the inode. 
  * Check permissions, then read the first 128 (BINPRM_BUF_SIZE) bytes
@@ -1275,36 +1322,9 @@ static int check_unsafe_exec(struct linux_binprm *bprm)
  */
 int prepare_binprm(struct linux_binprm *bprm)
 {
-	struct inode *inode = file_inode(bprm->file);
-	umode_t mode = inode->i_mode;
 	int retval;
 
-
-	/* clear any previous set[ug]id data from a previous binary */
-	bprm->cred->euid = current_euid();
-	bprm->cred->egid = current_egid();
-
-	if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) &&
-	    !current->no_new_privs &&
-	    kuid_has_mapping(bprm->cred->user_ns, inode->i_uid) &&
-	    kgid_has_mapping(bprm->cred->user_ns, inode->i_gid)) {
-		/* Set-uid? */
-		if (mode & S_ISUID) {
-			bprm->per_clear |= PER_CLEAR_ON_SETID;
-			bprm->cred->euid = inode->i_uid;
-		}
-
-		/* Set-gid? */
-		/*
-		 * If setgid is set but no group execute bit then this
-		 * is a candidate for mandatory locking, not a setgid
-		 * executable.
-		 */
-		if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
-			bprm->per_clear |= PER_CLEAR_ON_SETID;
-			bprm->cred->egid = inode->i_gid;
-		}
-	}
+	bprm_fill_uid(bprm);
 
 	/* fill in binprm security blob */
 	retval = security_bprm_set_creds(bprm);
-- 
1.9.1

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