[request for 3.10 inclusion][PATCH 1/3] fs: take i_mutex during prepare_binprm for set[ug]id executables

[Sheng Yong <shengyong1@xxxxxxxxxx>]

From: Jann Horn <jann@xxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[shengyong: backport to 3.10
 - FIX: CVE-2015-3339
 - adjust context
 - task_no_new_privs() was added in commit 1d4457f ('sched: move no_new_privs
   into new atomic flags), which is not availiable in 3.10-stable, so we
   still use task_struct->no_new_privs
 - READ_ONCE() was added in v3.19, so use ACCESS_ONCE instead of READ_ONCE
]
Signed-off-by: Sheng Yong <shengyong1@xxxxxxxxxx>
---
 fs/exec.c | 79 ++++++++++++++++++++++++++++++++++++++-------------------------
 1 file changed, 48 insertions(+), 31 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index dd6aa61..8507abd 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1265,6 +1265,53 @@ 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
@@ -1273,39 +1320,9 @@ static int check_unsafe_exec(struct linux_binprm *bprm)
  */
 int prepare_binprm(struct linux_binprm *bprm)
 {
-	umode_t mode;
-	struct inode * inode = file_inode(bprm->file);
 	int retval;
 
-	mode = inode->i_mode;
-	if (bprm->file->f_op == NULL)
-		return -EACCES;
-
-	/* 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);
-- 
1.8.3.4

--
To unsubscribe from this list: send the line "unsubscribe stable" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html