linux-kernel - [PATCH 3/4] exec: Relocate path

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Date:   Sun, 17 May 2020 22:54:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Eric Biggers <ebiggers3@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 3/4] exec: Relocate path_noexec() check

The path_noexec() check, like the regular file check, was happening too
late, letting LSMs see impossible execve()s. Check it earlier as well
and collect the redundant fs/exec.c path_noexec() test under the same
robustness comment as the S_ISREG() check.

My notes on the call path, and related arguments, checks, etc:

do_open_execat()
    struct open_flags open_exec_flags = {
        .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC, ...
    do_filp_open(dfd, filename, open_flags)
        path_openat(nameidata, open_flags, flags)
            /* f_mode populated from open_flags in alloc_empty_file() */
            file = alloc_empty_file(open_flags, current_cred());
            do_open(nameidata, file, open_flags)
                /* new location of FMODE_EXEC vs path_noexec() test */
                may_open(path, acc_mode, open_flag)
                    inode_permission(inode, MAY_OPEN | acc_mode)
                        security_inode_permission(inode, acc_mode)
                vfs_open(path, file)
                    do_dentry_open(file, path->dentry->d_inode, open)
                        security_file_open(f)
                        open()
    /* old location of path_noexec() test */

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/exec.c  | 6 ++----
 fs/namei.c | 5 +++--
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index f0c80a8b9ccd..a34093323aa1 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -140,13 +140,12 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
 		goto out;
 
 	/*
-	 * do_open() has already checked for this, but we can be extra
+	 * do_open() has already checked for these, but we can be extra
 	 * cautious and check again at the very end too.
 	 */
 	error = -EACCES;
 	if (!S_ISREG(file_inode(file)->i_mode))
 		goto exit;
-
 	if (path_noexec(&file->f_path))
 		goto exit;
 
@@ -865,13 +864,12 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
 		goto out;
 
 	/*
-	 * do_open() has already checked for this, but we can be extra
+	 * do_open() has already checked for these, but we can be extra
 	 * cautious and check again at the very end too.
 	 */
 	err = -EACCES;
 	if (!S_ISREG(file_inode(file)->i_mode))
 		goto exit;
-
 	if (path_noexec(&file->f_path))
 		goto exit;
 
diff --git a/fs/namei.c b/fs/namei.c
index b9408aacaaa4..6bb1b6624bad 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3212,8 +3212,9 @@ static int do_open(struct nameidata *nd,
 	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
 		return -ENOTDIR;
 
-	/* Any file opened for execution has to be a regular file. */
-	if ((file->f_flags & FMODE_EXEC) && !d_is_reg(nd->path.dentry))
+	/* Opening for execution requires a regular file on an exec mnt. */
+	if ((file->f_flags & FMODE_EXEC) && (!d_is_reg(nd->path.dentry) ||
+					     path_noexec(&nd->path)))
 		return -EACCES;
 
 	do_truncate = false;
-- 
2.20.1