[PATCH 2/4] exec: Relocate S_ISREG() check

[Kees Cook]

The execve(2)/uselib(2) syscalls have always rejected non-regular
files. Recently, it was noticed that a deadlock was introduced when trying
to execute pipes, as the S_ISREG() test was happening too late. This was
fixed in commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files
during execve()"), but it was added after inode_permission() had already
run, which meant LSMs could see bogus attempts to execute non-regular
files. Move the test earlier.

Also include a comment with the redundant S_ISREG() checks at the end of
execve(2)/uselib(2) to note that they are present to avoid any mistakes.

Finally, instead of dereferencing the inode, use dcache for S_ISREG()
test.

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 S_ISREG() 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)
                        /* old location of FMODE_EXEC vs S_ISREG() test */
                        security_file_open(f)
                        open()

Signed-off-by: Kees Cook <keescook at chromium.org>
---
 fs/exec.c  | 8 ++++++++
 fs/namei.c | 4 ++++
 fs/open.c  | 6 ------
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 30735ce1dc0e..f0c80a8b9ccd 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -139,6 +139,10 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
 	if (IS_ERR(file))
 		goto out;
 
+	/*
+	 * do_open() has already checked for this, 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;
@@ -860,6 +864,10 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
 	if (IS_ERR(file))
 		goto out;
 
+	/*
+	 * do_open() has already checked for this, 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;
diff --git a/fs/namei.c b/fs/namei.c
index a320371899cf..b9408aacaaa4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3212,6 +3212,10 @@ 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))
+		return -EACCES;
+
 	do_truncate = false;
 	acc_mode = op->acc_mode;
 	if (file->f_mode & FMODE_CREATED) {
diff --git a/fs/open.c b/fs/open.c
index 719b320ede52..bb16e4e3cd57 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -753,12 +753,6 @@ static int do_dentry_open(struct file *f,
 		return 0;
 	}
 
-	/* Any file opened for execve()/uselib() has to be a regular file. */
-	if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) {
-		error = -EACCES;
-		goto cleanup_file;
-	}
-
 	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
 		error = get_write_access(inode);
 		if (unlikely(error))
-- 
2.20.1