exec: move S_ISREG() check earlier

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 into the other inode type checks (which already look for
other pathological conditions[1]).  Since there is no need to use
FMODE_EXEC while we still have access to "acc_mode", also switch the test
to MAY_EXEC.

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.

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,
        .acc_mode = MAY_EXEC,
        ...
    do_filp_open(dfd, filename, open_flags)
        path_openat(nameidata, open_flags, flags)
            file = alloc_empty_file(open_flags, current_cred());
            do_open(nameidata, file, open_flags)
                may_open(path, acc_mode, open_flag)
		    /* new location of MAY_EXEC vs S_ISREG() test */
                    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()

[1] https://lore.kernel.org/lkml/202006041910.9EF0C602@keescook/

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Link: http://lkml.kernel.org/r/20200605160013.3954297-3-keescook@chromium.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/exec.c b/fs/exec.c
index c56aad3070695f5ba06ec54c3e6df486c43fd0e9..cc7ec7b9000bb013ee8609f67abcdc5340249e23 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -141,8 +141,13 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
        if (IS_ERR(file))
                goto out;
 
+       /*
+        * may_open() has already checked for this, so it should be
+        * impossible to trip now. But we need to be extra cautious
+        * and check again at the very end too.
+        */
        error = -EACCES;
-       if (!S_ISREG(file_inode(file)->i_mode))
+       if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
                goto exit;
 
        if (path_noexec(&file->f_path))
@@ -908,8 +913,13 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
        if (IS_ERR(file))
                goto out;
 
+       /*
+        * may_open() has already checked for this, so it should be
+        * impossible to trip now. But we need to be extra cautious
+        * and check again at the very end too.
+        */
        err = -EACCES;
-       if (!S_ISREG(file_inode(file)->i_mode))
+       if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
                goto exit;
 
        if (path_noexec(&file->f_path))

diff --git a/fs/namei.c b/fs/namei.c
index fde8fe086c090d8a4fe4b8c5ca7bb67d5ece3f77..9cc3b90abdffd01ee10107bf68dec89e2725416a 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2849,16 +2849,18 @@ static int may_open(const struct path *path, int acc_mode, int flag)
        case S_IFLNK:
                return -ELOOP;
        case S_IFDIR:
-               if (acc_mode & MAY_WRITE)
+               if (acc_mode & (MAY_WRITE | MAY_EXEC))
                        return -EISDIR;
                break;
        case S_IFBLK:
        case S_IFCHR:
                if (!may_open_dev(path))
                        return -EACCES;
-               /*FALLTHRU*/
+               fallthrough;
        case S_IFIFO:
        case S_IFSOCK:
+               if (acc_mode & MAY_EXEC)
+                       return -EACCES;
                flag &= ~O_TRUNC;
                break;
        }

diff --git a/fs/open.c b/fs/open.c
index c80e9f497e9bfee13a417a692d23a88eb25a2f60..9af548fb841b00aa5f5c9eb34f7e36150e090cfd 100644 (file)
--- a/fs/open.c
+++ b/fs/open.c
@@ -779,12 +779,6 @@ 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))