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    SubjectRe: [PATCH v7 2/7] exec: Move S_ISREG() check earlier
    Mickaël Salaün <mic@digikod.net> writes:

    > From: Kees Cook <keescook@chromium.org>
    >
    > 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.

    The comment is:
    > + /*
    > + * 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.
    > + */
    Those comments scare me. Why do you need to be extra cautious?
    How can the file type possibly change between may_open and anywhere?
    The type of a file is immutable after it's creation.

    If the comment said check just in case something went wrong with
    code maintenance I could understand but that isn't what the comment
    says.

    Also the fallthrough change below really should be broken out into
    it's own change.


    > 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: Mickaël Salaün <mic@digikod.net>
    > Signed-off-by: Kees Cook <keescook@chromium.org>
    > Link: https://lore.kernel.org/r/20200605160013.3954297-3-keescook@chromium.org
    > ---
    > fs/exec.c | 14 ++++++++++++--
    > fs/namei.c | 6 ++++--
    > fs/open.c | 6 ------
    > 3 files changed, 16 insertions(+), 10 deletions(-)
    >
    > diff --git a/fs/exec.c b/fs/exec.c
    > index d7c937044d10..bdc6a6eb5dce 100644
    > --- 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))
    > @@ -886,8 +891,13 @@ 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 72d4219c93ac..a559ad943970 100644
    > --- 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;
    ^^^^^^^^^^^
    That is an unrelated change and should be sent separately.

    > 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 6cd48a61cda3..623b7506a6db 100644
    > --- a/fs/open.c
    > +++ b/fs/open.c
    > @@ -784,12 +784,6 @@ 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))

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