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    SubjectRe: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()
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    On 05/08/2019 01:55, Andy Lutomirski wrote:
    > On Wed, Dec 12, 2018 at 6:43 AM Jan Kara <jack@suse.cz> wrote:
    >>
    >> On Wed 12-12-18 09:17:08, Mickaël Salaün wrote:
    >>> When the O_MAYEXEC flag is passed, sys_open() may be subject to
    >>> additional restrictions depending on a security policy implemented by an
    >>> LSM through the inode_permission hook.
    >>>
    >>> The underlying idea is to be able to restrict scripts interpretation
    >>> according to a policy defined by the system administrator. For this to
    >>> be possible, script interpreters must use the O_MAYEXEC flag
    >>> appropriately. To be fully effective, these interpreters also need to
    >>> handle the other ways to execute code (for which the kernel can't help):
    >>> command line parameters (e.g., option -e for Perl), module loading
    >>> (e.g., option -m for Python), stdin, file sourcing, environment
    >>> variables, configuration files... According to the threat model, it may
    >>> be acceptable to allow some script interpreters (e.g. Bash) to interpret
    >>> commands from stdin, may it be a TTY or a pipe, because it may not be
    >>> enough to (directly) perform syscalls.
    >>>
    >>> A simple security policy implementation is available in a following
    >>> patch for Yama.
    >>>
    >>> This is an updated subset of the patch initially written by Vincent
    >>> Strubel for CLIP OS:
    >>> https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch
    >>> This patch has been used for more than 10 years with customized script
    >>> interpreters. Some examples can be found here:
    >>> https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
    >>>
    >>> Signed-off-by: Mickaël Salaün <mic@digikod.net>
    >>> Signed-off-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
    >>> Signed-off-by: Vincent Strubel <vincent.strubel@ssi.gouv.fr>
    >>> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
    >>> Cc: Al Viro <viro@zeniv.linux.org.uk>
    >>> Cc: Kees Cook <keescook@chromium.org>
    >>> Cc: Mickaël Salaün <mickael.salaun@ssi.gouv.fr>
    >>
    >> ...
    >>
    >>> diff --git a/fs/open.c b/fs/open.c
    >>> index 0285ce7dbd51..75479b79a58f 100644
    >>> --- a/fs/open.c
    >>> +++ b/fs/open.c
    >>> @@ -974,6 +974,10 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
    >>> if (flags & O_APPEND)
    >>> acc_mode |= MAY_APPEND;
    >>>
    >>> + /* Check execution permissions on open. */
    >>> + if (flags & O_MAYEXEC)
    >>> + acc_mode |= MAY_OPENEXEC;
    >>> +
    >>> op->acc_mode = acc_mode;
    >>>
    >>> op->intent = flags & O_PATH ? 0 : LOOKUP_OPEN;
    >>
    >> I don't feel experienced enough in security to tell whether we want this
    >> functionality or not. But if we do this, shouldn't we also set FMODE_EXEC
    >> on the resulting struct file? That way also security_file_open() can be
    >> used to arbitrate such executable opens and in particular
    >> fanotify permission event FAN_OPEN_EXEC will get properly generated which I
    >> guess is desirable (support for it is sitting in my tree waiting for the
    >> merge window) - adding some audit people involved in FAN_OPEN_EXEC to
    >> CC. Just an idea...
    >>
    >
    > I would really like to land this patch. I'm fiddling with making
    > bpffs handle permissions intelligently, and the lack of a way to say
    > "hey, I want to open this bpf program so that I can run it" is
    > annoying.

    Are you OK with this series? What about Aleksa's work on openat2(), and
    Sean's work on SGX/noexec? Is it time to send a new patch series (with a
    dedicated LSM instead of Yama)?

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