Date: Sun, 4 Aug 2019 16:55:58 -0700 From: Andy Lutomirski <luto@...nel.org> To: Jan Kara <jack@...e.cz>, Song Liu <songliubraving@...com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net> Cc: Mickaël Salaün <mic@...ikod.net>, LKML <linux-kernel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>, James Morris <jmorris@...ei.org>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Matthew Garrett <mjg59@...gle.com>, Michael Kerrisk <mtk.manpages@...il.com>, Mickaël Salaün <mickael.salaun@....gouv.fr>, Mimi Zohar <zohar@...ux.ibm.com>, Philippe Trébuchet <philippe.trebuchet@....gouv.fr>, Shuah Khan <shuah@...nel.org>, Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>, Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>, LSM List <linux-security-module@...r.kernel.org>, Linux FS Devel <linux-fsdevel@...r.kernel.org>, Steve Grubb <sgrubb@...hat.com>, Matthew Bobrowski <mbobrowski@...browski.org> Subject: Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() On Wed, Dec 12, 2018 at 6:43 AM Jan Kara <jack@...e.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@...ikod.net> > > Signed-off-by: Thibaut Sautereau <thibaut.sautereau@....gouv.fr> > > Signed-off-by: Vincent Strubel <vincent.strubel@....gouv.fr> > > Reviewed-by: Philippe Trébuchet <philippe.trebuchet@....gouv.fr> > > Cc: Al Viro <viro@...iv.linux.org.uk> > > Cc: Kees Cook <keescook@...omium.org> > > Cc: Mickaël Salaün <mickael.salaun@....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. --Andy
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