On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote: > On 2019-09-06, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Fri, 2019-09-06 at 18:06 +0200, Mickaël Salaün wrote: > > > On 06/09/2019 17:56, Florian Weimer wrote: > > > > Let's assume I want to add support for this to the glibc dynamic loader, > > > > while still being able to run on older kernels. > > > > > > > > Is it safe to try the open call first, with O_MAYEXEC, and if that fails > > > > with EINVAL, try again without O_MAYEXEC? > > > > > > The kernel ignore unknown open(2) flags, so yes, it is safe even for > > > older kernel to use O_MAYEXEC. > > > > > > > Well...maybe. What about existing programs that are sending down bogus > > open flags? Once you turn this on, they may break...or provide a way to > > circumvent the protections this gives. > > It should be noted that this has been a valid concern for every new O_* > flag introduced (and yet we still introduced new flags, despite the > concern) -- though to be fair, O_TMPFILE actually does have a > work-around with the O_DIRECTORY mask setup. > > The openat2() set adds O_EMPTYPATH -- though in fairness it's also > backwards compatible because empty path strings have always given ENOENT > (or EINVAL?) while O_EMPTYPATH is a no-op non-empty strings. > > > Maybe this should be a new flag that is only usable in the new openat2() > > syscall that's still under discussion? That syscall will enforce that > > all flags are recognized. You presumably wouldn't need the sysctl if you > > went that route too. > > I'm also interested in whether we could add an UPGRADE_NOEXEC flag to > how->upgrade_mask for the openat2(2) patchset (I reserved a flag bit for > it, since I'd heard about this work through the grape-vine). > I rather like the idea of having openat2 fds be non-executable by default, and having userland request it specifically via O_MAYEXEC (or some similar openat2 flag) if it's needed. Then you could add an UPGRADE_EXEC flag instead? That seems like something reasonable to do with a brand new API, and might be very helpful for preventing certain classes of attacks. -- Jeff Layton <jlayton@xxxxxxxxxx>
