On 2019-09-06, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > On Fri, 2019-09-06 at 13:06 -0700, Andy Lutomirski wrote: > > > On Sep 6, 2019, at 12:43 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > > > > 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. > > > > > > > > > > There are at least four concepts of executability here: > > > > - Just check the file mode and any other relevant permissions. Return a normal fd. Makes sense for script interpreters, perhaps. > > > > - Make the fd fexecve-able. > > > > - Make the resulting fd mappable PROT_EXEC. > > > > - Make the resulting fd upgradable. > > > > I’m not at all convinced that the kernel needs to distinguish all these, but at least upgradability should be its own thing IMO. > > Good point. Upgradability is definitely orthogonal, though the idea > there is to alter the default behavior. If the default is NOEXEC then > UPGRADE_EXEC would make sense. > > In any case, I was mostly thinking about the middle two in your list > above. After more careful reading of the patches, I now get get that > Mickaël is more interested in the first, and that's really a different > sort of use-case. > > Most opens never result in the fd being fed to fexecve or mmapped with > PROT_EXEC, so having userland explicitly opt-in to allowing that during > the open sounds like a reasonable thing to do. > > But I get that preventing execution via script interpreters of files > that are not executable might be something nice to have. My first glance at the patch lead me to believe that this was about blocking at fexecve()-time (which was what my first attempt at this problem looked like) -- hence why I mentioned the upgrade_mask stuff (because of the dances you can do with O_PATH, if blocking at fexecve()-time was the goal then you seriously do need the upgrade_mask and "O_PATH mask" in order for it to be even slightly secure). But I also agree this is useful, and we can always add FMODE_EXEC, FMODE_MAP_EXEC, and FMODE_UPGRADE_EXEC (and the related bits) at a later date. > Perhaps we need two flags for openat2? > > OA2_MAYEXEC : test that permissions allow execution and that the file > doesn't reside on a noexec mount before allowing the open > > OA2_EXECABLE : only allow fexecve or mmapping with PROT_EXEC if the fd > was opened with this That seems reasonable to me. The only thing is that there currently isn't any code to restrict fexecve() or PROT_EXEC in that fashion (doubly so when you consider binfmt_script). So if we want to make certain things default behaviour (such as disallowing exec by default) we'd need to get the PROT_EXEC restriction work done first. -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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