Quoting Matt Brown (matt@xxxxxxxxx): > On 6/2/17 11:36 AM, Serge E. Hallyn wrote: > > Quoting Matt Brown (matt@xxxxxxxxx): > >> On 6/1/17 5:24 PM, Alan Cox wrote: > >>>> There's a difference between "bugs" and "security bugs". Letting > >>> > >>> Not really, it's merely a matter of severity of result. A non security > >>> bug that hoses your hard disk is to anyone but security nutcases at > >>> least as bad as a security hole. > >>> > >>>> security bugs continue to get exploited because we want to flush out > >>>> bugs seems insensitive to the people getting attacked. I'd rather > >>>> protect against a class of bug than have to endless fix each bug. > >>> > >>> The others are security bugs too to varying degree > >>> > >>>>> I'm not against doing something to protect the container folks, but that > >>>>> something as with Android is a whitelist of ioctls. And if we need to do > >>>>> this with a kernel hook lets do it properly. > >>>>> > >>>>> Remember the namespace of the tty on creation > >>>>> If the magic security flag is set then > >>>>> Apply a whitelist to *any* tty ioctl call where the ns doesn't > >>>>> match > >>>>> > >>>>> and we might as well just take the Android whitelist since they've kindly > >>>>> built it for us all! > >>>>> > >>>>> In the tty layer it ends up being something around 10 lines of code and > >>>>> some other file somewhere in security/ that's just a switch or similar > >>>>> with the whitelisted ioctl codes in it. > >>>>> > >>>>> That (or a similar SELinux ruleset) would actually fix the problem. > >>>>> SELinux would be better because it can also apply the rules when doing > >>>>> things like su/sudo/... > >>>> > >>>> Just to play devil's advocate, wouldn't such a system continue to not > >>>> address your physical-console concerns? I wouldn't want to limit the > >>> > >>> It would for the cases that a whitelist and container check covers - > >>> because the whitelist wouldn't allow you to do anything but boring stuff > >>> on the tty. TIOCSTI is just one of a whole range of differently stupid > >>> and annoying opportunities. Containers do not and should not be able to > >>> set the keymap, change the video mode, use console selection, make funny > >>> beepy noises, access video I/O registers and all the other stuff like > >>> that. Nothing is going to break if we have a fairly conservative > >>> whitelist. > >>> > >>>> protection to only containers (but it's a good start), since it > >>>> wouldn't protect people not using containers that still have a > >>>> privileged TTY attached badly somewhere. > >>> > >>> How are you going to magically fix the problem. I'm not opposed to fixing > >>> the real problem but right now it appears to be a product of wishful > >>> thinking not programming. What's the piece of security code that > >>> magically discerns the fact you are running something untrusted at the > >>> other end of your tty. SELinux can do it via labelling but I don't see > >>> any generic automatic way for the kernel to magically work out when to > >>> whitelist and when not to. If there is a better magic rule than > >>> differing-namespace then provide the code. > >>> > >>> You can't just disable TIOCSTI, it has users deal with it. You can > >>> get away with disabling it for namespace crossing I think but if you do > >>> that you need to disable a pile of others. > >>> > >>> (If it breaks containers blocking TIOCSTI then we need to have a good > >>> look at algorithms for deciding when to flush the input queue on exiting > >>> a container or somesuch) > >>> > >>>> If you're talking about wholistic SELinux policy, sure, I could > >>>> imagine a wholistic fix. But for the tons of people without a > >>>> comprehensive SELinux policy, the proposed protection continues to > >>>> make sense. > >>> > >>> No it doesn't. It's completely useless unless you actually bother to > >>> address the other exploit opportunities. > >>> > >>> Right now the proposal is a hack to do > >>> > >>> if (TIOCSTI && different_namespace && magic_flag) > >>> > >> > >> > >> This is not what my patch does. Mine is like: > >> > >> if (TIOCSTI && !ns_capable(tty->owner_user_ns, CAP_SYS_ADMIN) && > >> magic_flag) > >> > >> in other words: > >> if (TIOCSTI && (different_owner_user_ns || !CAP_SYS_ADMIN) && > >> magic_flag) > >> > >> can you specify what you mean by different_namespace? which namespace? > > > > I think you're focusing on the wrong thing. Your capable check (apart > > from the fact that I think I've been convinced CAP_SYS_ADMIN is wrong) > > is fine. > I'm cc:ing linux-api here because really we're designing an interesting API. > Can't we also have a sysctl that toggles if CAP_SYS_ADMIN is involved in > this whitelist check? Otherwise someone might leave things out of the > whitelist just because they want to use those ioctls as a privileged > process. I'm not quite sure what you're asking for here. Let me offer a precise strawman design. I'm sure there are problems with it, it's just a starting point. system-wide whitelist (for now 'may_push_chars') is full by default. By default, nothing changes - you can use those on your own tty, need CAP_SYS_ADMIN against init_user_ns otherwise. Introduce a new CAP_TTY_PRIVILEGED. When may_push_chars is removed from the whitelist, you lose the ability to use TIOCSTI on a tty - even your own - if you do not have CAP_TTY_PRIVILEGED against the tty's user_ns. > Also restricting a privileged user from ioctls with this > whitelist approach is going to be pointless because, if the whitelist is > configurable from userspace, they will just be able to modify the > whitelist. > > > > > The key point is to not only check for TIOCSTI, but instead check for > > a whitelisted ioctl. > > > > What would the whitelist look like? Should configuing that be the way > > that you enable/disable, instead of the sysctl in this patchset? So > > by default the whitelist includes all ioctls (no change), but things > > like sandboxes/sudo/container-starts can clear out the whitelist? > > > > I'm fine with moving this to an LSM that whitelists ioctls. I also want Right - what else would go into the whitelist? may_mmap? > to understand what a whitelist would like look and how you would > configure it? Does a sysctl that is a list of allowed ioctls work? I > don't want to just have a static whitelist that you can't change without > recompiling your kernel. > > just running a sysctl -a on a linux box shows me one thing that looks > like a list: net.core.flow_limit_cpu_bitmap -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
