On Fri, Oct 14, 2022 at 05:35:26PM +0200, Jann Horn wrote: > On Fri, Oct 14, 2022 at 5:18 AM Andy Lutomirski <[email protected]> wrote: > > On Thu, Oct 6, 2022, at 7:13 AM, Jann Horn wrote: > > > On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <[email protected]> > > > wrote: > > >> On Thu, Oct 06, 2022 at 01:27:34AM -0700, Kees Cook wrote: > > >> > The check_unsafe_exec() counting of n_fs would not add up under a > > >> > heavily > > >> > threaded process trying to perform a suid exec, causing the suid > > >> > portion > > >> > to fail. This counting error appears to be unneeded, but to catch any > > >> > possible conditions, explicitly unshare fs_struct on exec, if it ends > > >> > up > > >> > > >> Isn't this a potential uapi break? Afaict, before this change a call to > > >> clone{3}(CLONE_FS) followed by an exec in the child would have the > > >> parent and child share fs information. So if the child e.g., changes the > > >> working directory post exec it would also affect the parent. But after > > >> this change here this would no longer be true. So a child changing a > > >> workding directoro would not affect the parent anymore. IOW, an exec is > > >> accompanied by an unshare(CLONE_FS). Might still be worth trying ofc but > > >> it seems like a non-trivial uapi change but there might be few users > > >> that do clone{3}(CLONE_FS) followed by an exec. > > > > > > I believe the following code in Chromium explicitly relies on this > > > behavior, but I'm not sure whether this code is in active use anymore: > > > > > > https://source.chromium.org/chromium/chromium/src/+/main:sandbox/linux/suid/sandbox.c;l=101?q=CLONE_FS&sq=&ss=chromium > > > > Wait, this is absolutely nucking futs. On a very quick inspection, the > > sharable things like this are fs, files, sighand, and io. files and > > sighand get unshared, which makes sense. fs supposedly checks for extra > > refs and prevents gaining privilege. io is... ignored! At least it's not > > immediately obvious that io is a problem. > > > > But seriously, this makes no sense at all. It should not be possible to > > exec a program and then, without ptrace, change its cwd out from under it. > > Do we really need to preserve this behavior? > > I agree that this is pretty wild. > > The single user I'm aware of is Chrome, and as far as I know, they use > it for establishing their sandbox on systems where unprivileged user > namespaces are disabled - see > <https://chromium.googlesource.com/chromium/src/+/main/docs/linux/suid_sandbox.md>. > They also have seccomp-based sandboxing, but IIRC there are some small > holes that mean it's still useful for them to be able to set up > namespaces, like how sendmsg() on a unix domain socket can specify a > file path as the destination address. > > (By the way, I think maybe Chrome wouldn't need this wacky trick with > the shared fs_struct if the "NO_NEW_PRIVS permits chroot()" thing had > ever landed that you > (https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.l...@amacapital.net/) > and Mickaël Salaün proposed in the past... or alternatively, if there > was a way to properly filter all the syscalls that Chrome has to > permit for renderers.) > > (But also, to be clear, I don't speak for Chrome, this is just my > understanding of how their stuff works.)
Chrome seems to just want a totally empty filesystem view, yes? Let's land the nnp+chroot change. :P Only 10 years late! Then we can have Chrome use this and we can unshare fs on exec... -- Kees Cook
