On Mon, May 6, 2019 at 6:56 PM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > The need to be able to scope path resolution of interpreters became > clear with one of the possible vectors used in CVE-2019-5736 (which > most major container runtimes were vulnerable to). > > Naively, it might seem that openat(2) -- which supports path scoping -- > can be combined with execveat(AT_EMPTY_PATH) to trivially scope the > binary being executed. Unfortunately, a "bad binary" (usually a symlink) > could be written as a #!-style script with the symlink target as the > interpreter -- which would be completely missed by just scoping the > openat(2). An example of this being exploitable is CVE-2019-5736. > > In order to get around this, we need to pass down to each binfmt_* > implementation the scoping flags requested in execveat(2). In order to > maintain backwards-compatibility we only pass the scoping AT_* flags. > > To avoid breaking userspace (in the exceptionally rare cases where you > have #!-scripts with a relative path being execveat(2)-ed with dfd != > AT_FDCWD), we only pass dfd down to binfmt_* if any of our new flags are > set in execveat(2). This seems extremely dangerous. I like the overall series, but not this patch. > @@ -1762,6 +1774,12 @@ static int __do_execve_file(int fd, struct filename *filename, > > sched_exec(); > > + bprm->flags = flags & (AT_XDEV | AT_NO_MAGICLINKS | AT_NO_SYMLINKS | > + AT_THIS_ROOT); [...] > +#define AT_THIS_ROOT 0x100000 /* - Scope ".." resolution to dirfd (like chroot(2)). */ So now what happens if there is a setuid root ELF binary with program interpreter "/lib64/ld-linux-x86-64.so.2" (like /bin/su), and an unprivileged user runs it with execveat(..., AT_THIS_ROOT)? Is that going to let the unprivileged user decide which interpreter the setuid-root process should use? From a high-level perspective, opening the interpreter should be controlled by the program that is being loaded, not by the program that invoked it. In my opinion, CVE-2019-5736 points out two different problems: The big problem: The __ptrace_may_access() logic has a special-case short-circuit for "introspection" that you can't opt out of; this makes it possible to open things in procfs that are related to the current process even if the credentials of the process wouldn't permit accessing another process like it. I think the proper fix to deal with this would be to add a prctl() flag for "set whether introspection is allowed for this process", and if userspace has manually un-set that flag, any introspection special-case logic would be skipped. An additional problem: /proc/*/exe can be used to open a file for writing; I think it may have been Andy Lutomirski who pointed out some time ago that it would be nice if you couldn't use /proc/*/fd/* to re-open files with more privileges, which is sort of the same thing.
