Date: Fri, 14 Aug 2020 17:41:38 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Restrictions on child context after multithreaded fork musl 1.2.1 has exposed bugs in several applications and libraries caused by async-signal-unsafe code between (multithreaded) fork and subsequent exec. So far, dbus library code, pulseaudio library code, and libvirt have been found to be affected. A couple of the bug reports (with incomplete information) are: https://gitlab.alpinelinux.org/alpine/aports/-/issues/11602 https://gitlab.alpinelinux.org/alpine/aports/-/issues/11815 Fixing the affected library code looks very straightforward; it's just a matter of doing proper iterations of existing data/state rather than allocating lists and using opendir on procfs and such. I've discussed fixes with Alpine Linux folks and I believe fixes have been tested, but I don't see any patches in aports yet. I've seen suspicions that the switch to mallocng exposed this, but I'm pretty sure it was: https://git.musl-libc.org/cgit/musl/commit/?id=e01b5939b38aea5ecbe41670643199825874b26c Before this commit, the (incorrect) lock skipping logic allowed the child process to access inconsistent state left from the parent if it violated the requirement not to call AS-unsafe functions. Now, the lock attempt in the child rightly deadlocks before accessing state that was being modified under control of the lock in the parent. This is not specific to malloc but common with anything using libc-internal locks. I'll follow up on this thread once there are patches for the known affected libraries. Note that this is a type of bug that's possibly hard to get upstreams to take seriously. libvirt in particular, despite having multiple comments throughout the source warning developers that they can't do anything AS-unsafe between fork and exec, is somehow deeming malloc an exception to that rule because they want to use it (despite it clearly not being necessary). And the dbus issue has been known for a long time; see open bug: https://gitlab.freedesktop.org/dbus/dbus/-/issues/173 (originally: https://bugs.freedesktop.org/show_bug.cgi?id=100843) This is largely because glibc attempts to make the erroneous usage by these libraries work (more on that below). The next issue of POSIX (Issue 8) will drop the requirement that fork be AS-safe, as a result of Austin Group tracker issue #62. This makes the glibc behavior permissible/conforming, but there does not seem to be any effort on the POSIX side to drop the requirement on applications not to do AS-unsafe things in the child before exec, so regardless of this change, what these libraries are doing is still wrong. In order to make the child environment unrestricted after fork, either fork must hold *all* locks at the time the actual fork syscall takes place, or it must be able to reset any state protected by a lock that was held in the parent (or some mix of the two). It's fundamentally impossible to do this completely (in a way that lets the child run unrestricted), since some locks in the parent may be held arbitrarily long such that fork waiting on them would deadlock. In particular, any stdio FILE lock may be held indefinitely because there's a blocking operation in progress on the underlying fd, or because the application has called flockfile. Thus, at best, the implementation can give the child an environment where fflush(0) and exit() still deadlock. In case we do want to follow a direction of trying to provide some degree of relaxation of restrictions on the child (taking the liberty of POSIX-future drop of fork's AS-safety requirement), I did a quick survey of libc-internal locks, and found: - at_quick_exit - atexit - dlerror - gettext - malloc - pthread_atfork (already necessarily held at fork) - random - sem_open - stdio open file list (vs individual FILEs) - syslog - timezone This list looks tractable. Aside from malloc, whose locks would need to be taken last since the others may call malloc, these don't seem to have any lock order dependencies between them, and each one's lock functions could be provided as strong overrides to weak no-op definitions in fork.c.
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