From 51891498f2da78ee64dfad88fa53c9e85fb50abf Mon Sep 17 00:00:00 2001 From: Tycho Andersen Date: Wed, 4 Mar 2020 11:05:17 -0700 Subject: seccomp: allow TSYNC and USER_NOTIF together The restriction introduced in 7a0df7fbc145 ("seccomp: Make NEW_LISTENER and TSYNC flags exclusive") is mostly artificial: there is enough information in a seccomp user notification to tell which thread triggered a notification. The reason it was introduced is because TSYNC makes the syscall return a thread-id on failure, and NEW_LISTENER returns an fd, and there's no way to distinguish between these two cases (well, I suppose the caller could check all fds it has, then do the syscall, and if the return value was an fd that already existed, then it must be a thread id, but bleh). Matthew would like to use these two flags together in the Chrome sandbox which wants to use TSYNC for video drivers and NEW_LISTENER to proxy syscalls. So, let's fix this ugliness by adding another flag, TSYNC_ESRCH, which tells the kernel to just return -ESRCH on a TSYNC error. This way, NEW_LISTENER (and any subsequent seccomp() commands that want to return positive values) don't conflict with each other. Suggested-by: Matthew Denton Signed-off-by: Tycho Andersen Link: https://lore.kernel.org/r/20200304180517.23867-1-tycho@tycho.ws Signed-off-by: Kees Cook --- kernel/seccomp.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) (limited to 'kernel/seccomp.c') diff --git a/kernel/seccomp.c b/kernel/seccomp.c index b6ea3dcb57bf..29022c1bbe18 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -528,8 +528,12 @@ static long seccomp_attach_filter(unsigned int flags, int ret; ret = seccomp_can_sync_threads(); - if (ret) - return ret; + if (ret) { + if (flags & SECCOMP_FILTER_FLAG_TSYNC_ESRCH) + return -ESRCH; + else + return ret; + } } /* Set log flag, if present. */ @@ -1288,10 +1292,12 @@ static long seccomp_set_mode_filter(unsigned int flags, * In the successful case, NEW_LISTENER returns the new listener fd. * But in the failure case, TSYNC returns the thread that died. If you * combine these two flags, there's no way to tell whether something - * succeeded or failed. So, let's disallow this combination. + * succeeded or failed. So, let's disallow this combination if the user + * has not explicitly requested no errors from TSYNC. */ if ((flags & SECCOMP_FILTER_FLAG_TSYNC) && - (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER)) + (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) && + ((flags & SECCOMP_FILTER_FLAG_TSYNC_ESRCH) == 0)) return -EINVAL; /* Prepare the new filter before holding any locks. */ -- cgit