From 39680f50ae54cbbb6e72ac38b8329dd3eb9105f4 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Tue, 1 Mar 2016 11:56:22 -0800 Subject: [PATCH] userfaultfd: don't block on the last VM updates at exit time The exit path will do some final updates to the VM of an exiting process to inform others of the fact that the process is going away. That happens, for example, for robust futex state cleanup, but also if the parent has asked for a TID update when the process exits (we clear the child tid field in user space). However, at the time we do those final VM accesses, we've already stopped accepting signals, so the usual "stop waiting for userfaults on signal" code in fs/userfaultfd.c no longer works, and the process can become an unkillable zombie waiting for something that will never happen. To solve this, just make handle_userfault() abort any user fault handling if we're already in the exit path past the signal handling state being dead (marked by PF_EXITING). This VM special case is pretty ugly, and it is possible that we should look at finalizing signals later (or move the VM final accesses earlier). But in the meantime this is a fairly minimally intrusive fix. Reported-and-tested-by: Dmitry Vyukov Acked-by: Andrea Arcangeli Signed-off-by: Linus Torvalds --- fs/userfaultfd.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 50311703135b..66cdb44616d5 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -286,6 +286,12 @@ int handle_userfault(struct vm_area_struct *vma, unsigned long address, if (unlikely(ACCESS_ONCE(ctx->released))) goto out; + /* + * We don't do userfault handling for the final child pid update. + */ + if (current->flags & PF_EXITING) + goto out; + /* * Check that we can return VM_FAULT_RETRY. * -- 2.30.2