• Linus Torvalds's avatar
    userfaultfd: don't block on the last VM updates at exit time · 39680f50
    Linus Torvalds authored
    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
    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: default avatarDmitry Vyukov <dvyukov@google.com>
    Acked-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
userfaultfd.c 35.1 KB