From: Andrea Arcangeli <aarcange@xxxxxxxxxx> Subject: userfaultfd: shmem: handle coredumping in handle_userfault() Anon and hugetlbfs handle FOLL_DUMP set by get_dump_page() internally to __get_user_pages(). shmem as opposed has no special FOLL_DUMP handling there so handle_mm_fault() is invoked without mmap_sem and ends up calling handle_userfault() that isn't expecting to be invoked without mmap_sem held. This makes handle_userfault() fail immediately if invoked through shmem_vm_ops->fault during coredumping and solves the problem. The side effect is a BUG_ON with no lock held triggered by the coredumping process which exits. Only 4.11 is affected, pre-4.11 anon memory holes are skipped in __get_user_pages by checking FOLL_DUMP explicitly against empty pagetables (mm/gup.c:no_page_table()). It's zero cost as we already had a check for current->flags to prevent futex to trigger userfaults during exit (PF_EXITING). Link: http://lkml.kernel.org/r/20170615214838.27429-1-aarcange@xxxxxxxxxx Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Reported-by: "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> [4.11+] Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/userfaultfd.c | 29 +++++++++++++++++++++-------- 1 file changed, 21 insertions(+), 8 deletions(-) diff -puN fs/userfaultfd.c~userfaultfd-shmem-handle-coredumping-in-handle_userfault fs/userfaultfd.c --- a/fs/userfaultfd.c~userfaultfd-shmem-handle-coredumping-in-handle_userfault +++ a/fs/userfaultfd.c @@ -340,9 +340,28 @@ int handle_userfault(struct vm_fault *vm bool must_wait, return_to_userland; long blocking_state; - BUG_ON(!rwsem_is_locked(&mm->mmap_sem)); - ret = VM_FAULT_SIGBUS; + + /* + * We don't do userfault handling for the final child pid update. + * + * We also don't do userfault handling during + * coredumping. hugetlbfs has the special + * follow_hugetlb_page() to skip missing pages in the + * FOLL_DUMP case, anon memory also checks for FOLL_DUMP with + * the no_page_table() helper in follow_page_mask(), but the + * shmem_vm_ops->fault method is invoked even during + * coredumping without mmap_sem and it ends up here. + */ + if (current->flags & (PF_EXITING|PF_DUMPCORE)) + goto out; + + /* + * Coredumping runs without mmap_sem so we can only check that + * the mmap_sem is held, if PF_DUMPCORE was not set. + */ + WARN_ON_ONCE(!rwsem_is_locked(&mm->mmap_sem)); + ctx = vmf->vma->vm_userfaultfd_ctx.ctx; if (!ctx) goto out; @@ -361,12 +380,6 @@ int handle_userfault(struct vm_fault *vm 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. * * NOTE: it should become possible to return VM_FAULT_RETRY _