On Thu, Aug 24, 2017 at 04:02:49PM +0100, Mark Rutland wrote: > On Wed, Aug 23, 2017 at 02:14:08PM -0700, Eric Biggers wrote: > > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > Commit 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for > > write killable") made it possible to kill a forking task while it is > > waiting to acquire its ->mmap_sem for write, in dup_mmap(). However, it > > was overlooked that this introduced an new error path before a reference > > is taken on the mm_struct's ->exe_file. Since the ->exe_file of the new > > mm_struct was already set to the old ->exe_file by the memcpy() in > > dup_mm(), it was possible for the mmput() in the error path of dup_mm() > > to drop a reference to ->exe_file which was never taken. This caused > > the struct file to later be freed prematurely. > > > > Fix it by updating mm_init() to NULL out the ->exe_file, in the same > > place it clears other things like the list of mmaps. > > > diff --git a/kernel/fork.c b/kernel/fork.c > > index e075b7780421..cbbea277b3fb 100644 > > --- a/kernel/fork.c > > +++ b/kernel/fork.c > > @@ -806,6 +806,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p, > > mm_init_cpumask(mm); > > mm_init_aio(mm); > > mm_init_owner(mm, p); > > + RCU_INIT_POINTER(mm->exe_file, NULL); > > mmu_notifier_mm_init(mm); > > init_tlb_flush_pending(mm); > > #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS > > I've been seeing similar issues on arm64 with use-after-free of a file > and other memory corruption [1]. > > This patch seems to fix that; a test that normally fired in a few > minutes has been happily running for hours with this applied. Those haven't triggered after 24 hours, and in 16+ hours of fuzzing with this applied, I haven't seen new issues. FWIW: Tested-by: Mark Rutland <mark.rutland@xxxxxxx> Thanks, Mark.