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    SubjectRe: [PATCH] fork: fix incorrect fput of ->exe_file causing use-after-free
    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@google.com>
    > >
    > > 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@arm.com>

    Thanks,
    Mark.

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