fork: fix incorrect fput of ->exe_file causing use-after-free

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.

This bug was found by syzkaller.  It can be reproduced using the
following C program:

    #define _GNU_SOURCE
    #include <pthread.h>
    #include <stdlib.h>
    #include <sys/mman.h>
    #include <sys/syscall.h>
    #include <sys/wait.h>
    #include <unistd.h>

    static void *mmap_thread(void *_arg)
    {
        for (;;) {
            mmap(NULL, 0x1000000, PROT_READ,
                 MAP_POPULATE|MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
        }
    }

    static void *fork_thread(void *_arg)
    {
        usleep(rand() % 10000);
        fork();
    }

    int main(void)
    {
        fork();
        fork();
        fork();
        for (;;) {
            if (fork() == 0) {
                pthread_t t;

                pthread_create(&t, NULL, mmap_thread, NULL);
                pthread_create(&t, NULL, fork_thread, NULL);
                usleep(rand() % 10000);
                syscall(__NR_exit_group, 0);
            }
            wait(NULL);
        }
    }

No special kernel config options are needed.  It usually causes a NULL
pointer dereference in __remove_shared_vm_struct() during exit, or in
dup_mmap() (which is usually inlined into copy_process()) during fork.
Both are due to a vm_area_struct's ->vm_file being used after it's
already been freed.

Google Bug Id: 64772007

Link: http://lkml.kernel.org/r/20170823211408.31198-1-ebiggers3@gmail.com
Fixes: 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for write killable")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>	[v4.7+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
