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

[Eric Biggers <ebiggers3@xxxxxxxxx>]

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.

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.

Fixes: 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for write killable")
Google-Bug-Id: 64772007
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.7+
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

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
-- 
2.14.1.342.g6490525c54-goog