Patch "fork: fix incorrect fput of ->exe_file causing use-after-free" has been added to the 4.12-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fork-fix-incorrect-fput-of-exe_file-causing-use-after-free.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Eric Biggers <[email protected]>
Date: Fri, 25 Aug 2017 15:55:43 -0700
Subject: fork: fix incorrect fput of ->exe_file causing use-after-free

From: Eric Biggers <[email protected]>

commit 2b7e8665b4ff51c034c55df3cff76518d1a9ee3a upstream.

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

---
 kernel/fork.c |    1 +
 1 file changed, 1 insertion(+)

@@ -802,6 +802,7 @@ static struct mm_struct *mm_init(struct
 	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);
 	clear_tlb_flush_pending(mm);
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/mm-madvise.c-fix-freeing-of-locked-page-with-madv_free.patch queue-4.12/x86-mm-fix-use-after-free-of-ldt_struct.patch queue-4.12/fork-fix-incorrect-fput-of-exe_file-causing-use-after-free.patch