Re: [PATCH] mm, uprobes: fix multiple free of ->uprobes_state.xol_area

[Michal Hocko]

On Tue 29-08-17 22:33:03, 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
> the new mm_struct's ->uprobes_state.xol_area has been set to NULL after
> being copied from the old mm_struct by the memcpy in dup_mm().  For a
> task that has previously hit a uprobe tracepoint, this resulted in the
> 'struct xol_area' being freed multiple times if the task was killed at
> just the right time while forking.
> 
> Fix it by setting ->uprobes_state.xol_area to NULL in mm_init() rather
> than in uprobe_dup_mmap().
> 
> With CONFIG_UPROBE_EVENTS=y, the bug can be reproduced by the same C
> program given by commit 2b7e8665b4ff ("fork: fix incorrect fput of
> ->exe_file causing use-after-free"), provided that a uprobe tracepoint
> has been set on the fork_thread() function.  For example:
> 
>     $ gcc reproducer.c -o reproducer -lpthread
>     $ nm reproducer | grep fork_thread
>     0000000000400719 t fork_thread
>     $ echo "p $PWD/reproducer:0x719" > /sys/kernel/debug/tracing/uprobe_events
>     $ echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable
>     $ ./reproducer
> 
> Here is the use-after-free reported by KASAN:
> 
>     BUG: KASAN: use-after-free in uprobe_clear_state+0x1c4/0x200
>     Read of size 8 at addr ffff8800320a8b88 by task reproducer/198
> 
>     CPU: 1 PID: 198 Comm: reproducer Not tainted 4.13.0-rc7-00015-g36fde05f3fb5 #255
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-20170228_101828-anatol 04/01/2014
>     Call Trace:
>      dump_stack+0xdb/0x185
>      print_address_description+0x7e/0x290
>      kasan_report+0x23b/0x350
>      __asan_report_load8_noabort+0x19/0x20
>      uprobe_clear_state+0x1c4/0x200
>      mmput+0xd6/0x360
>      do_exit+0x740/0x1670
>      do_group_exit+0x13f/0x380
>      get_signal+0x597/0x17d0
>      do_signal+0x99/0x1df0
>      exit_to_usermode_loop+0x166/0x1e0
>      syscall_return_slowpath+0x258/0x2c0
>      entry_SYSCALL_64_fastpath+0xbc/0xbe
> 
>     ...
> 
>     Allocated by task 199:
>      save_stack_trace+0x1b/0x20
>      kasan_kmalloc+0xfc/0x180
>      kmem_cache_alloc_trace+0xf3/0x330
>      __create_xol_area+0x10f/0x780
>      uprobe_notify_resume+0x1674/0x2210
>      exit_to_usermode_loop+0x150/0x1e0
>      prepare_exit_to_usermode+0x14b/0x180
>      retint_user+0x8/0x20
> 
>     Freed by task 199:
>      save_stack_trace+0x1b/0x20
>      kasan_slab_free+0xa8/0x1a0
>      kfree+0xba/0x210
>      uprobe_clear_state+0x151/0x200
>      mmput+0xd6/0x360
>      copy_process.part.8+0x605f/0x65d0
>      _do_fork+0x1a5/0xbd0
>      SyS_clone+0x19/0x20
>      do_syscall_64+0x22f/0x660
>      return_from_SYSCALL_64+0x0/0x7a
> 
> Note: without KASAN, you may instead see a "Bad page state" message, or
> simply a general protection fault.
> 
> Fixes: 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for write killable")
> Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>    [v4.7+]
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

I am not very much familiar with this code but the patch makes sense to
me. I do not feel quialified to ack this but it seems like the right
thing to do.

Thanks!

> ---
>  kernel/events/uprobes.c | 2 --
>  kernel/fork.c           | 8 ++++++++
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 0e137f98a50c..267f6ef91d97 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1262,8 +1262,6 @@ void uprobe_end_dup_mmap(void)
>  
>  void uprobe_dup_mmap(struct mm_struct *oldmm, struct mm_struct *newmm)
>  {
> -	newmm->uprobes_state.xol_area = NULL;
> -
>  	if (test_bit(MMF_HAS_UPROBES, &oldmm->flags)) {
>  		set_bit(MMF_HAS_UPROBES, &newmm->flags);
>  		/* unconditionally, dup_mmap() skips VM_DONTCOPY vmas */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index cbbea277b3fb..b7e9e57b71ea 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -785,6 +785,13 @@ static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
>  #endif
>  }
>  
> +static void mm_init_uprobes_state(struct mm_struct *mm)
> +{
> +#ifdef CONFIG_UPROBES
> +	mm->uprobes_state.xol_area = NULL;
> +#endif
> +}
> +
>  static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	struct user_namespace *user_ns)
>  {
> @@ -812,6 +819,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
>  	mm->pmd_huge_pte = NULL;
>  #endif
> +	mm_init_uprobes_state(mm);
>  
>  	if (current->mm) {
>  		mm->flags = current->mm->flags & MMF_INIT_MASK;
> -- 
> 2.14.1
> 

-- 
Michal Hocko
SUSE Labs