Re: [PATCH 3/3] usercopy: Add find_vmap_area_try() to avoid deadlocks

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Sep 16, 2022 at 08:15:24PM +0100, Matthew Wilcox wrote:
> On Fri, Sep 16, 2022 at 08:09:16AM -0700, Kees Cook wrote:
> > On Fri, Sep 16, 2022 at 03:46:07PM +0100, Matthew Wilcox wrote:
> > > On Fri, Sep 16, 2022 at 06:59:57AM -0700, Kees Cook wrote:
> > > > The check_object_size() checks under CONFIG_HARDENED_USERCOPY need to be
> > > > more defensive against running from interrupt context. Use a best-effort
> > > > check for VMAP areas when running in interrupt context
> > > 
> > > I had something more like this in mind:
> > 
> > Yeah, I like -EAGAIN. I'd like to keep the interrupt test to choose lock
> > vs trylock, otherwise it's trivial to bypass the hardening test by having
> > all the other CPUs beating on the spinlock.
> 
> I was thinking about this:
> 
> +++ b/mm/vmalloc.c
> @@ -1844,12 +1844,19 @@
>  {
>  	struct vmap_area *va;
> 
> -	if (!spin_lock(&vmap_area_lock))
> -		return ERR_PTR(-EAGAIN);
> +	/*
> +	 * It's safe to walk the rbtree under the RCU lock, but we may
> +	 * incorrectly find no vmap_area if the tree is being modified.
> +	 */
> +	rcu_read_lock();
>  	va = __find_vmap_area(addr, &vmap_area_root);
> -	spin_unlock(&vmap_area_lock);
> +	if (!va && in_interrupt())
> +		va = ERR_PTR(-EAGAIN);
> +	rcu_read_unlock();
> 
> -	return va;
> +	if (va)
> +		return va;
> +	return find_vmap_area(addr);
>  }
> 
>  /*** Per cpu kva allocator ***/
> 
> ... but I don't think that works since vmap_areas aren't freed by RCU,
> and I think they're reused without going through an RCU cycle.
>
Right you are. It should be freed via RCU-core. So wrapping by rcu_* is
useless here.

> 
> So here's attempt #4, which actually compiles, and is, I think, what you
> had in mind.
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 096d48aa3437..2b7c52e76856 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -215,7 +215,7 @@ extern struct vm_struct *__get_vm_area_caller(unsigned long size,
>  void free_vm_area(struct vm_struct *area);
>  extern struct vm_struct *remove_vm_area(const void *addr);
>  extern struct vm_struct *find_vm_area(const void *addr);
> -struct vmap_area *find_vmap_area(unsigned long addr);
> +struct vmap_area *find_vmap_area_try(unsigned long addr);
>  
>  static inline bool is_vm_area_hugepages(const void *addr)
>  {
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index c1ee15a98633..e0fb605c1b38 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -173,7 +173,11 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
>  	}
>  
>  	if (is_vmalloc_addr(ptr)) {
> -		struct vmap_area *area = find_vmap_area(addr);
> +		struct vmap_area *area = find_vmap_area_try(addr);
> +
> +		/* We may be in NMI context */
> +		if (area == ERR_PTR(-EAGAIN))
> +			return;
>  
>  		if (!area)
>  			usercopy_abort("vmalloc", "no area", to_user, 0, n);
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index dd6cdb201195..c47b3b5d1c2d 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1829,7 +1829,7 @@ static void free_unmap_vmap_area(struct vmap_area *va)
>  	free_vmap_area_noflush(va);
>  }
>  
> -struct vmap_area *find_vmap_area(unsigned long addr)
> +static struct vmap_area *find_vmap_area(unsigned long addr)
>  {
>  	struct vmap_area *va;
>  
> @@ -1840,6 +1840,26 @@ struct vmap_area *find_vmap_area(unsigned long addr)
>  	return va;
>  }
>  
> +/*
> + * The vmap_area_lock is not interrupt-safe, and we can end up here from
> + * NMI context, so it's not worth even trying to make it IRQ-safe.
> + */
> +struct vmap_area *find_vmap_area_try(unsigned long addr)
> +{
> +	struct vmap_area *va;
> +
> +	if (in_interrupt()) {
> +		if (!spin_trylock(&vmap_area_lock))
> +			return ERR_PTR(-EAGAIN);
> +	} else {
> +		spin_lock(&vmap_area_lock);
> +	}
> +	va = __find_vmap_area(addr, &vmap_area_root);
> +	spin_unlock(&vmap_area_lock);
> +
> +	return va;
> +}
> +
>
If we look at it other way around. There is a user that tries to access
the tree from IRQ context. Can we just extend the find_vmap_area() with?

   - in_interrupt();
   - use trylock if so.

--
Uladzislau Rezki