Re: [PATCH] usercopy: Remove HARDENED_USERCOPY_PAGESPAN

[Kees Cook <keescook@chromium.org>]

On Fri, May 10, 2019 at 01:43:36PM -0700, Kees Cook wrote:
> This feature continues to cause more problems than it solves[1]. Its
> intention was to check the bounds of page-allocator allocations by using
> __GFP_COMP, for which we would need to find all missing __GFP_COMP
> markings. This work has been on hold and there is an argument[2]
> that such markings are not even the correct signal for checking for
> same-allocation pages. Instead of depending on BROKEN, this just removes
> it entirely. It can be trivially reverted if/when a better solution for
> tracking page allocator sizes is found.
> 
> [1] https://www.mail-archive.com/linux-crypto@vger.kernel.org/msg37479.html
> [2] https://lkml.kernel.org/r/20190415022412.GA29714@bombadil.infradead.org
> 
> Suggested-by: Eric Biggers <ebiggers@kernel.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>

So, after looking at this more, I think I'm going to keep this patch,
and we can add new sanity checks on a per-Page flag check. (See below.)

Andrew, can you apply this to -mm please?

> ---
>  mm/usercopy.c    | 67 ------------------------------------------------
>  security/Kconfig | 11 --------
>  2 files changed, 78 deletions(-)
> 
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 14faadcedd06..15dc1bf03303 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -159,70 +159,6 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
>  		usercopy_abort("null address", NULL, to_user, ptr, n);
>  }
>  
> -/* Checks for allocs that are marked in some way as spanning multiple pages. */
> -static inline void check_page_span(const void *ptr, unsigned long n,
> -				   struct page *page, bool to_user)
> -{
> -#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
> -	const void *end = ptr + n - 1;
> -	struct page *endpage;
> -	bool is_reserved, is_cma;
> -
> -	/*
> -	 * Sometimes the kernel data regions are not marked Reserved (see
> -	 * check below). And sometimes [_sdata,_edata) does not cover
> -	 * rodata and/or bss, so check each range explicitly.
> -	 */
> -
> -	/* Allow reads of kernel rodata region (if not marked as Reserved). */
> -	if (ptr >= (const void *)__start_rodata &&
> -	    end <= (const void *)__end_rodata) {
> -		if (!to_user)
> -			usercopy_abort("rodata", NULL, to_user, 0, n);
> -		return;
> -	}
> -
> -	/* Allow kernel data region (if not marked as Reserved). */
> -	if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
> -		return;
> -
> -	/* Allow kernel bss region (if not marked as Reserved). */
> -	if (ptr >= (const void *)__bss_start &&
> -	    end <= (const void *)__bss_stop)
> -		return;
> -
> -	/* Is the object wholly within one base page? */
> -	if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
> -		   ((unsigned long)end & (unsigned long)PAGE_MASK)))
> -		return;
> -
> -	/* Allow if fully inside the same compound (__GFP_COMP) page. */
> -	endpage = virt_to_head_page(end);
> -	if (likely(endpage == page))
> -		return;
> -
> -	/*
> -	 * Reject if range is entirely either Reserved (i.e. special or
> -	 * device memory), or CMA. Otherwise, reject since the object spans
> -	 * several independently allocated pages.
> -	 */
> -	is_reserved = PageReserved(page);
> -	is_cma = is_migrate_cma_page(page);
> -	if (!is_reserved && !is_cma)
> -		usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
> -
> -	for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
> -		page = virt_to_head_page(ptr);
> -		if (is_reserved && !PageReserved(page))
> -			usercopy_abort("spans Reserved and non-Reserved pages",
> -				       NULL, to_user, 0, n);
> -		if (is_cma && !is_migrate_cma_page(page))
> -			usercopy_abort("spans CMA and non-CMA pages", NULL,
> -				       to_user, 0, n);
> -	}
> -#endif
> -}
> -
>  static inline void check_heap_object(const void *ptr, unsigned long n,
>  				     bool to_user)
>  {
> @@ -236,9 +172,6 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
>  	if (PageSlab(page)) {
>  		/* Check slab allocator for flags and size. */
>  		__check_heap_object(ptr, n, page, to_user);
> -	} else {
> -		/* Verify object does not incorrectly span multiple pages. */
> -		check_page_span(ptr, n, page, to_user);
>  	}

In the future, instead of this catch-all "else", we can add things like:

	} else if (PageCompound(page)) {
		... do some check for compound pages ...
	} else if (PageReserved(page))
		... etc ...
	}

But for 5.3, I think we need to just entirely drop the PAGESPAN thing.

-Kees

>  }
>  
> diff --git a/security/Kconfig b/security/Kconfig
> index 353cfef71d4e..8392647f5a4c 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -176,17 +176,6 @@ config HARDENED_USERCOPY_FALLBACK
>  	  Booting with "slab_common.usercopy_fallback=Y/N" can change
>  	  this setting.
>  
> -config HARDENED_USERCOPY_PAGESPAN
> -	bool "Refuse to copy allocations that span multiple pages"
> -	depends on HARDENED_USERCOPY
> -	depends on EXPERT
> -	help
> -	  When a multi-page allocation is done without __GFP_COMP,
> -	  hardened usercopy will reject attempts to copy it. There are,
> -	  however, several cases of this in the kernel that have not all
> -	  been removed. This config is intended to be used only while
> -	  trying to find such users.
> -
>  config FORTIFY_SOURCE
>  	bool "Harden common str/mem functions against buffer overflows"
>  	depends on ARCH_HAS_FORTIFY_SOURCE
> -- 
> 2.17.1
> 
> 
> -- 
> Kees Cook

-- 
Kees Cook