linux-kernel - RE: [PATCH RFC] usercopy: optimize stack check flow when the page-spanning test is disabled

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Date:   Tue, 14 Aug 2018 13:02:55 +0000
From:   "Yuanxiaofeng (XiAn)" <yuanxiaofeng1@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     "keescook@...omium.org" <keescook@...omium.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC] usercopy: optimize stack check flow when the
 page-spanning test is disabled

1, When the THREAD_SIZE is less than PAGE_SIZE, the stack will allocate memory by kmem_cache_alloc_node(), it's slab memory and will execute __check_heap_object().
2, When CONFIG_HARDENED_USERCOPY_PAGESPAN is enabled, the multiple-pages stacks will do some check in check_page_span().

So, I set some restrictions to make sure the useful check will not be skipped.

-----Original Message-----
From: Matthew Wilcox [mailto:willy@...radead.org] 
Sent: Tuesday, August 14, 2018 8:35 PM
To: Yuanxiaofeng (XiAn)
Cc: keescook@...omium.org; linux-mm@...ck.org; linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] usercopy: optimize stack check flow when the

On Tue, Aug 14, 2018 at 08:17:31PM +0800, Xiaofeng Yuan wrote:
> The check_heap_object() checks the spanning multiple pages and slab.
> When the page-spanning test is disabled, the check_heap_object() is
> redundant for spanning multiple pages. However, the kernel stacks are
> multiple pages under certain conditions: CONFIG_ARCH_THREAD_STACK_ALLOCATOR
> is not defined and (THREAD_SIZE >= PAGE_SIZE). At this point, We can skip
> the check_heap_object() for kernel stacks to improve performance.
> Similarly, the virtually-mapped stack can skip check_heap_object() also,
> beacause virt_addr_valid() will return.

Why not just check_stack_object() first, then check_heap_object() second?