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

From: Yuanxiaofeng (XiAn)
Date: Tue Aug 14 2018 - 09:28:39 EST

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@xxxxxxxxxxxxx]
Sent: Tuesday, August 14, 2018 8:35 PM
To: Yuanxiaofeng (XiAn)
Cc: keescook@xxxxxxxxxxxx; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
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?