RE: page allocation/attributes question (i386/x86_64 specific)

[<Stuart_Hayes@Dell.com>]

>> 
>> That should be already fixed.
> 
> It doesn't appear to be fixed (in the i386 arch).  The
> change_page_attr()/split_large_page() code will still still set all
> the 4K PTEs to PAGE_KERNEL (setting the _PAGE_NX bit) when a large
> page needs to be split.   
> 
> This wouldn't be a problem for the bulk of the kernel memory, but
> there are pages in the lower 4MB of memory that's free, and are part
> of large executable pages that also contain kernel code.  If
> change_page_attr() is called on these, it will set the _PAGE_NX bit
> on the whole 2MB region that was covered by the large page, causing a
> large chunk of kernel code to be non-executable.     
> 
> I can only think of a few ways to fix this:
> 
> (1) Make sure that any free pages that share a large-page-aligned
> area with kernel code are not executable.  This would cause a big
> part of the kernel code to be in small pages, which might have an
> effect on performance.   
> 
> (2) Make sure that there aren't any free pages in the
> large-page-aligned areas that contain kernel executable code, by
> reserving it somehow.  This seems kind of wasteful of
> memory--ZONE_DMA memory, at that.   
> 
> or (3) Allow the free pages in the 2MB large pages that also contain
> kernel code to be executable, but somehow fix change_page_attr() so
> it doesn't set the _PAGE_NX bit on the whole 2MB large page region
> when called.  This would require one of the changes that I suggested
> earlier, like ignoring the NX bit in the "prot" that's passed to
> change_page_attr(), or just accepting the fact that calling
> change_page_attr() on a large executable page will not be reverted
> back to a large page if the calling function uses
> change_page_attr(page,PAGE_KERNEL) when it is done.  Or changing the
> interface, so that there's a change_page_attr() and an
> unchange_page_attr() that automatically reverts the page back to
> whatever the previous attributes were.           
> 
> Am I missing something?
> 

(Fixed Andrea's email address)

This is definitely broken in 2.6.12.  I wrote a test module that does
__get_free_pages(GFP_DMA,0) until it sees that the page it gets is
executable (it got about 30 pages before it got one that's executable),
and then it does a change_page_attr(page,PAGE_KERNEL_NOCACHE) on the
page.  Shortly thereafter the system reboots--presumably it got a triple
fault because the exception handler was no longer executable, or
something like that.

My personal opinion of the best way to fix this would be option 3
above--just rewrite change_page_attr() so that the 511 other little
pages that are created inheret the _PAGE_NX bit from the large page that
was split--but make sure that it doesn't revert these PTEs back into a
large page if change_page_attr(page,PAGE_KERNEL) is called, since the
other pages in this 2MB area aren't PAGE_KERNEL.

Option 1 seems more correct to me, but not necessarily worth the
performance cost.

What do you think?

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