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Re: sg_io HARDENED_USERCOPY_PAGESPAN trace

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  Re: sg_io HARDENED_USERCOPY_PAGESPAN trace Kees Cook <keescook@chromium.org> - 2017-01-03 22:50 +0100
    Re: sg_io HARDENED_USERCOPY_PAGESPAN trace Christoph Hellwig <hch@infradead.org> - 2017-01-08 10:50 +0100

#1550230 — Re: sg_io HARDENED_USERCOPY_PAGESPAN trace

On Fri, Dec 30, 2016 at 7:10 AM, Christoph Hellwig <hch@infradead.org> wrote:
> On Fri, Dec 30, 2016 at 10:01:39AM -0500, Dave Jones wrote:
>> I threw this debug printk into the pagespan code to see what exactly
>> it was complaining about..
>>
>> ptr:ffff88042614cff8 end:ffff88042614d003 n:c
>>
>> so it was copying 12 bytes that spanned two pages.
>> >From my reading of the config option help text, this thing is
>> complaining that wasn't allocated with __GFP_COMP maybe ?

There are a lot of cases of "missing" __GFP_COMP, which is why
HARDENED_USERCOPY_PAGESPAN defaults to "n".

> If this is on a devie using blk-mq the block core will use high
> order allocations (as high as possible) to allocate the requests
> for each queue, so struct request could very well span multiple
> pages.  But I don't see what __GFP_COMP would have to do with
> user copy annoations.  As all requests for a queue are freed
> togeth again there is no point in setting __GFP_COMP for the
> request allocations.

Does it hurt anything to mark these pages as allocated "together" via
__GFP_COMP?

-Kees

-- 
Kees Cook
Nexus Security

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#1553819

On Tue, Jan 03, 2017 at 01:48:03PM -0800, Kees Cook wrote:
> There are a lot of cases of "missing" __GFP_COMP, which is why
> HARDENED_USERCOPY_PAGESPAN defaults to "n".
> 
> > If this is on a devie using blk-mq the block core will use high
> > order allocations (as high as possible) to allocate the requests
> > for each queue, so struct request could very well span multiple
> > pages.  But I don't see what __GFP_COMP would have to do with
> > user copy annoations.  As all requests for a queue are freed
> > togeth again there is no point in setting __GFP_COMP for the
> > request allocations.
> 
> Does it hurt anything to mark these pages as allocated "together" via
> __GFP_COMP?

It don't think it would hurt the block code - it only allocates the
pages once, and frees them once.  But I think hijacking your feature
on top of a totally unrelated flag is a horrible idea.  __GFP_COMP
is about refcounting the allocation, not about anything else.  The prime
use case of high order allocations is to use them as a single memory
object, which might include user copies.

So as-is I think HARDENED_USERCOPY_PAGESPAN is a misfeature, it needs to
be opt-in for allocations where we might not copy over the span of
pages, not opt-out.  And I suspect there aren't going to be all that
many opt-out candidates.

> 
> -Kees
> 
> -- 
> Kees Cook
> Nexus Security
---end quoted text---

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