Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

[Andrew Morton]

On Wed, 30 Jan 2019 13:44:18 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:

> From: Jiri Kosina <jkosina@xxxxxxx>
> 
> The semantics of what mincore() considers to be resident is not completely
> clear, but Linux has always (since 2.3.52, which is when mincore() was
> initially done) treated it as "page is available in page cache".
> 
> That's potentially a problem, as that [in]directly exposes meta-information
> about pagecache / memory mapping state even about memory not strictly belonging
> to the process executing the syscall, opening possibilities for sidechannel
> attacks.
> 
> Change the semantics of mincore() so that it only reveals pagecache information
> for non-anonymous mappings that belog to files that the calling process could
> (if it tried to) successfully open for writing.

"for writing" comes as a bit of a surprise.  Why not for reading?

Could we please explain the reasoning in the changelog and in the
(presently absent) comments which describe can_do_mincore()?

> @@ -189,8 +197,13 @@ static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *v
>  	vma = find_vma(current->mm, addr);
>  	if (!vma || addr < vma->vm_start)
>  		return -ENOMEM;
> -	mincore_walk.mm = vma->vm_mm;
>  	end = min(vma->vm_end, addr + (pages << PAGE_SHIFT));
> +	if (!can_do_mincore(vma)) {
> +		unsigned long pages = (end - addr) >> PAGE_SHIFT;

I'm not sure this is correct in all cases.   If

	addr = 4095
	vma->vm_end = 4096
	pages = 1000

then `end' is 4096 and `(end - addr) << PAGE_SHIFT' is zero, but it
should have been 1.

Please check?

A mincore test suite in tools/testing/selftests would be useful,
methinks.  To exercise such corner cases, check for future breakage,
etc.

> +		memset(vec, 1, pages);
> +		return pages;
> +	}
> +	mincore_walk.mm = vma->vm_mm;
>  	err = walk_page_range(addr, end, &mincore_walk);
>  	if (err < 0)
>  		return err;