Date:   Fri, 23 Sep 2016 14:42:45 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     zijun_hu <zijun_hu@...o.com>
Cc:     zijun_hu@....com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, tj@...nel.org,
        mingo@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH 1/1] lib/ioremap.c: avoid endless loop under ioremapping
 page unaligned ranges

On Fri 23-09-16 20:29:20, zijun_hu wrote:
> On 2016/9/23 16:45, Michal Hocko wrote:
> > On Thu 22-09-16 23:13:17, zijun_hu wrote:
> >> On 2016/9/22 20:47, Michal Hocko wrote:
> >>> On Wed 21-09-16 12:19:53, zijun_hu wrote:
> >>>> From: zijun_hu <zijun_hu@....com>
> >>>>
> >>>> endless loop maybe happen if either of parameter addr and end is not
> >>>> page aligned for kernel API function ioremap_page_range()
> >>>
> >>> Does this happen in practise or this you found it by reading the code?
> >>>
> >> i found it by reading the code, this is a kernel API function and there
> >> are no enough hint for parameter requirements, so any parameters
> >> combination maybe be used by user, moreover, it seems appropriate for
> >> many bad parameter combination, for example, provided  PMD_SIZE=2M and
> >> PAGE_SIZE=4K, 0x00 is used for aligned very well address
> >> a user maybe want to map virtual range[0x1ff800, 0x200800) to physical address
> >> 0x300800, it will cause endless loop
> > 
> > Well, we are relying on the kernel to do the sane thing otherwise we
> > would be screwed anyway. If this can be triggered by a userspace then it
> > would be a different story. Just look at how we are doing mmap, we
> > sanitize the page alignment at the high level and the lower level
> > functions just assume sane values.
> > 
> ioremap_page_range() is exported by EXPORT_SYMBOL_GPL() as a kernel interface
> so perhaps it is called by not only any kernel module authors but also other
> kernel parts
> 
> if the bad range is used by a careless kernel user really, it seems a better
> choice to alert the warning message or panic the kernel than hanging the system
> due to endless loop, it can help them locate problem usefully

I absolutely do not want to panic my system just because a crapy module
or whatnot doesn't provide an aligned address. Warning and a fixup
sounds much more sane to me.

[...]

> >> no, it don't work for many special case
> >> for example, provided  PMD_SIZE=2M
> >> mapping [0x1f8800, 0x208800) virtual range will be split to two ranges
> >> [0x1f8800, 0x200000) and [0x200000,0x208800) and map them separately
> >> the first range will cause dead loop
> > 
> > I am not sure I see your point. How can we deadlock if _both_ addresses
> > get aligned to the page boundary and how does PMD_SIZE make any
> > difference.
> > 
> i will take a example to illustrate my considerations
> provided PUD_SIZE == 1G, PMD_SIZE == 2M, PAGE_SIZE == 4K
> it is used by arm64 normally
> 
> we want to map virtual range [0xffffffff_ffc08800, 0xffffffff_fffff800) by
> ioremap_page_range(),ioremap_pmd_range() is called to map the range
> finally, ioremap_pmd_range() will call
> ioremap_pte_range(pmd, 0xffffffff_ffc08800, 0xffffffff_fffe0000) and
> ioremap_pte_range(pmd, 0xffffffff_fffe0000, 0xffffffff fffff800) separately

but those ranges are not aligned and it ioremap_page_range fix them up
to _be_ aligned then there is no problem, right? So either I am missing
something or we are talking past each other.

-- 
Michal Hocko
SUSE Labs

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