Re: Have any influence on set_memory_** about below patch ??

[Mark Rutland]

On Wed, Jan 13, 2016 at 12:10:29PM +0800, Xishi Qiu wrote:
> On 2016/1/12 19:15, Mark Rutland wrote:
> 
> > On Tue, Jan 12, 2016 at 09:20:54AM +0800, Xishi Qiu wrote:
> >> On 2016/1/11 21:31, Mark Rutland wrote:
> >>
> >>> Hi,
> >>>
> >>> On Mon, Jan 11, 2016 at 08:59:44PM +0800, zhong jiang wrote:
> >>>>
> >>>> http://www.spinics.net/lists/arm-kernel/msg472090.html
> >>>>
> >>>> Hi, Can I ask you a question? Say, This patch tells that the section spilting
> >>>> and merging wiil produce confilct in the liner mapping area. Based on the
> >>>> situation, Assume that set up page table in 4kb page table way in the liner
> >>>> mapping area, Does the set_memroy_** will work without any conplict??
> >>>
> >>> I'm not sure I understand the question.
> >>>
> >>> I'm also not a fan of responding to off-list queries as information gets
> >>> lost.
> >>>
> >>> Please ask your question on the mailing list. I am more than happy to
> >>> respond there.
> >>>
> >>> Thanks,
> >>> Mark.
> >>>
> >>
> >> Hi Mark,
> >>
> >> In your patch it said "The presence of conflicting TLB entries may result in
> >> a variety of behaviours detrimental to the system " and "but this(break-before-make
> >> approach) cannot work for modifications to the swapper page tables that cover the
> >> kernel text and data."
> >>
> >> I'm not quite understand this, why the direct mapping can't work?
> > 
> > The problem is that the TLB hardware can operate asynchronously to the
> > rest of the CPU. At any point in time, for any reason, it can decide to
> > destroy TLB entries, to allocate new ones, or to perform a walk based on
> > the existing contents of the TLB.
> > 
> > When the TLB contains conflicting entries, TLB lookups may result in TLB
> > conflict aborts, or may return an "amalgamation" of the conflicting
> > entries (e.g. you could get an erroneous output address).
> > 
> > The direct mapping is in active use (and hence live in TLBs). Modifying
> > it without break-before-make (BBM) risks the allocation of conflicting
> > TLB entries. Modifying it with BBM risks unmapping the portion of the
> > kernel performing the modification, resulting in an unrecoverable abort.
> > 
> >> flush tlb can't resolve it?
> > 
> > Flushing the TLB doesn't help because the page table update, TLB
> > invalidate, and corresponding barrier(s) are separate operations. The
> > TLB can allocate or destroy entries at any point during the sequence.
> > 
> > For example, without BBM a page table update would look something like:
> > 
> > 1)	str	<newpte>, [<*pte>]
> > 2)	dsb	ish
> > 3)	tlbi	vmalle1is
> > 4)	dsb	ish
> > 5)	isb
> > 
> > After step 1, the new pte value may become visible to the TLBs, and the
> > TLBs may allocate a new entry for it. Until step 4 completes, this entry
> > may remain active in the TLB, and may conflict with an existing entry.
> > 
> > If that entry covers the kernel text for steps 2-5, executing the
> > sequence may result in an unrecoverable TLB conflict abort, or some
> > other behaviour resulting from an amalgamated TLB, e.g. the I-cache
> > might fetch instructions from the wrong address such that steps 2-5
> > cannot be executed.
> > 
> > If the kernel doesn't explicitly access the address covered by that pte,
> > there may still be a problem. The TLB may perform an internal lookup
> > when performing a page table walk, and could then use an erroneous
> > result to continue the walk, resulting in a variety of potential issues
> > (e.g. reading from an MMIO peripheral register).
> > 
> > BBM avoids the conflict, but as that would mean kernel text and/or data
> > would be unmapped, you can't execute the code to finish the update.
> > 
> >> I find x86 does not have this limit. e.g. set_memory_r*.
> > 
> > I don't know much about x86; it's probably worth asking the x86 guys
> > about that. It may be that the x86 architecture requires that a conflict
> > or amalgamation is never visible to software, or it could be that
> > contemporary implementations happen to provide that property.
> > 
> > Thanks,
> > Mark.
> > 
> 
> Hi Mark,

Hi,

> Thank you for your reply, I find this code in /arch/arm64/mm/mmu.c
> 
> ...
> #ifdef CONFIG_DEBUG_RODATA
> void mark_rodata_ro(void)
> {
> 	create_mapping_late(__pa(_stext), (unsigned long)_stext,
> 				(unsigned long)_etext - (unsigned long)_stext,
> 				PAGE_KERNEL_EXEC | PTE_RDONLY);
> 
> }
> #endif
> ...
> 
> So does it also have this problem?

Currently, yes.

I've addressed the splitting/merging problem with my pagetable rework
series [1,2]. The RO region is initially mapped at the same granularity
as it will be modified with, so only the permissions bits will change
when mark_rodata_ro is called.

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/397095.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/397114.html