Re: ptrace and pfn mappings

[Nick Piggin <npiggin@suse.de>]

On Tue, Oct 10, 2006 at 01:40:56PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2006-10-10 at 04:58 +0200, Nick Piggin wrote:
> > On Tue, Oct 10, 2006 at 12:47:46PM +1000, Benjamin Herrenschmidt wrote:
> > > 
> > > > Switch the mm and do a copy_from_user? (rather than the GUP).
> > > > Sounds pretty ugly :P
> > > > 
> > > > Can you do a get_user_pfns, and do a copy_from_user on the pfn
> > > > addresses? In other words, is the memory / mmio at the end of a
> > > > given address the same from the perspective of any process? It
> > > > is for physical memory of course, which is why get_user_pages
> > > > works...
> > > 
> > > Doesn't help with the racyness.
> > 
> > I don't understand what the racyness is that you can solve by accessing
> > it from the target process's mm?
> 
> You get a struct page or a pfn, you race with the migration, and access
> something that isn't the "current" one. Doing an actual access goes
> through the normal mmu path which guarantees that after the migration
> has finished its unmap_mapping_ranges(), no access via those old PTEs is
> possible (tlbs have been flushed etc...). We don't get such guarantee if
> we get a struct page or a pfn and go peek at it.

OK, so it is a matter of preventing the migration while this is going on.
BTW. I think you need to disallow get_user_pages to this region entirely,
regardless of whether it is backed by a page or not: there is no guarantee
of when the caller will release the page.

> > > > What if you hold your per-object lock over the operation? (I guess
> > > > it would have to nest *inside* mmap_sem, but that should be OK).
> > > 
> > > Over the ptrace operation ? how so ?
> > 
> > You just have to hold it over access_process_vm, AFAIKS. Once it
> > is copied into the kernel buffer that's done. Maybe I misunderstood
> > what the race is?
> 
> But since when ptrace knows about various private locks of objects that
> are backing vma's ?

Since we decided it would be better to make a new function or some arch
specfic hooks rather than switch mm's in the kernel? ;)

No, I don't know. Your idea might be reasonable, but I really haven't
thought about it much.

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