Re: [PATCH] mm/highmem: Align-down to page the address for kunmap_flush_on_unmap()

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jan 26, 2023 at 09:04:02PM +0000, Al Viro wrote:
> On Thu, Jan 26, 2023 at 08:48:03PM +0000, Matthew Wilcox wrote:
> > On Thu, Jan 26, 2023 at 12:38:58PM -0800, Andrew Morton wrote:
> > > On Thu, 26 Jan 2023 15:33:46 +0100 "Fabio M. De Francesco" <fmdefrancesco@gmail.com> wrote:
> > > 
> > > > If ARCH_HAS_FLUSH_ON_KUNMAP is defined (PA-RISC case), __kunmap_local()
> > > > calls kunmap_flush_on_unmap(). The latter currently flushes the wrong
> > > > address (as confirmed by Matthew Wilcox and Helge Deller). Al Viro
> > > > proposed to call kunmap_flush_on_unmap() on an aligned-down to page
> > > > address in order to fix this issue. Consensus has been reached on this
> > > > solution.
> > > 
> > > What are the user-visible runtime effects of this flaw?
> > 
> > The version of this patch I sent out includes this information,
> > as well as the missed alignment for kunmap_atomic().
> 
> One point: AFAICS, the situation right now is
> 	* all callers of kunmap_local() pass page-aligned pointers

Ah, no.  kmap_local_folio() accepts a byte offset within the folio
and returns a pointer to that byte.  I hadn't noticed the parisc
case and thought it was already allowed to pass a misaligned pointer
to kunmap_local() since it is allowed for the highmem case.  It
simplified the callers, so it looked like a good tradeoff.

See, eg 338f379cf7c2:

-               src_addr = kmap_atomic(src_page);
-               dest_addr = kmap_atomic(dest_page);
+               src_addr = kmap_local_folio(src_folio,
+                                       offset_in_folio(src_folio, srcoff));
+               dst_addr = kmap_local_folio(dst_folio,
+                                       offset_in_folio(dst_folio, dstoff));

-               if (memcmp(src_addr + src_poff, dest_addr + dest_poff, cmp_len))
+               if (memcmp(src_addr, dst_addr, cmp_len))

-               kunmap_atomic(dest_addr);
-               kunmap_atomic(src_addr);
+               kunmap_local(dst_addr);
+               kunmap_local(src_addr);