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@xxxxxxxxx> 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);
