On Fri, Apr 04, 2014 at 01:28:22AM -0700, Hugh Dickins wrote: > get_user_pages(write=1, force=1) has always had odd behaviour on write- > protected shared mappings: although it demands FMODE_WRITE-access to the > underlying object (do_mmap_pgoff sets neither VM_SHARED nor VM_MAYWRITE > without that), it ends up with do_wp_page substituting private anonymous > Copied-On-Write pages for the shared file pages in the area. > > That was long ago intentional, as a safety measure to prevent ptrace > setting a breakpoint (or POKETEXT or POKEDATA) from inadvertently > corrupting the underlying executable. Yet exec and dynamic loaders > open the file read-only, and use MAP_PRIVATE rather than MAP_SHARED. > > The traditional odd behaviour still causes surprises and bugs in mm, > and is probably not what any caller wants - even the comment on the flag > says "You do not want this" (although it's undoubtedly necessary for > overriding userspace protections in some contexts, and good when !write). > > Let's stop doing that. But it would be dangerous to remove the long- > standing safety at this stage, so just make get_user_pages(write,force) > fail with EFAULT when applied to a write-protected shared area. > Infiniband may in future want to force write through to underlying > object: we can add another FOLL_flag later to enable that if required. > > Odd though the old behaviour was, there is no doubt that we may turn > out to break userspace with this change, and have to revert it quickly. > Issue a WARN_ON_ONCE to help debug the changed case (easily triggered > by userspace, so only once to prevent spamming the logs); and delay a > few associated cleanups until this change is proved. > > get_user_pages callers who might see trouble from this change: > ptrace poking, or writing to /proc/<pid>/mem > drivers/infiniband/ > drivers/media/v4l2-core/ > drivers/gpu/drm/exynos/exynos_drm_gem.c > drivers/staging/tidspbridge/core/tiomap3430.c > if they ever apply get_user_pages to write-protected shared mappings > of an object which was opened for writing. > > I went to apply the same change to mm/nommu.c, but retreated. NOMMU > has no place for COW, and its VM_flags conventions are not the same: > I'd be more likely to screw up NOMMU than make an improvement there. > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> There's comment in do_wp_page() which is not true anymore with patch applied. It should be fixed. Otherwise, looks good to me: Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
