Re: [PATCH] mm: get_user_pages(write,force) refuse to COW in shared areas

["Kirill A. Shutemov" <kirill@shutemov.name>]

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@linux-foundation.org>
> Signed-off-by: Hugh Dickins <hughd@google.com>

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@linux.intel.com>

-- 
 Kirill A. Shutemov

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