Re: [PATCH v4.19.x] make 'user_access_begin()' do 'access_ok()'

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, May 13, 2020 at 07:19:21AM +0530, ashwin-h wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
> 
> commit 594cc251fdd0d231d342d88b2fdff4bc42fb0690 upstream.
> 
> Originally, the rule used to be that you'd have to do access_ok()
> separately, and then user_access_begin() before actually doing the
> direct (optimized) user access.
> 
> But experience has shown that people then decide not to do access_ok()
> at all, and instead rely on it being implied by other operations or
> similar.  Which makes it very hard to verify that the access has
> actually been range-checked.
> 
> If you use the unsafe direct user accesses, hardware features (either
> SMAP - Supervisor Mode Access Protection - on x86, or PAN - Privileged
> Access Never - on ARM) do force you to use user_access_begin().  But
> nothing really forces the range check.
> 
> By putting the range check into user_access_begin(), we actually force
> people to do the right thing (tm), and the range check vill be visible
> near the actual accesses.  We have way too long a history of people
> trying to avoid them.
> 
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Ashwin H <ashwinh@vmware.com>
> ---
>  arch/x86/include/asm/uaccess.h             | 11 ++++++++++-
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 15 +++++++++++++--
>  include/linux/uaccess.h                    |  2 +-
>  kernel/compat.c                            |  6 ++----
>  kernel/exit.c                              |  6 ++----
>  lib/strncpy_from_user.c                    |  9 +++++----
>  lib/strnlen_user.c                         |  9 +++++----
>  7 files changed, 38 insertions(+), 20 deletions(-)

Are you wanting this merged to a specific stable kernel tree?  If so, why?

thanks,

greg k-h