Re: [git pull] vfs.git pile 11

From: Kees Cook
Date: Thu Jul 06 2017 - 15:45:58 EST

On Thu, Jul 6, 2017 at 2:12 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> iov_iter/uaccess/hardening pile. For one thing, it trims the
> inline part of copy_to_user/copy_from_user to the minimum that *does*
> need to be inlined - object size checks, basically. For another,
> it sanitizes the checks for iov_iter primitives. There are 4 groups
> of checks: access_ok(), might_fault(), object size and KASAN.
> * access_ok() had been verified by whoever had set the iov_iter
> up. However, that has happened in a function far away, so proving that
> there's no path to actual copying bypassing those checks is hard and
> proving that iov_iter has not been buggered in the meanwhile is also
> not pleasant. So we want those redone in actual copyin/copyout.
> * might_fault() is better off consolidated - we know whether
> it needs to be checked as soon as we enter iov_iter primitive and
> observe the iov_iter flavour. No need to wait until the copyin/copyout.
> The call chains are short enough to make sure we won't miss anything -
> in fact, it's more robust that way, since there are cases where we do
> e.g. forced fault-in before getting to copyin/copyout.
> * KASAN checks belong in copyin/copyout - at the same level
> where other iov_iter flavours would've hit them in memcpy().
> * object size checks should apply to *all* iov_iter flavours,
> not just iovec-backed ones.
> There are two groups of primitives - one gets the kernel object
> described as pointer + size (copy_to_iter(), etc.) while another gets
> it as page + offset + size (copy_page_to_iter(), etc.)
> For the first group the checks are best done where we actually
> have a chance to find the object size. In other words, those belong in
> inline wrappers in uio.h, before calling into iov_iter.c. Same kind
> as we have for inlined part of copy_to_user().
> For the second group there is no object to look at - offset in
> page is just a number, it bears no type information. So we do them
> in the common helper called by iov_iter.c primitives of that kind.
> All it currently does is checking that we are not trying to access
> outside of the compound page; eventually we might want to add some
> sanity checks on the page involved.
> So the things we need in copyin/copyout part of iov_iter.c
> do not quite match anything in uaccess.h (we want no zeroing, we *do*
> want access_ok() and KASAN and we want no might_fault() or object size
> checks done on that level). OTOH, these needs are simple enough to
> provide a couple of helpers (static in iov_iter.c) doing just what
> we need...
> The following changes since commit 2ea659a9ef488125eb46da6eb571de5eae5c43f6:
> Linux 4.12-rc1 (2017-05-13 13:19:49 -0700)
> are available in the git repository at:
> git:// uaccess-work.iov_iter
> for you to fetch changes up to ea93a426af164d346a0b4fe0836143bf32177330:
> iov_iter: saner checks on copyin/copyout (2017-06-29 22:29:36 -0400)
> ----------------------------------------------------------------
> Al Viro (5):
> copy_{from,to}_user(): move kasan checks and might_fault() out-of-line
> copy_{to,from}_user(): consolidate object size checks

We still need to fix the missed-zeroing-on-overflow corner-case:

> iov_iter/hardening: move object size checks to inlined part

+ if (unlikely(!check_copy_size(addr, bytes, false)))
+ return false;
+ else
+ return _copy_from_iter_full(addr, bytes, i);

Can these be rewritten to avoid the double-negative?

> iov_iter: sanity checks for copy to/from page primitives

Nice to see these!

> iov_iter: saner checks on copyin/copyout

+ might_fault();

Should this be might_sleep()? Just from reading the patch it looked
like you were adding might_sleep()s in the other cases.

> include/linux/thread_info.h | 27 +++++++++++++
> include/linux/uaccess.h | 44 +++++----------------
> include/linux/uio.h | 58 ++++++++++++++++++++++++---
> lib/iov_iter.c | 96 ++++++++++++++++++++++++++++++++-------------
> lib/usercopy.c | 10 ++++-
> 5 files changed, 167 insertions(+), 68 deletions(-)


Kees Cook
Pixel Security