Re: [PATCH for-rc v5] IB/hfi1: Ensure correct mm is used at all times

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Nov 25, 2020 at 04:01:12PM -0500, Dennis Dalessandro wrote:
> Two earlier bug fixes have created a security problem in the hfi1
> driver. One fix aimed to solve an issue where current->mm was not valid
> when closing the hfi1 cdev. It attempted to do this by saving a cached
> value of the current->mm pointer at file open time. This is a problem if
> another process with access to the FD calls in via write() or ioctl() to
> pin pages via the hfi driver. The other fix tried to solve a use after
> free by taking a reference on the mm.
> 
> To fix this correctly we use the existing cached value of the mm in the
> mmu notifier. Now we can check in the insert, evict, etc. routines that
> current->mm matched what the notifier was registered for. If not, then
> don't allow access. The register of the mmu notifier will save the mm
> pointer.
> 
> Note the check in the unregister is not needed in the event that
> current->mm is empty. This means the tear down is happening due to a
> SigKill or OOM Killer, something along those lines. If current->mm has a
> value then it must be checked and only the task that did the register
> can do the unregister.

I deleted this paragraph, it doesn't apply any more

> Since in do_exit() the exit_mm() is called before exit_files(), which
> would call our close routine a reference is needed on the mm. We rely on
> the mmgrab done by the registration of the notifier, whereas before it
> was explicit. The mmu notifier deregistration happens when the user
> context is torn down, the creation of which triggered the registration.
> 
> Also of note is we do not do any explicit work to protect the interval
> tree notifier. It doesn't seem that this is going to be needed since we
> aren't actually doing anything with current->mm. The interval tree
> notifier stuff still has a FIXME noted from a previous commit that will
> be addressed in a follow on patch.
> 
> Fixes: e0cf75deab81 ("IB/hfi1: Fix mm_struct use after free")
> Fixes: 3faa3d9a308e ("IB/hfi1: Make use of mm consistent")
> Cc: <stable@vger.kernel.org>
> Suggested-by: Jann Horn <jannh@google.com>
> Reported-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Mike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>
> Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>

Applied to for-rc

Thanks,
Jason