linux-hardening - Re: [PATCH] RDMA/mlx5: Use memset_after() to zero struct mlx5_ib

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Date:   Wed, 12 Jan 2022 20:32:20 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Leon Romanovsky <leon@...nel.org>, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] RDMA/mlx5: Use memset_after() to zero struct mlx5_ib_mr

On Wed, Jan 12, 2022 at 12:49:13PM -0800, Kees Cook wrote:
> On Tue, Dec 07, 2021 at 03:45:25PM -0400, Jason Gunthorpe wrote:
> > On Tue, Dec 07, 2021 at 11:41:07AM -0800, Kees Cook wrote:
> > > On Tue, Dec 07, 2021 at 02:47:29PM -0400, Jason Gunthorpe wrote:
> > > > On Sun, Nov 21, 2021 at 03:54:55PM +0200, Leon Romanovsky wrote:
> > > > > On Thu, Nov 18, 2021 at 12:31:38PM -0800, Kees Cook wrote:
> > > > > > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > > > > > field bounds checking for memset(), avoid intentionally writing across
> > > > > > neighboring fields.
> > > > > > 
> > > > > > Use memset_after() to zero the end of struct mlx5_ib_mr that should
> > > > > > be initialized.
> > > > > > 
> > > > > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > > > >  drivers/infiniband/hw/mlx5/mlx5_ib.h | 5 ++---
> > > > > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/infiniband/hw/mlx5/mlx5_ib.h b/drivers/infiniband/hw/mlx5/mlx5_ib.h
> > > > > > index e636e954f6bf..af94c9fe8753 100644
> > > > > > +++ b/drivers/infiniband/hw/mlx5/mlx5_ib.h
> > > > > > @@ -665,8 +665,7 @@ struct mlx5_ib_mr {
> > > > > >  	/* User MR data */
> > > > > >  	struct mlx5_cache_ent *cache_ent;
> > > > > >  	struct ib_umem *umem;
> > > > > > -
> > > > > > -	/* This is zero'd when the MR is allocated */
> > > > > > +	/* Everything after umem is zero'd when the MR is allocated */
> > > > > >  	union {
> > > > > >  		/* Used only while the MR is in the cache */
> > > > > >  		struct {
> > > > > > @@ -718,7 +717,7 @@ struct mlx5_ib_mr {
> > > > > >  /* Zero the fields in the mr that are variant depending on usage */
> > > > > >  static inline void mlx5_clear_mr(struct mlx5_ib_mr *mr)
> > > > > >  {
> > > > > > -	memset(mr->out, 0, sizeof(*mr) - offsetof(struct mlx5_ib_mr, out));
> > > > > > +	memset_after(mr, 0, umem);
> > > > > 
> > > > > I think that it is not equivalent change and you need "memset_after(mr, 0, cache_ent);"
> > > > > to clear umem pointer too.
> > > > 
> > > > Kees?
> > > 
> > > Oops, sorry, I missed the ealrier reply!
> > > 
> > > I don't think that matches -- the original code wipes from the start of
> > > "out" to the end of the struct. "out" is the first thing in the union
> > > after "umem", so "umem" was not wiped before. I retained that behavior
> > > ("wipe everything after umem").
> > > 
> > > Am I misunderstanding the desired behavior here?
> > 
> > Ah, it is this patch:
> > 
> > commit f0ae4afe3d35e67db042c58a52909e06262b740f
> > Author: Alaa Hleihel <alaa@...dia.com>
> > Date:   Mon Nov 22 13:41:51 2021 +0200
> > 
> >     RDMA/mlx5: Fix releasing unallocated memory in dereg MR flow
> > 
> > Which moved umem into the union that is causing the confusion
> > 
> > It hasn't quite made it to a rc release yet, so I suppose the answer
> > is to rebase this on that then it is as Leon  says about cache_ent
> 
> The umem patch appears to have been reverted. Should I send an updated
> patch for memset_after()? I think it would simply be:

No, I'll fix it in the merge. It is still correct to zero everything
after cache_ent

Thanks,
Jason