Re: [PATCH rdma-next 3/6] RDMA/ucontext: Do not allow BAR mappings to be executable

[Jason Gunthorpe <jgg@mellanox.com>]

On Wed, Apr 17, 2019 at 07:05:37PM +0000, Ruhl, Michael J wrote:

> >diff --git a/drivers/infiniband/core/uverbs_main.c
> >b/drivers/infiniband/core/uverbs_main.c
> >index fef4519d1241..3ef6474cd201 100644
> >+++ b/drivers/infiniband/core/uverbs_main.c
> >@@ -889,6 +889,10 @@ static struct rdma_umap_priv
> >*rdma_user_mmap_pre(struct ib_ucontext *ucontext,
> > 	struct ib_uverbs_file *ufile = ucontext->ufile;
> > 	struct rdma_umap_priv *priv;
> >
> >+	if (vma->vm_flags & VM_EXEC)
> >+		return ERR_PTR(-EINVAL);
> >+	vma->vm_flags &= ~VM_MAYEXEC;
> >+
> 
> A change like this was made in HFI with:
> 
> commit 12220267645cb7d1f3f699218e0098629e932e1f
> IB/hfi: Protect against writable mmap
> 
> This caused user applications that use the stack for execution to fail.
> The VM_EXEC flag is passed down during mmaps.
> 
> We had to remove this patch with:
> 
> commit 7709b0dc265f28695487712c45f02bbd1f98415d
> IB/hfi1: Remove overly conservative VM_EXEC flag check
> 
> to resolve this issue.
> 
> I am not sure if this is an equivalent issue, but the code path
> appears very similar.

It does seem problematic here too

Kees: You have worked in this W^X area in other parts of the kernel,
what should drivers do here?

The situation is we have a driver providing mmap against BAR memory
that is absolutely not intended for execution, so we would prefer to
block VM_EXEC in the driver's mmap fops callback

However READ_IMPLIES_EXEC forces VM_EXEC on for everything with no way
to opt out..

Jason