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Re: [Xen-devel] [PATCH v7 10/10] xen/common: do not implicitly permit access to mapped I/O memory

On 26/05/14 12:37, Jan Beulich wrote:
On 26.05.14 at 13:24, <julien.grall@xxxxxxxxxx> wrote:
On 26/05/14 12:14, Jan Beulich wrote:
On 26.05.14 at 12:53, <julien.grall@xxxxxxxxxx> wrote:
On 26/05/14 11:14, Jan Beulich wrote:

Or maybe I wasn't wrong - the patch context doesn't really make
clear whether it's the granting or mapping operation that gets
adjusted here (since an earlier patch moved the mapping one into
this function).

            ret = -EPERM;
-        if ( !iomem_access_permitted(current->domain, mfn, mfn_end) )
+        if ( !iomem_access_permitted(d, mfn, mfn_end) )

            ret = xsm_iomem_mapping(XSM_HOOK, d, mfn, mfn_end, add);

There is an xsm_iomem_mapping just after, so the change has been done in

In which case I indeed stick to my original comment - it's perhaps
best to check _both_.

Why? We may want to map the region in the guest P2M without giving the
permission to the guest (I'm thinking about ARM passthrough case).

How can you put a mapping of memory into a guest's P2M for which
that guest has no access permission? To me this reads like you're
intending to create a security issue here.

iomem_access_permitted is used to check if we allow the current guest to map a region in another guest P2M.

Once the mapping is done, at least on ARM, we don't use anymore the permission check. This is because there is no trap involved afterwards.

In a such case, I don't see any posssible security issue.

With your requirements, we have to call 2 hypercalls rather than one for
memory mapping, even if we don't want to allow the guest modifying iomem

While I can see you not allowing modification, even r/o access may
(and likely will) be problematic for MMIO.

AFAIU, iomem_access_permitted is only here to allow modification of this range via hypercall.


Julien Grall

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