[Xen-devel] [PATCH for-4.13 v2] xen/arm: Don't use _end in is_xen_fixed_mfn()

[Julien Grall <julien.grall@xxxxxxx>]

virt_to_maddr() is using the hardware page-table walk instructions to
translate a virtual address to physical address. The function should
only be called on virtual address mapped.

_end points past the end of Xen binary and may not be mapped when the
binary size is page-aligned. This means virt_to_maddr() will not be able
to do the translation and therefore crash Xen.

Note there is also an off-by-one issue in this code, but the panic will
trump that.

Both issues can be fixed by using _end - 1 in the check.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
Release-acked-by: Juergen Gross <jgross@xxxxxxxx>

---

Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Julien Grall <julien@xxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>

x86 seems to be affected by the off-by-one issue. Jan, Andrew?

This could be reached by a domain via XEN_SYSCTL_page_offline_op.
However, the operation is not security supported (see XSA-77). So we are
fine here.

    Changes in v2:
        - Cast _end to vaddr_t to prevent UB.
        - Add Juergen's released-acked-by
---
 xen/include/asm-arm/mm.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
index 262d92f18d..333efd3a60 100644
--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -153,7 +153,7 @@ extern unsigned long xenheap_base_pdx;
 
 #define is_xen_fixed_mfn(mfn)                                   \
     ((mfn_to_maddr(mfn) >= virt_to_maddr(&_start)) &&           \
-     (mfn_to_maddr(mfn) <= virt_to_maddr(&_end)))
+     (mfn_to_maddr(mfn) <= virt_to_maddr((vaddr_t)_end - 1)))
 
 #define page_get_owner(_p)    (_p)->v.inuse.domain
 #define page_set_owner(_p,_d) ((_p)->v.inuse.domain = (_d))
-- 
2.11.0


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