[Xen-devel] [PATCH 21/22] libxc: range checks in xc_dom_p2m_host and _guest

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

These functions take guest pfns and look them up in the p2m.  They did
no range checking.

However, some callers, notably xc_dom_boot.c:setup_hypercall_page want
to pass untrusted guest-supplied value(s).  It is most convenient to
detect this here and return INVALID_MFN.

This is part of the fix to a security issue, XSA-55.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>

v6: Check for underflow too (thanks to Andrew Cooper).
---
 tools/libxc/xc_dom.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/tools/libxc/xc_dom.h b/tools/libxc/xc_dom.h
index 5968e7b..86e23ee 100644
--- a/tools/libxc/xc_dom.h
+++ b/tools/libxc/xc_dom.h
@@ -342,6 +342,8 @@ static inline xen_pfn_t xc_dom_p2m_host(struct xc_dom_image 
*dom, xen_pfn_t pfn)
 {
     if (dom->shadow_enabled)
         return pfn;
+    if (pfn < dom->rambase_pfn || pfn >= dom->rambase_pfn + dom->total_pages)
+        return INVALID_MFN;
     return dom->p2m_host[pfn - dom->rambase_pfn];
 }
 
@@ -350,6 +352,8 @@ static inline xen_pfn_t xc_dom_p2m_guest(struct 
xc_dom_image *dom,
 {
     if (xc_dom_feature_translated(dom))
         return pfn;
+    if (pfn < dom->rambase_pfn || pfn >= dom->rambase_pfn + dom->total_pages)
+        return INVALID_MFN;
     return dom->p2m_host[pfn - dom->rambase_pfn];
 }
 
-- 
1.7.2.5


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