Xen devel - Dec 2013 - [PATCH v2] xen: arm: do not BUG on guest paddrs which are very high

The BUG_ON in p2m_map_first was over aggressive since the paddr_t can have
come from the guest, via add_to_physmap. Instead return failure to the caller.

Also the check was simultaneously too lose. The valid offsets are
0..P2M_FIRST_ENTRIES-1 inclusive.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
---
v2: Don''t access uninitialised pte on the error path
---
 xen/arch/arm/p2m.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 1d5c841..083f8bf 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -58,7 +58,8 @@ static lpae_t *p2m_map_first(struct p2m_domain *p2m, paddr_t
addr)
 {
     struct page_info *page;
 
-    BUG_ON(first_linear_offset(addr) > P2M_FIRST_ENTRIES);
+    if ( first_linear_offset(addr) >= P2M_FIRST_ENTRIES )
+        return NULL;
 
     page = p2m->first_level + p2m_first_level_index(addr);
 
@@ -80,6 +81,8 @@ paddr_t p2m_lookup(struct domain *d, paddr_t paddr)
     spin_lock(&p2m->lock);
 
     first = p2m_map_first(p2m, paddr);
+    if ( !first )
+        goto err;
 
     pte = first[first_table_offset(paddr)];
     if ( !pte.p2m.valid || !pte.p2m.table )
@@ -105,6 +108,7 @@ done:
     if (second) unmap_domain_page(second);
     if (first) unmap_domain_page(first);
 
+err:
     spin_unlock(&p2m->lock);
 
     return maddr;
@@ -181,6 +185,11 @@ static int create_p2m_entries(struct domain *d,
         {
             if ( first ) unmap_domain_page(first);
             first = p2m_map_first(p2m, addr);
+            if ( !first )
+            {
+                rc = -EINVAL;
+                goto out;
+            }
             cur_first_page = p2m_first_level_index(addr);
         }
 
-- 
1.7.10.4

On 12/09/2013 11:09 AM, Ian Campbell wrote:
> The BUG_ON in p2m_map_first was over aggressive since the paddr_t can have
> come from the guest, via add_to_physmap. Instead return failure to the
caller.
>
> Also the check was simultaneously too lose. The valid offsets are
> 0..P2M_FIRST_ENTRIES-1 inclusive.
>
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
> ---
> v2: Don''t access uninitialised pte on the error path
> ---
>   xen/arch/arm/p2m.c |   11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 1d5c841..083f8bf 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -58,7 +58,8 @@ static lpae_t *p2m_map_first(struct p2m_domain *p2m,
paddr_t addr)
>   {
>       struct page_info *page;
>
> -    BUG_ON(first_linear_offset(addr) > P2M_FIRST_ENTRIES);
> +    if ( first_linear_offset(addr) >= P2M_FIRST_ENTRIES )
> +        return NULL;
>
>       page = p2m->first_level + p2m_first_level_index(addr);
>
> @@ -80,6 +81,8 @@ paddr_t p2m_lookup(struct domain *d, paddr_t paddr)
>       spin_lock(&p2m->lock);
>
>       first = p2m_map_first(p2m, paddr);
> +    if ( !first )
> +        goto err;
>
>       pte = first[first_table_offset(paddr)];
>       if ( !pte.p2m.valid || !pte.p2m.table )
> @@ -105,6 +108,7 @@ done:
>       if (second) unmap_domain_page(second);
>       if (first) unmap_domain_page(first);
>
> +err:
>       spin_unlock(&p2m->lock);
>
>       return maddr;
> @@ -181,6 +185,11 @@ static int create_p2m_entries(struct domain *d,
>           {
>               if ( first ) unmap_domain_page(first);
>               first = p2m_map_first(p2m, addr);
> +            if ( !first )
> +            {
> +                rc = -EINVAL;
> +                goto out;
> +            }
>               cur_first_page = p2m_first_level_index(addr);
>           }
>
>
-- 
Julien Grall