[xen staging] x86/p2m: fix p2m_add_foreign error path

[patchbot@xxxxxxx]

commit 62dde2c1eff6d4e5d45883b78bdae5447e7d2d9c
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Mon Jan 4 10:03:23 2021 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Jan 4 10:41:59 2021 +0000

    x86/p2m: fix p2m_add_foreign error path
    
    One of the error paths in p2m_add_foreign could call put_page with a
    NULL page, thus triggering a fault.
    
    Split the checks into two different if statements, so the appropriate
    error path can be taken.
    
    Fixes: 173ae325026bd ('x86/p2m: tidy p2m_add_foreign() a little')
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/mm/p2m.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 487959b121..a4ebfc9b21 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -2601,8 +2601,13 @@ static int p2m_add_foreign(struct domain *tdom, unsigned 
long fgfn,
      *     ram_rw | ram_logdirty | ram_ro | paging_out.
      */
     page = get_page_from_gfn(fdom, fgfn, &p2mt, P2M_ALLOC);
-    if ( !page ||
-         !p2m_is_ram(p2mt) || p2m_is_shared(p2mt) || p2m_is_hole(p2mt) )
+    if ( !page )
+    {
+        rc = -EINVAL;
+        goto out;
+    }
+
+    if ( !p2m_is_ram(p2mt) || p2m_is_shared(p2mt) || p2m_is_hole(p2mt) )
     {
         rc = -EINVAL;
         goto put_one;
--
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