[xen stable-4.15] xen/gnttab: fix gnttab_acquire_resource()

[patchbot@xxxxxxx]

commit 3ac64b3751837a117ee3dfb3e2cc27057a83d0f7
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Tue Oct 11 15:16:53 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Oct 11 15:16:53 2022 +0200

    xen/gnttab: fix gnttab_acquire_resource()
    
    Commit 9dc46386d89d ("gnttab: work around "may be used uninitialized"
    warning") was wrong, as vaddrs can legitimately be NULL in case
    XENMEM_resource_grant_table_id_status was specified for a grant table
    v1. This would result in crashes in debug builds due to
    ASSERT_UNREACHABLE() triggering.
    
    Check vaddrs only to be NULL in the rc == 0 case.
    
    Expand the tests in tools/tests/resource to tickle this path, and verify 
that
    using XENMEM_resource_grant_table_id_status on a v1 grant table fails.
    
    Fixes: 9dc46386d89d ("gnttab: work around "may be used uninitialized" 
warning")
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> # xen
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 52daa6a8483e4fbd6757c9d1b791e23931791608
    master date: 2022-09-09 16:28:38 +0100
---
 tools/tests/resource/test-resource.c | 15 +++++++++++++++
 xen/common/grant_table.c             |  2 +-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/tools/tests/resource/test-resource.c 
b/tools/tests/resource/test-resource.c
index 1caaa60e62..bf485baff2 100644
--- a/tools/tests/resource/test-resource.c
+++ b/tools/tests/resource/test-resource.c
@@ -63,6 +63,21 @@ static void test_gnttab(uint32_t domid, unsigned int 
nr_frames)
     rc = xenforeignmemory_unmap_resource(fh, res);
     if ( rc )
         return fail("    Fail: Unmap %d - %s\n", errno, strerror(errno));
+
+    /*
+     * Verify that an attempt to map the status frames fails, as the domain is
+     * in gnttab v1 mode.
+     */
+    res = xenforeignmemory_map_resource(
+        fh, domid, XENMEM_resource_grant_table,
+        XENMEM_resource_grant_table_id_status, 0, 1,
+        (void **)&gnttab, PROT_READ | PROT_WRITE, 0);
+
+    if ( res )
+    {
+        fail("    Fail: Managed to map gnttab v2 status frames in v1 mode\n");
+        xenforeignmemory_unmap_resource(fh, res);
+    }
 }
 
 static void test_domain_configurations(void)
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 0523beb9b7..01e426c67f 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -4138,7 +4138,7 @@ int gnttab_acquire_resource(
      * on non-error paths, and hence it needs setting to NULL at the top of the
      * function.  Leave some runtime safety.
      */
-    if ( !vaddrs )
+    if ( !rc && !vaddrs )
     {
         ASSERT_UNREACHABLE();
         rc = -ENODATA;
--
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