[Xen-devel] [Patch v2 3/4] AMD/microcode: Avoid use-after-free for the microcode buffer

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

It is possible to free the mc_old buffer and then store it for use in the case
of resume.

This keeps the old semantics of being able to return an error even after a
successful microcode application.

Coverity-ID 1146953
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
Acked-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>

---
Changes in v2:
 * Fix commit message.  No code change.
---
 xen/arch/x86/microcode_amd.c |   17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/microcode_amd.c b/xen/arch/x86/microcode_amd.c
index a3ceef8..8ea4e63 100644
--- a/xen/arch/x86/microcode_amd.c
+++ b/xen/arch/x86/microcode_amd.c
@@ -275,7 +275,7 @@ static int cpu_request_microcode(int cpu, const void *buf, 
size_t bufsize)
     struct microcode_amd *mc_amd, *mc_old;
     size_t offset = bufsize;
     size_t last_offset, applied_offset = 0;
-    int error = 0;
+    int error = 0, save_error = 1;
     struct ucode_cpu_info *uci = &per_cpu(ucode_cpu_info, cpu);
 
     /* We should bind the task to the CPU */
@@ -338,19 +338,20 @@ static int cpu_request_microcode(int cpu, const void 
*buf, size_t bufsize)
      */
     if ( applied_offset )
     {
-        int ret = get_ucode_from_buffer_amd(mc_amd, buf, bufsize,
-                                            &applied_offset);
-        if ( ret == 0 )
-            xfree(mc_old);
-        else
-            error = ret;
+        save_error = get_ucode_from_buffer_amd(
+            mc_amd, buf, bufsize, &applied_offset);
+
+        if ( save_error )
+            error = save_error
     }
 
-    if ( !applied_offset || error )
+    if ( save_error )
     {
         xfree(mc_amd);
         uci->mc.mc_amd = mc_old;
     }
+    else
+        xfree(mc_old);
 
   out:
     svm_host_osvw_init();
-- 
1.7.10.4


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