[PATCH for-4.15] x86/ucode/amd: Handle length sanity check failures more gracefully

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Currently, a failure of verify_patch_size() causes an early abort of the
microcode blob loop, which in turn causes a second go around the main
container loop, ultimately failing the UCODE_MAGIC check.

First, check for errors after the blob loop.  An error here is unrecoverable,
so avoid going around the container loop again and printing an
unhelpful-at-best error concerning bad UCODE_MAGIC.

Second, split the verify_patch_size() check out of the microcode blob header
check.  In the case that the sanity check fails, we can still use the
known-to-be-plausible header length to continue walking the container to
potentially find other applicable microcode blobs.

Before:
  (XEN) microcode: Bad microcode data
  (XEN) microcode: Wrong microcode patch file magic
  (XEN) Parsing microcode blob error -22

After:
  (XEN) microcode: Bad microcode length 0x000015c0 for cpu 0xa000
  (XEN) microcode: Bad microcode length 0x000015c0 for cpu 0xa010
  (XEN) microcode: Bad microcode length 0x000015c0 for cpu 0xa011
  (XEN) microcode: Bad microcode length 0x000015c0 for cpu 0xa200
  (XEN) microcode: Bad microcode length 0x000015c0 for cpu 0xa210
  (XEN) microcode: Bad microcode length 0x000015c0 for cpu 0xa500
  (XEN) microcode: couldn't find any matching ucode in the provided blob!

Fixes: 4de936a38a ("x86/ucode/amd: Rework parsing logic in 
cpu_request_microcode()")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Ian Jackson <iwj@xxxxxxxxxxxxxx>

For 4.15.  Found when putting a test together to prove the correctness of the
"x86/ucode: Fix microcode payload size for Fam19 processors" fix.

This allows microcode loading to still function even if the length magic
numbers aren't correct for a subset of blobs within the container(s).
---
 xen/arch/x86/cpu/microcode/amd.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu/microcode/amd.c b/xen/arch/x86/cpu/microcode/amd.c
index c4ab395799..fe7b79bd0a 100644
--- a/xen/arch/x86/cpu/microcode/amd.c
+++ b/xen/arch/x86/cpu/microcode/amd.c
@@ -348,8 +348,7 @@ static struct microcode_patch *cpu_request_microcode(const 
void *buf, size_t siz
 
             if ( size < sizeof(*mc) ||
                  (mc = buf)->type != UCODE_UCODE_TYPE ||
-                 size - sizeof(*mc) < mc->len ||
-                 (!skip_ucode && !verify_patch_size(mc->len)) )
+                 size - sizeof(*mc) < mc->len )
             {
                 printk(XENLOG_ERR "microcode: Bad microcode data\n");
                 error = -EINVAL;
@@ -359,6 +358,19 @@ static struct microcode_patch *cpu_request_microcode(const 
void *buf, size_t siz
             if ( skip_ucode )
                 goto skip;
 
+            if ( !verify_patch_size(mc->len) )
+            {
+                printk(XENLOG_WARNING
+                       "microcode: Bad microcode length 0x%08x for cpu 
0x%04x\n",
+                       mc->len, mc->patch->processor_rev_id);
+                /*
+                 * If the blob size sanity check fails, trust the container
+                 * length which has already been checked to be at least
+                 * plausible at this point.
+                 */
+                goto skip;
+            }
+
             /*
              * If the new ucode covers current CPU, compare ucodes and store 
the
              * one with higher revision.
@@ -382,6 +394,14 @@ static struct microcode_patch *cpu_request_microcode(const 
void *buf, size_t siz
             if ( size >= 4 && *(const uint32_t *)buf == UCODE_MAGIC )
                 break;
         }
+
+        /*
+         * Any error means we didn't get cleanly to the end of the microcode
+         * container.  There isn't an overall length field, so we've got no
+         * way of skipping to the next container in the stream.
+         */
+        if ( error )
+            break;
     }
 
     if ( saved )
-- 
2.11.0