[PATCH 06/10] x86/ucode: Enforce invariant about module selection

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The work to add the `ucode=nmi` cmdline option left a subtle corner case.
Both scan and an explicit index could be selected, and we could really find a
CPIO archive and explicit microcode file.

Worse, because the if/else chains for processing ucode_{blob,mod} are opposite
ways around in early_microcode_load() and microcode_init_cache(), we can
genuinely perform early microcode loading from the CPIO archive, then cache
from the explicit file.

Therefore, enforce that only one selection method can be active.

Rename ucode_{scan,mod_idx} to have an opt_ prefix.  This is both for
consistency with the rest of Xen, and to guarantee that we've got all
instances of the variables covered in this change.  Explain how they're use.
During cmdline/config parsing, always update both variables in pairs.

In early_microcode_load(), ASSERT() the invariant just in case.  Use a local
variable for idx rather than operating on the static; we're the only consume
of the value.

Expand the index selection logic with comments and warnings to the user when
something went wrong.

Fixes: 5ed12565aa32 ("microcode: rendezvous CPUs in NMI handler and load ucode")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>

Despite the fixes tag, this can't be backported (at least not in this form).
---
 xen/arch/x86/cpu/microcode/core.c | 76 +++++++++++++++++++++++--------
 1 file changed, 57 insertions(+), 19 deletions(-)

diff --git a/xen/arch/x86/cpu/microcode/core.c 
b/xen/arch/x86/cpu/microcode/core.c
index 4c4003bf9687..fc31ab35c3c8 100644
--- a/xen/arch/x86/cpu/microcode/core.c
+++ b/xen/arch/x86/cpu/microcode/core.c
@@ -60,7 +60,6 @@
 #define MICROCODE_UPDATE_TIMEOUT_US 1000000
 
 static module_t __initdata ucode_mod;
-static signed int __initdata ucode_mod_idx;
 static bool __initdata ucode_mod_forced;
 static unsigned int nr_cores;
 
@@ -97,11 +96,6 @@ struct patch_with_flags {
 };
 
 static struct ucode_mod_blob __initdata ucode_blob;
-/*
- * By default we will NOT parse the multiboot modules to see if there is
- * cpio image with the microcode images.
- */
-static bool __initdata ucode_scan;
 
 /* By default, ucode loading is done in NMI handler */
 static bool ucode_in_nmi = true;
@@ -109,13 +103,28 @@ static bool ucode_in_nmi = true;
 /* Protected by microcode_mutex */
 static const struct microcode_patch *microcode_cache;
 
+/*
+ * opt_mod_idx and opt_scan have subtle semantics.
+ *
+ * The cmdline can either identify a module by number (inc -ve back-reference)
+ * containing a raw microcode container, or select scan which instructs Xen to
+ * search all modules for an uncompressed CPIO archive containing a file with
+ * a vendor-dependent name.
+ *
+ * These options do not make sense when combined, so for the benefit of module
+ * location we require that they are not both active together.
+ */
+static int __initdata opt_mod_idx;
+static bool __initdata opt_scan;
+
 /*
  * Used by the EFI path only, when xen.cfg identifies an explicit microcode
  * file.  Overrides ucode=<int>|scan on the regular command line.
  */
 void __init microcode_set_module(unsigned int idx)
 {
-    ucode_mod_idx = idx;
+    opt_mod_idx = idx;
+    opt_scan = false;
     ucode_mod_forced = 1;
 }
 
@@ -139,12 +148,16 @@ static int __init cf_check parse_ucode(const char *s)
         else if ( !ucode_mod_forced ) /* Not forced by EFI */
         {
             if ( (val = parse_boolean("scan", s, ss)) >= 0 )
-                ucode_scan = val;
+            {
+                opt_scan = val;
+                opt_mod_idx = 0;
+            }
             else
             {
                 const char *q;
 
-                ucode_mod_idx = simple_strtol(s, &q, 0);
+                opt_scan = false;
+                opt_mod_idx = simple_strtol(s, &q, 0);
                 if ( q != ss )
                     rc = -EINVAL;
             }
@@ -166,7 +179,7 @@ static void __init microcode_scan_module(struct boot_info 
*bi)
     int i;
 
     ucode_blob.size = 0;
-    if ( !ucode_scan )
+    if ( !opt_scan )
         return;
 
     if ( boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
@@ -792,7 +805,7 @@ static int __init cf_check microcode_init_cache(void)
     if ( !ucode_ops.apply_microcode )
         return -ENODEV;
 
-    if ( ucode_scan )
+    if ( opt_scan )
         /* Need to rescan the modules because they might have been relocated */
         microcode_scan_module(bi);
 
@@ -817,17 +830,42 @@ static int __init early_microcode_load(struct boot_info 
*bi)
     const void *data = NULL;
     size_t size;
     struct microcode_patch *patch;
+    int idx = opt_mod_idx;
+
+    /*
+     * Cmdline parsing ensures this invariant holds, so that we don't end up
+     * trying to mix multiple ways of finding the microcode.
+     */
+    ASSERT(idx == 0 || !opt_scan);
 
-    if ( ucode_mod_idx < 0 )
-        ucode_mod_idx += bi->nr_modules;
-    if ( ucode_mod_idx <= 0 || ucode_mod_idx >= bi->nr_modules ||
-         !__test_and_clear_bit(ucode_mod_idx, bi->module_map) )
-        goto scan;
-    ucode_mod = *bi->mods[ucode_mod_idx].mod;
- scan:
-    if ( ucode_scan )
+    if ( opt_scan ) /* Scan for a CPIO archive */
         microcode_scan_module(bi);
 
+    if ( idx ) /* Specific module nominated */
+    {
+        /*
+         * Negative indicies can be used to reference from the end of the
+         * modules.  e.g. ucode=-1 refers to the last module.
+         */
+        if ( idx < 0 )
+            idx += bi->nr_modules;
+
+        if ( idx <= 0 || idx >= bi->nr_modules )
+        {
+            printk(XENLOG_WARNING "Microcode: Chosen module %d out of range 
[1, %u)\n",
+                   idx, bi->nr_modules);
+            return -ENODEV;
+        }
+
+        if ( !__test_and_clear_bit(idx, bi->module_map) )
+        {
+            printk(XENLOG_WARNING "Microcode: Chosen module %d already 
used\n", idx);
+            return -ENODEV;
+        }
+
+        ucode_mod = *bi->mods[idx].mod;
+    }
+
     if ( !ucode_mod.mod_end && !ucode_blob.size )
         return 0;
 
-- 
2.39.5