x86/boot: Fix microcode module handling during PVH boot

As detailed in commit 0fe607b2a1 ("x86/boot: Fix PVH boot during boot_info
transition period"), the use of __va(mbi->mods_addr) constitutes a
use-after-free on the PVH boot path.

This pattern has been in use since before PVH support was added.  Inside a PVH
VM, it will go unnoticed as long as the microcode container parser doesn't
choke on the random data it finds.

The use within early_microcode_init() happens to be safe because it's prior to
move_xen().  microcode_init_cache() is after move_xen(), and therefore unsafe.

Plumb the boot_info pointer down, replacing module_map and mbi.  Importantly,
bi->mods[].mod is a safe way to access the module list during PVH boot.

Note: microcode_scan_module() is still bogusly stashing a bootstrap_map()'d
      pointer in ucode_blob.data, which constitutes a different
      use-after-free, and only works in general because of a second bug.  This
      is unrelated to PVH, and needs untangling differently.

Signed-off-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 8ddf63a252
master date: 2024-10-23 18:14:24 +0100

xen/arch/x86/cpu/microcode/core.c

@@ -151,9 +151,9 @@ custom_param("ucode", parse_ucode);
 
 static void __init microcode_scan_module(
     unsigned long *module_map,
-    const multiboot_info_t *mbi)
+    const multiboot_info_t *mbi,
+    const module_t mod[])
 {
-    module_t *mod = (module_t *)__va(mbi->mods_addr);
     uint64_t *_blob_start;
     unsigned long _blob_size;
     struct cpio_data cd;
@@ -203,10 +203,9 @@ static void __init microcode_scan_module(
 
 static void __init microcode_grab_module(
     unsigned long *module_map,
-    const multiboot_info_t *mbi)
+    const multiboot_info_t *mbi,
+    const module_t mod[])
 {
-    module_t *mod = (module_t *)__va(mbi->mods_addr);
-
     if ( ucode_mod_idx < 0 )
         ucode_mod_idx += mbi->mods_count;
     if ( ucode_mod_idx <= 0 || ucode_mod_idx >= mbi->mods_count ||
@@ -215,7 +214,7 @@ static void __init microcode_grab_module(
     ucode_mod = mod[ucode_mod_idx];
 scan:
     if ( ucode_scan )
-        microcode_scan_module(module_map, mbi);
+        microcode_scan_module(module_map, mbi, mod);
 }
 
 static struct microcode_ops __ro_after_init ucode_ops;
@@ -801,7 +800,8 @@ static int __init early_update_cache(const void *data, size_t len)
 }
 
 int __init microcode_init_cache(unsigned long *module_map,
-                                const struct multiboot_info *mbi)
+                                const struct multiboot_info *mbi,
+                                const module_t mods[])
 {
     int rc = 0;
 
@@ -810,7 +810,7 @@ int __init microcode_init_cache(unsigned long *module_map,
 
     if ( ucode_scan )
         /* Need to rescan the modules because they might have been relocated */
-        microcode_scan_module(module_map, mbi);
+        microcode_scan_module(module_map, mbi, mods);
 
     if ( ucode_mod.mod_end )
         rc = early_update_cache(bootstrap_map(&ucode_mod),
@@ -857,7 +857,8 @@ static int __init early_microcode_update_cpu(void)
 }
 
 int __init early_microcode_init(unsigned long *module_map,
-                                const struct multiboot_info *mbi)
+                                const struct multiboot_info *mbi,
+                                const module_t mods[])
 {
     const struct cpuinfo_x86 *c = &boot_cpu_data;
     int rc = 0;
@@ -906,7 +907,7 @@ int __init early_microcode_init(unsigned long *module_map,
         return -ENODEV;
     }
 
-    microcode_grab_module(module_map, mbi);
+    microcode_grab_module(module_map, mbi, mods);
 
     if ( ucode_mod.mod_end || ucode_blob.size )
         rc = early_microcode_update_cpu();

xen/arch/x86/include/asm/microcode.h

@@ -3,6 +3,7 @@
 
 #include <xen/types.h>
 #include <xen/percpu.h>
+#include <xen/multiboot.h>
 
 #include <public/xen.h>
 
@@ -24,9 +25,11 @@ DECLARE_PER_CPU(struct cpu_signature, cpu_sig);
 void microcode_set_module(unsigned int idx);
 int microcode_update(XEN_GUEST_HANDLE(const_void), unsigned long len);
 int early_microcode_init(unsigned long *module_map,
-                         const struct multiboot_info *mbi);
+                         const struct multiboot_info *mbi,
+                         const module_t mods[]);
 int microcode_init_cache(unsigned long *module_map,
-                         const struct multiboot_info *mbi);
+                         const struct multiboot_info *mbi,
+                         const module_t mods[]);
 int microcode_update_one(void);
 
 #endif /* ASM_X86__MICROCODE_H */

xen/arch/x86/setup.c

@@ -1316,7 +1316,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
      * TODO: load ucode earlier once multiboot modules become accessible
      * at an earlier stage.
      */
-    early_microcode_init(module_map, mbi);
+    early_microcode_init(module_map, mbi, mod);
 
     if ( xen_phys_start )
     {
@@ -1842,7 +1842,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
     timer_init();
 
-    microcode_init_cache(module_map, mbi); /* Needs xmalloc() */
+    microcode_init_cache(module_map, mbi, mod); /* Needs xmalloc() */
 
     tsx_init(); /* Needs microcode.  May change HLE/RTM feature bits. */