x86/alternatives: do not BUG during apply

alternatives is used both at boot time, and when loading livepatch payloads.
While for the former it makes sense to panic, it's not useful for the later, as
for livepatches it's possible to fail to load the livepatch if alternatives
cannot be resolved and continue operating normally.

Relax the BUGs in _apply_alternatives() to instead return an error code.  The
caller will figure out whether the failures are fatal and panic.

Print an error message to provide some user-readable information about what
went wrong.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: aa5a06d5d6
master date: 2024-09-26 14:18:03 +0100

xen/arch/x86/alternative.c

@@ -175,9 +175,9 @@ extern void *const __initdata_cf_clobber_end[];
  * invocation, such that no CALLs/JMPs to NULL pointers will be left
  * around. See also the further comment below.
  */
-static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
-                                                  struct alt_instr *end,
-                                                  bool force)
+static int init_or_livepatch _apply_alternatives(struct alt_instr *start,
+                                                 struct alt_instr *end,
+                                                 bool force)
 {
     struct alt_instr *a, *base;
 
@@ -198,9 +198,29 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
         uint8_t buf[MAX_PATCH_LEN];
         unsigned int total_len = a->orig_len + a->pad_len;
 
-        BUG_ON(a->repl_len > total_len);
-        BUG_ON(total_len > sizeof(buf));
-        BUG_ON(a->cpuid >= NCAPINTS * 32);
+        if ( a->repl_len > total_len )
+        {
+            printk(XENLOG_ERR
+                   "Alt for %ps, replacement size %#x larger than origin %#x\n",
+                    ALT_ORIG_PTR(a), a->repl_len, total_len);
+            return -ENOSPC;
+        }
+
+        if ( total_len > sizeof(buf) )
+        {
+            printk(XENLOG_ERR
+                   "Alt for %ps, origin size %#x bigger than buffer %#zx\n",
+                   ALT_ORIG_PTR(a), total_len, sizeof(buf));
+            return -ENOSPC;
+        }
+
+        if ( a->cpuid >= NCAPINTS * 32 )
+        {
+             printk(XENLOG_ERR
+                   "Alt for %ps, feature %#x outside of featureset range %#x\n",
+                   ALT_ORIG_PTR(a), a->cpuid, NCAPINTS * 32);
+            return -ERANGE;
+        }
 
         /*
          * Detect sequences of alt_instr's patching the same origin site, and
@@ -356,12 +376,14 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
 
         printk("altcall: Optimised away %u endbr64 instructions\n", clobbered);
     }
+
+    return 0;
 }
 
 #ifdef CONFIG_LIVEPATCH
-void apply_alternatives(struct alt_instr *start, struct alt_instr *end)
+int apply_alternatives(struct alt_instr *start, struct alt_instr *end)
 {
-    _apply_alternatives(start, end, true);
+    return _apply_alternatives(start, end, true);
 }
 #endif
 
@@ -383,6 +405,8 @@ static int __init cf_check nmi_apply_alternatives(
      */
     if ( !(alt_done & alt_todo) )
     {
+        int rc;
+
         /*
          * Relax perms on .text to be RWX, so we can modify them.
          *
@@ -394,8 +418,10 @@ static int __init cf_check nmi_apply_alternatives(
                                  PAGE_HYPERVISOR_RWX);
         flush_local(FLUSH_TLB_GLOBAL);
 
-        _apply_alternatives(__alt_instructions, __alt_instructions_end,
-                            alt_done);
+        rc = _apply_alternatives(__alt_instructions, __alt_instructions_end,
+                                 alt_done);
+        if ( rc )
+            panic("Unable to apply alternatives: %d\n", rc);
 
         /*
          * Reinstate perms on .text to be RX.  This also cleans out the dirty

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

@@ -24,7 +24,7 @@ struct __packed alt_instr {
 
 extern void add_nops(void *insns, unsigned int len);
 /* Similar to alternative_instructions except it can be run with IRQs enabled. */
-extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
+extern int apply_alternatives(struct alt_instr *start, struct alt_instr *end);
 extern void alternative_instructions(void);
 extern void alternative_branches(void);
 

xen/common/livepatch.c

@@ -901,7 +901,15 @@ static int prepare_payload(struct payload *payload,
                 return -EINVAL;
             }
         }
-        apply_alternatives(start, end);
+
+        rc = apply_alternatives(start, end);
+        if ( rc )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s applying alternatives failed: %d\n",
+                   elf->name, rc);
+            return rc;
+        }
+
     alt_done:;
 #else
         printk(XENLOG_ERR LIVEPATCH "%s: We don't support alternative patching\n",