livepatch/arm/x86: Check payload for for unwelcomed symbols.

Certain platforms, such as ARM [32|64] add extra mapping symbols
such as $x (for ARM64 instructions), or more interesting to
this patch: $t (for Thumb instructions). These symbols are supposed
to help the final linker to make any adjustments (such as
add an veneer). But more importantly - we do not compile Xen
with any Thumb instructions (which are variable length) - and
if we find these mapping symbols we should disallow such payload.

Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

xen/arch/arm/arm32/livepatch.c

@@ -20,6 +20,19 @@ int arch_livepatch_verify_elf(const struct livepatch_elf *elf)
     return -EOPNOTSUPP;
 }
 
+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym)
+{
+    /*
+     * Xen does not use Thumb instructions - and we should not see any of
+     * them. If we do, abort.
+     */
+    if ( sym->name && sym->name[0] == '$' && sym->name[1] == 't' )
+        return ( !sym->name[2] || sym->name[2] == '.' );
+
+    return false;
+}
+
 int arch_livepatch_perform_rela(struct livepatch_elf *elf,
                                 const struct livepatch_elf_sec *base,
                                 const struct livepatch_elf_sec *rela)

xen/arch/arm/arm64/livepatch.c

@@ -90,6 +90,13 @@ int arch_livepatch_verify_elf(const struct livepatch_elf *elf)
     return 0;
 }
 
+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym)
+{
+    /* No special checks on ARM 64. */
+    return false;
+}
+
 enum aarch64_reloc_op {
     RELOC_OP_NONE,
     RELOC_OP_ABS,

xen/arch/x86/livepatch.c

@@ -131,6 +131,13 @@ bool arch_livepatch_symbol_ok(const struct livepatch_elf *elf,
     return true;
 }
 
+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym)
+{
+    /* No special checks on x86. */
+    return false;
+}
+
 int arch_livepatch_perform_rel(struct livepatch_elf *elf,
                                const struct livepatch_elf_sec *base,
                                const struct livepatch_elf_sec *rela)

xen/common/livepatch_elf.c

@@ -251,6 +251,12 @@ static int elf_get_sym(struct livepatch_elf *elf, const void *data)
 
         sym[i].sym = s;
         sym[i].name = strtab_sec->data + delta;
+        if ( arch_livepatch_symbol_deny(elf, &sym[i]) )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: Symbol '%s' should not be in payload!\n",
+                    elf->name, sym[i].name);
+            return -EINVAL;
+        }
     }
     elf->nsym = nsym;
 

xen/include/xen/livepatch.h

@@ -50,6 +50,8 @@ bool_t is_patch(const void *addr);
 int arch_livepatch_verify_elf(const struct livepatch_elf *elf);
 bool arch_livepatch_symbol_ok(const struct livepatch_elf *elf,
                               const struct livepatch_elf_sym *sym);
+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym);
 int arch_livepatch_perform_rel(struct livepatch_elf *elf,
                                const struct livepatch_elf_sec *base,
                                const struct livepatch_elf_sec *rela);