powerpc/64/module: REL32 relocation range check

The recent module relocation overflow crash demonstrated that we
have no range checking on REL32 relative relocations. This patch
implements a basic check, the same kernel that previously oopsed
and rebooted now continues with some of these errors when loading
the module:

  module_64: x_tables: REL32 527703503449812 out of range!

Possibly other relocations (ADDR32, REL16, TOC16, etc.) should also have
overflow checks.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/module_64.c

@@ -680,7 +680,14 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 
 		case R_PPC64_REL32:
 			/* 32 bits relative (used by relative exception tables) */
-			*(u32 *)location = value - (unsigned long)location;
+			/* Convert value to relative */
+			value -= (unsigned long)location;
+			if (value + 0x80000000 > 0xffffffff) {
+				pr_err("%s: REL32 %li out of range!\n",
+				       me->name, (long int)value);
+				return -ENOEXEC;
+			}
+			*(u32 *)location = value;
 			break;
 
 		case R_PPC64_TOCSAVE: