[PATCH] x86/module: Detect corrupt relocations against nonzero data

[Josh Poimboeuf]

There have been some cases where external tooling (e.g., kpatch-build)
creates a corrupt relocation which targets the wrong address.  This is a
silent failure which can corrupt memory in unexpected places.

On x86, the bytes of data being overwritten by relocations are always
initialized to zero beforehand.  Use that knowledge to add sanity checks
to detect such cases before they corrupt memory.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/module.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 62e7d70aadd5..a69b12617820 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -172,19 +172,27 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 		case R_X86_64_NONE:
 			break;
 		case R_X86_64_64:
+			if (*(u64 *)loc != 0)
+				goto nonzero;
 			*(u64 *)loc = val;
 			break;
 		case R_X86_64_32:
+			if (*(u32 *)loc != 0)
+				goto nonzero;
 			*(u32 *)loc = val;
 			if (val != *(u32 *)loc)
 				goto overflow;
 			break;
 		case R_X86_64_32S:
+			if (*(s32 *)loc != 0)
+				goto nonzero;
 			*(s32 *)loc = val;
 			if ((s64)val != *(s32 *)loc)
 				goto overflow;
 			break;
 		case R_X86_64_PC32:
+			if (*(u64 *)loc != 0)
+				goto nonzero;
 			val -= (u64)loc;
 			*(u32 *)loc = val;
 #if 0
@@ -200,6 +208,11 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 	}
 	return 0;
 
+nonzero:
+	pr_err("corrupt relocation: existing data is nonzero for type %d loc %p val %Lx\n",
+	       (int)ELF64_R_TYPE(rel[i].r_info), loc, val);
+	return -ENOEXEC;
+
 overflow:
 	pr_err("overflow in relocation type %d val %Lx\n",
 	       (int)ELF64_R_TYPE(rel[i].r_info), val);
-- 
2.13.6