x86/module: Detect and skip invalid relocations

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@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: jeyu@kernel.org
Cc: live-patching@vger.kernel.org
Link: http://lkml.kernel.org/r/37450d6c6225e54db107fba447ce9e56e5f758e9.1509713553.git.jpoimboe@redhat.com
[ Restructured the messages, as it's unclear whether the relocation or the target is corrupted. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 62e7d70aadd5..da0c160e5589 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 invalid_relocation;
                         *(u64 *)loc = val;
                         break;
                 case R_X86_64_32:
+                        if (*(u32 *)loc != 0)
+                                goto invalid_relocation;
                         *(u32 *)loc = val;
                         if (val != *(u32 *)loc)
                                 goto overflow;
                         break;
                 case R_X86_64_32S:
+                        if (*(s32 *)loc != 0)
+                                goto invalid_relocation;
                         *(s32 *)loc = val;
                         if ((s64)val != *(s32 *)loc)
                                 goto overflow;
                         break;
                 case R_X86_64_PC32:
+                        if (*(u32 *)loc != 0)
+                                goto invalid_relocation;
                         val -= (u64)loc;
                         *(u32 *)loc = val;
 #if 0
@@ -200,6 +208,11 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
         }
         return 0;
 
+invalid_relocation:
+        pr_err("x86/modules: Skipping invalid relocation target, existing value 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);