objtool: Add relocation check for alternative sections

Relocations in alternative code can be dangerous, because the code is
copy/pasted to the text section after relocations have been resolved,
which can corrupt PC-relative addresses.

However, relocations might be acceptable in some cases, depending on the
architecture.  For example, the x86 alternatives code manually fixes up
the target addresses for PC-relative jumps and calls.

So disallow relocations in alternative code, except where the x86 arch
code allows it.

This code may need to be tweaked for other arches when objtool gets
support for them.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Julien Thierry <jthierry@redhat.com>
Link: https://lkml.kernel.org/r/7b90b68d093311e4e8f6b504a9e1c758fd7e0002.1581359535.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -768,6 +768,27 @@ static int handle_group_alt(struct objtool_file *file,
 		insn->ignore = orig_insn->ignore_alts;
 		insn->func = orig_insn->func;
 
+		/*
+		 * Since alternative replacement code is copy/pasted by the
+		 * kernel after applying relocations, generally such code can't
+		 * have relative-address relocation references to outside the
+		 * .altinstr_replacement section, unless the arch's
+		 * alternatives code can adjust the relative offsets
+		 * accordingly.
+		 *
+		 * The x86 alternatives code adjusts the offsets only when it
+		 * encounters a branch instruction at the very beginning of the
+		 * replacement group.
+		 */
+		if ((insn->offset != special_alt->new_off ||
+		    (insn->type != INSN_CALL && !is_static_jump(insn))) &&
+		    find_rela_by_dest_range(insn->sec, insn->offset, insn->len)) {
+
+			WARN_FUNC("unsupported relocation in alternatives section",
+				  insn->sec, insn->offset);
+			return -1;
+		}
+
 		if (!is_static_jump(insn))
 			continue;