[PATCH v2 19/19] objtool: Detect loading function pointers across noinstr

[Peter Zijlstra]

Detect if noinstr text loads functions pointers from regular text,
doing so is a definite sign that indirect function calls are unsafe.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/arch.h  |    2 +
 tools/objtool/check.c |   68 ++++++++++++++++++++++++++++++++++++++++++++++++++
 tools/objtool/check.h |    2 -
 3 files changed, 71 insertions(+), 1 deletion(-)

--- a/tools/objtool/arch.h
+++ b/tools/objtool/arch.h
@@ -75,4 +75,6 @@ int arch_decode_instruction(struct elf *
 
 bool arch_callee_saved_reg(unsigned char reg);
 
+#define MAX_INSN_SIZE 15
+
 #endif /* _ARCH_H */
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -42,6 +42,25 @@ struct instruction *find_insn(struct obj
 	return NULL;
 }
 
+static struct instruction *find_insn_containing(struct objtool_file *file, struct section *sec,
+					 unsigned long offset)
+{
+	struct instruction *insn;
+	unsigned long o;
+
+	for_offset_range(o, offset - MAX_INSN_SIZE - 1, offset) {
+		hash_for_each_possible(file->insn_hash, insn, hash, sec_offset_hash(sec, o)) {
+			if (insn->sec != sec)
+				continue;
+
+			if (insn->offset <= offset && insn->offset + insn->len > offset)
+				return insn;
+		}
+	}
+
+	return NULL;
+}
+
 static struct instruction *next_insn_same_sec(struct objtool_file *file,
 					      struct instruction *insn)
 {
@@ -2102,6 +2121,29 @@ static int validate_return(struct symbol
 	return 0;
 }
 
+static int validate_rela(struct instruction *insn, struct insn_state *state)
+{
+	/*
+	 * Assume that any text rela that's not a CALL or JMP is a load of a
+	 * function pointer.
+	 */
+
+	switch (insn->type) {
+	case INSN_CALL:
+	case INSN_CALL_DYNAMIC:
+	case INSN_JUMP_CONDITIONAL:
+	case INSN_JUMP_UNCONDITIONAL:
+		return 0;
+	}
+
+	if (state->noinstr && state->instr <= 0 && insn->has_text_rela) {
+		WARN_FUNC("loading non-noinstr function pointer", insn->sec, insn->offset);
+		return 1;
+	}
+
+	return 0;
+}
+
 /*
  * Follow the branch starting at the given instruction, and recursively follow
  * any other branches (jumps).  Meanwhile, track the frame pointer state at
@@ -2217,6 +2259,10 @@ static int validate_branch(struct objtoo
 				return 0;
 		}
 
+		ret = validate_rela(insn, &state);
+		if (ret)
+			return ret;
+
 		switch (insn->type) {
 
 		case INSN_RETURN:
@@ -2485,6 +2531,25 @@ static bool ignore_unreachable_insn(stru
 	return false;
 }
 
+static void prepare_insn_rela(struct objtool_file *file, struct section *sec)
+{
+	struct instruction *insn;
+	struct rela *rela;
+
+	if (!sec->rela)
+		return;
+
+	list_for_each_entry(rela, &sec->rela->rela_list, list) {
+		insn = find_insn_containing(file, sec, rela->offset);
+		if (!insn)
+			continue;
+
+		insn->has_text_rela = rela->sym && rela->sym->sec &&
+				      rela->sym->sec->text &&
+				      !rela->sym->sec->noinstr;
+	}
+}
+
 static int validate_sec_functions(struct objtool_file *file, struct section *sec)
 {
 	struct symbol *func;
@@ -2507,6 +2572,9 @@ static int validate_sec_functions(struct
 	if (vmlinux)
 		state.noinstr = sec->noinstr;
 
+	if (state.noinstr)
+		prepare_insn_rela(file, sec);
+
 	list_for_each_entry(func, &sec->symbol_list, list) {
 		if (func->type != STT_FUNC)
 			continue;
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -36,7 +36,7 @@ struct instruction {
 	enum insn_type type;
 	unsigned long immediate;
 	bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
-	bool retpoline_safe;
+	bool retpoline_safe, has_text_rela;
 	s8 instr;
 	u8 visited;
 	struct symbol *call_dest;