arm64: Reject modules with internal alternative callbacks

During module loading, check if a callback function used by the
alternatives specified in the '.altinstruction' ELF section (if present)
is located in core kernel .text. If not fail module loading before
callback is called.

Reported-by: Fanqin Cui <cuifq1@chinatelecom.cn>
Closes: https://lore.kernel.org/all/20250807072700.348514-1-fanqincui@163.com/
Signed-off-by: Adrian Barnaś <abarnas@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
[will: Folded in 'noinstr' tweak from Mark]
Signed-off-by: Will Deacon <will@kernel.org>

Author: abarnas-usys

arch/arm64/include/asm/alternative.h

@@ -26,9 +26,12 @@ void __init apply_alternatives_all(void);
 bool alternative_is_applied(u16 cpucap);
 
 #ifdef CONFIG_MODULES
-void apply_alternatives_module(void *start, size_t length);
+int apply_alternatives_module(void *start, size_t length);
 #else
-static inline void apply_alternatives_module(void *start, size_t length) { }
+static inline int apply_alternatives_module(void *start, size_t length)
+{
+	return 0;
+}
 #endif
 
 void alt_cb_patch_nops(struct alt_instr *alt, __le32 *origptr,

arch/arm64/kernel/alternative.c

@@ -139,9 +139,9 @@ static noinstr void clean_dcache_range_nopatch(u64 start, u64 end)
 	} while (cur += d_size, cur < end);
 }
 
-static void __apply_alternatives(const struct alt_region *region,
-				 bool is_module,
-				 unsigned long *cpucap_mask)
+static int __apply_alternatives(const struct alt_region *region,
+				bool is_module,
+				unsigned long *cpucap_mask)
 {
 	struct alt_instr *alt;
 	__le32 *origptr, *updptr;
@@ -166,10 +166,13 @@ static void __apply_alternatives(const struct alt_region *region,
 		updptr = is_module ? origptr : lm_alias(origptr);
 		nr_inst = alt->orig_len / AARCH64_INSN_SIZE;
 
-		if (ALT_HAS_CB(alt))
+		if (ALT_HAS_CB(alt)) {
 			alt_cb  = ALT_REPL_PTR(alt);
-		else
+			if (is_module && !core_kernel_text((unsigned long)alt_cb))
+				return -ENOEXEC;
+		} else {
 			alt_cb = patch_alternative;
+		}
 
 		alt_cb(alt, origptr, updptr, nr_inst);
 
@@ -193,6 +196,8 @@ static void __apply_alternatives(const struct alt_region *region,
 		bitmap_and(applied_alternatives, applied_alternatives,
 			   system_cpucaps, ARM64_NCAPS);
 	}
+
+	return 0;
 }
 
 static void __init apply_alternatives_vdso(void)
@@ -277,7 +282,7 @@ void __init apply_boot_alternatives(void)
 }
 
 #ifdef CONFIG_MODULES
-void apply_alternatives_module(void *start, size_t length)
+int apply_alternatives_module(void *start, size_t length)
 {
 	struct alt_region region = {
 		.begin	= start,
@@ -287,7 +292,7 @@ void apply_alternatives_module(void *start, size_t length)
 
 	bitmap_fill(all_capabilities, ARM64_NCAPS);
 
-	__apply_alternatives(&region, true, &all_capabilities[0]);
+	return __apply_alternatives(&region, true, &all_capabilities[0]);
 }
 #endif
 

arch/arm64/kernel/module.c

@@ -489,8 +489,13 @@ int module_finalize(const Elf_Ehdr *hdr,
 	int ret;
 
 	s = find_section(hdr, sechdrs, ".altinstructions");
-	if (s)
-		apply_alternatives_module((void *)s->sh_addr, s->sh_size);
+	if (s) {
+		ret = apply_alternatives_module((void *)s->sh_addr, s->sh_size);
+		if (ret < 0) {
+			pr_err("module %s: error occurred when applying alternatives\n", me->name);
+			return ret;
+		}
+	}
 
 	if (scs_is_dynamic()) {
 		s = find_section(hdr, sechdrs, ".init.eh_frame");