[PATCH 2/2] arch: arm64: Reject modules with internal alternative callbacks

["Adrian Barnaś" <abarnas@google.com>]

During module loading, check if there is a callback function used by the
alternatives specified in the '.altinstruction' ELF section and block
loading the module if such a function is present.

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>
---
 arch/arm64/include/asm/alternative.h |  4 ++--
 arch/arm64/kernel/alternative.c      | 15 ++++++++++-----
 arch/arm64/kernel/module.c           |  8 ++++++--
 3 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index 00d97b8a757f..6eab2517c809 100644
--- a/arch/arm64/include/asm/alternative.h
+++ b/arch/arm64/include/asm/alternative.h
@@ -26,9 +26,9 @@ 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) { }
 #endif
 
 void alt_cb_patch_nops(struct alt_instr *alt, __le32 *origptr,
diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 8ff6610af496..69fae94c843a 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -139,15 +139,18 @@ 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;
 	alternative_cb_t alt_cb;
 
 	for (alt = region->begin; alt < region->end; alt++) {
+		if (ALT_HAS_CB(alt) && is_module)
+			return -EPERM;
+
 		int nr_inst;
 		int cap = ALT_CAP(alt);
 
@@ -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
 
diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
index 5d6d228c6156..aeefc50229e3 100644
--- a/arch/arm64/kernel/module.c
+++ b/arch/arm64/kernel/module.c
@@ -478,8 +478,12 @@ 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");
-- 
2.51.0.534.gc79095c0ca-goog