arm64: mm: add support for WXN memory translation attribute

The AArch64 virtual memory system supports a global WXN control, which
can be enabled to make all writable mappings implicitly no-exec. This is
a useful hardening feature, as it prevents mistakes in managing page
table permissions from being exploited to attack the system.

When enabled at EL1, the restrictions apply to both EL1 and EL0. EL1 is
completely under our control, and has been cleaned up to allow WXN to be
enabled from boot onwards. EL0 is not under our control, but given that
widely deployed security features such as selinux or PaX already limit
the ability of user space to create mappings that are writable and
executable at the same time, the impact of enabling this for EL0 is
expected to be limited. (For this reason, common user space libraries
that have a legitimate need for manipulating executable code already
carry fallbacks such as [0].)

If enabled at compile time, the feature can still be disabled at boot if
needed, by passing arm64.nowxn on the kernel command line.

[0] https://github.com/libffi/libffi/blob/master/src/closures.c#L440

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-88-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

3 files changed, 73 insertions, 1 deletions

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index a8f97690ce1f..ee33b7e52da7 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -18,6 +18,7 @@
 #define ARM64_SW_FEATURE_OVERRIDE_NOKASLR	0
 #define ARM64_SW_FEATURE_OVERRIDE_HVHE		4
 #define ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF	8
+#define ARM64_SW_FEATURE_OVERRIDE_NOWXN		12
 
 #ifndef __ASSEMBLY__
 
@@ -962,6 +963,13 @@ static inline bool kaslr_disabled_cmdline(void)
 	return arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOKASLR);
 }
 
+static inline bool arm64_wxn_enabled(void)
+{
+	if (!IS_ENABLED(CONFIG_ARM64_WXN))
+		return false;
+	return !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOWXN);
+}
+
 u32 get_kvm_ipa_limit(void);
 void dump_cpu_features(void);
 
diff --git a/arch/arm64/include/asm/mman.h b/arch/arm64/include/asm/mman.h
index 5966ee4a6154..6d4940342ba7 100644
--- a/arch/arm64/include/asm/mman.h
+++ b/arch/arm64/include/asm/mman.h
@@ -35,11 +35,40 @@ static inline unsigned long arch_calc_vm_flag_bits(unsigned long flags)
 }
 #define arch_calc_vm_flag_bits(flags) arch_calc_vm_flag_bits(flags)
 
+static inline bool arm64_check_wx_prot(unsigned long prot,
+				       struct task_struct *tsk)
+{
+	/*
+	 * When we are running with SCTLR_ELx.WXN==1, writable mappings are
+	 * implicitly non-executable. This means we should reject such mappings
+	 * when user space attempts to create them using mmap() or mprotect().
+	 */
+	if (arm64_wxn_enabled() &&
+	    ((prot & (PROT_WRITE | PROT_EXEC)) == (PROT_WRITE | PROT_EXEC))) {
+		/*
+		 * User space libraries such as libffi carry elaborate
+		 * heuristics to decide whether it is worth it to even attempt
+		 * to create writable executable mappings, as PaX or selinux
+		 * enabled systems will outright reject it. They will usually
+		 * fall back to something else (e.g., two separate shared
+		 * mmap()s of a temporary file) on failure.
+		 */
+		pr_info_ratelimited(
+			"process %s (%d) attempted to create PROT_WRITE+PROT_EXEC mapping\n",
+			tsk->comm, tsk->pid);
+		return false;
+	}
+	return true;
+}
+
 static inline bool arch_validate_prot(unsigned long prot,
 	unsigned long addr __always_unused)
 {
 	unsigned long supported = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM;
 
+	if (!arm64_check_wx_prot(prot, current))
+		return false;
+
 	if (system_supports_bti())
 		supported |= PROT_BTI;
 
@@ -50,6 +79,13 @@ static inline bool arch_validate_prot(unsigned long prot,
 }
 #define arch_validate_prot(prot, addr) arch_validate_prot(prot, addr)
 
+static inline bool arch_validate_mmap_prot(unsigned long prot,
+					   unsigned long addr)
+{
+	return arm64_check_wx_prot(prot, current);
+}
+#define arch_validate_mmap_prot arch_validate_mmap_prot
+
 static inline bool arch_validate_flags(unsigned long vm_flags)
 {
 	if (!system_supports_mte())
diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index c768d16b81a4..f0fe2d09d139 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -20,13 +20,41 @@
 #include <asm/cpufeature.h>
 #include <asm/daifflags.h>
 #include <asm/proc-fns.h>
-#include <asm-generic/mm_hooks.h>
 #include <asm/cputype.h>
 #include <asm/sysreg.h>
 #include <asm/tlbflush.h>
 
 extern bool rodata_full;
 
+static inline int arch_dup_mmap(struct mm_struct *oldmm,
+				struct mm_struct *mm)
+{
+	return 0;
+}
+
+static inline void arch_exit_mmap(struct mm_struct *mm)
+{
+}
+
+static inline void arch_unmap(struct mm_struct *mm,
+			unsigned long start, unsigned long end)
+{
+}
+
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+		bool write, bool execute, bool foreign)
+{
+	if (IS_ENABLED(CONFIG_ARM64_WXN) && execute &&
+	    (vma->vm_flags & (VM_WRITE | VM_EXEC)) == (VM_WRITE | VM_EXEC)) {
+		pr_warn_ratelimited(
+			"process %s (%d) attempted to execute from writable memory\n",
+			current->comm, current->pid);
+		/* disallow unless the nowxn override is set */
+		return !arm64_wxn_enabled();
+	}
+	return true;
+}
+
 static inline void contextidr_thread_switch(struct task_struct *next)
 {
 	if (!IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR))