Revert "arm64: mm: add support for WXN memory translation attribute"

This reverts commit 50e3ed0.

The SCTLR_EL1.WXN control forces execute-never when a page has write
permissions. While the idea of hardening such write/exec combinations is
good, with permissions indirection enabled (FEAT_PIE) this control
becomes RES0. FEAT_PIE introduces a slightly different form of WXN which
only has an effect when the base permission is RWX and the write is
toggled by the permission overlay (FEAT_POE, not yet supported by the
arm64 kernel). Revert the patch for now.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/ZfGESD3a91lxH367@arm.com

Author: ctmarinas

arch/arm64/Kconfig

@@ -1606,17 +1606,6 @@ config RODATA_FULL_DEFAULT_ENABLED
 	  This requires the linear region to be mapped down to pages,
 	  which may adversely affect performance in some cases.
 
-config ARM64_WXN
-	bool "Enable WXN attribute so all writable mappings are non-exec"
-	help
-	  Set the WXN bit in the SCTLR system register so that all writable
-	  mappings are treated as if the PXN/UXN bit is set as well.
-	  If this is set to Y, it can still be disabled at runtime by
-	  passing 'arm64.nowxn' on the kernel command line.
-
-	  This should only be set if no software needs to be supported that
-	  relies on being able to execute from writable mappings.
-
 config ARM64_SW_TTBR0_PAN
 	bool "Emulate Privileged Access Never using TTBR0_EL1 switching"
 	help

arch/arm64/include/asm/cpufeature.h

@@ -18,7 +18,6 @@
 #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__
 
@@ -968,13 +967,6 @@ 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);
 

arch/arm64/include/asm/mman.h

@@ -35,40 +35,11 @@ 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;
 
@@ -79,13 +50,6 @@ 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())

arch/arm64/include/asm/mmu_context.h

@@ -20,41 +20,13 @@
 #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))

arch/arm64/kernel/pi/idreg-override.c

@@ -189,7 +189,6 @@ static const struct ftr_set_desc sw_features __prel64_initconst = {
 		FIELD("nokaslr", ARM64_SW_FEATURE_OVERRIDE_NOKASLR, NULL),
 		FIELD("hvhe", ARM64_SW_FEATURE_OVERRIDE_HVHE, hvhe_filter),
 		FIELD("rodataoff", ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF, NULL),
-		FIELD("nowxn", ARM64_SW_FEATURE_OVERRIDE_NOWXN, NULL),
 		{}
 	},
 };
@@ -222,9 +221,8 @@ static const struct {
 	{ "arm64.nomops",		"id_aa64isar2.mops=0" },
 	{ "arm64.nomte",		"id_aa64pfr1.mte=0" },
 	{ "nokaslr",			"arm64_sw.nokaslr=1" },
-	{ "rodata=off",			"arm64_sw.rodataoff=1 arm64_sw.nowxn=1" },
+	{ "rodata=off",			"arm64_sw.rodataoff=1" },
 	{ "arm64.nolva",		"id_aa64mmfr2.varange=0" },
-	{ "arm64.nowxn",		"arm64_sw.nowxn=1" },
 };
 
 static int __init parse_hexdigit(const char *p, u64 *v)

arch/arm64/kernel/pi/map_kernel.c

@@ -132,25 +132,6 @@ static void __init map_kernel(u64 kaslr_offset, u64 va_offset, int root_level)
 	idmap_cpu_replace_ttbr1(swapper_pg_dir);
 }
 
-static void noinline __section(".idmap.text") disable_wxn(void)
-{
-	u64 sctlr = read_sysreg(sctlr_el1) & ~SCTLR_ELx_WXN;
-
-	/*
-	 * We cannot safely clear the WXN bit while the MMU and caches are on,
-	 * so turn the MMU off, flush the TLBs and turn it on again but with
-	 * the WXN bit cleared this time.
-	 */
-	asm("	msr	sctlr_el1, %0		;"
-	    "	isb				;"
-	    "	tlbi    vmalle1			;"
-	    "	dsb     nsh			;"
-	    "	isb				;"
-	    "	msr     sctlr_el1, %1		;"
-	    "	isb				;"
-	    ::	"r"(sctlr & ~SCTLR_ELx_M), "r"(sctlr));
-}
-
 static void noinline __section(".idmap.text") set_ttbr0_for_lpa2(u64 ttbr)
 {
 	u64 sctlr = read_sysreg(sctlr_el1);
@@ -248,10 +229,6 @@ asmlinkage void __init early_map_kernel(u64 boot_status, void *fdt)
 	if (va_bits > VA_BITS_MIN)
 		sysreg_clear_set(tcr_el1, TCR_T1SZ_MASK, TCR_T1SZ(va_bits));
 
-	if (IS_ENABLED(CONFIG_ARM64_WXN) &&
-	    arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOWXN))
-		disable_wxn();
-
 	/*
 	 * The virtual KASLR displacement modulo 2MiB is decided by the
 	 * physical placement of the image, as otherwise, we might not be able

arch/arm64/mm/proc.S

@@ -546,12 +546,6 @@ alternative_else_nop_endif
 	 * Prepare SCTLR
 	 */
 	mov_q	x0, INIT_SCTLR_EL1_MMU_ON
-#ifdef CONFIG_ARM64_WXN
-	ldr_l	x1, arm64_sw_feature_override + FTR_OVR_VAL_OFFSET
-	tst	x1, #0xf << ARM64_SW_FEATURE_OVERRIDE_NOWXN
-	orr	x1, x0, #SCTLR_ELx_WXN
-	csel	x0, x0, x1, ne
-#endif
 	ret					// return to head.S
 
 	.unreq	mair