[PATCH 2/2] arm64: mm: add support for WXN memory translation attribute
Ard Biesheuvel
ardb+git at google.com
Thu Mar 14 02:48:07 PDT 2024
From: Ard Biesheuvel <ardb at kernel.org>
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. So as far as EL1 is concerned, there is no
reason not to enable this.
Since there is no practical way to make this restriction apply to EL1
only, or selectively to some user processes and not others, this will
apply to the kernel and all of user space when it is enabled. This
constitutes an ABI change and is therefore not recommended in general.
Note that mmap()/mprotect() calls requesting writable executable
mappings will fail gracefully under this policy, and many software
components and libraries have already been updated to deal with this
limitation, given that hardening schemes such as PaX or SeLinux have
already been imposing such restrictions for years.
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
arch/arm64/Kconfig | 11 +++++++
arch/arm64/include/asm/cpufeature.h | 8 ++++++
arch/arm64/include/asm/mman.h | 16 +++++++++++
arch/arm64/include/asm/mmu_context.h | 30 +++++++++++++++++++-
arch/arm64/kernel/pi/idreg-override.c | 4 ++-
arch/arm64/kernel/pi/map_kernel.c | 23 +++++++++++++++
arch/arm64/mm/proc.S | 6 ++++
7 files changed, 96 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 4869265ace2d..24dfd87fab93 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1606,6 +1606,17 @@ 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
diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 6d86ad37c615..66ba0801f7b7 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__
@@ -967,6 +968,13 @@ 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..20ac42d645c5 100644
--- a/arch/arm64/include/asm/mman.h
+++ b/arch/arm64/include/asm/mman.h
@@ -35,6 +35,22 @@ 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 arch_deny_write_exec(void)
+{
+ if (!arm64_wxn_enabled())
+ return false;
+
+ /*
+ * 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().
+ */
+ pr_info_ratelimited("process %s (%d) attempted to create PROT_WRITE+PROT_EXEC mapping\n",
+ current->comm, current->pid);
+ return true;
+}
+#define arch_deny_write_exec arch_deny_write_exec
+
static inline bool arch_validate_prot(unsigned long prot,
unsigned long addr __always_unused)
{
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))
diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
index aad399796e81..bccfee34f62f 100644
--- a/arch/arm64/kernel/pi/idreg-override.c
+++ b/arch/arm64/kernel/pi/idreg-override.c
@@ -189,6 +189,7 @@ 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),
{}
},
};
@@ -221,8 +222,9 @@ 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" },
+ { "rodata=off", "arm64_sw.rodataoff=1 arm64_sw.nowxn=1" },
{ "arm64.nolva", "id_aa64mmfr2.varange=0" },
+ { "arm64.nowxn", "arm64_sw.nowxn=1" },
};
static int __init parse_hexdigit(const char *p, u64 *v)
diff --git a/arch/arm64/kernel/pi/map_kernel.c b/arch/arm64/kernel/pi/map_kernel.c
index 5fa08e13e17e..cac1e1f63c44 100644
--- a/arch/arm64/kernel/pi/map_kernel.c
+++ b/arch/arm64/kernel/pi/map_kernel.c
@@ -132,6 +132,25 @@ 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);
@@ -229,6 +248,10 @@ 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
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 9d40f3ffd8d2..bfd2ad896108 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -546,6 +546,12 @@ 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
--
2.44.0.278.ge034bb2e1d-goog
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