diff mbox series

[v12,21/39] arm64/gcs: Ensure that new threads have a GCS

Message ID 20240829-arm64-gcs-v12-21-42fec947436a@kernel.org (mailing list archive)
State New
Headers show
Series arm64/gcs: Provide support for GCS in userspace | expand

Commit Message

Mark Brown Aug. 28, 2024, 11:27 p.m. UTC 
When a new thread is created by a thread with GCS enabled the GCS needs
to be specified along with the regular stack.

Unfortunately plain clone() is not extensible and existing clone3()
users will not specify a stack so all existing code would be broken if
we mandated specifying the stack explicitly.  For compatibility with
these cases and also x86 (which did not initially implement clone3()
support for shadow stacks) if no GCS is specified we will allocate one
so when a thread is created which has GCS enabled allocate one for it.
We follow the extensively discussed x86 implementation and allocate
min(RLIMIT_STACK, 2G).  Since the GCS only stores the call stack and not
any variables this should be more than sufficient for most applications.

GCSs allocated via this mechanism will be freed when the thread exits.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/include/asm/gcs.h |  9 ++++++
 arch/arm64/kernel/process.c  | 26 ++++++++++++++++
 arch/arm64/mm/gcs.c          | 70 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 105 insertions(+)

Comments

Yury Khrustalev Sept. 3, 2024, 9:04 a.m. UTC | #1 
On Thu, Aug 29, 2024 at 12:27:37AM +0100, Mark Brown wrote:
> When a new thread is created by a thread with GCS enabled the GCS needs
> to be specified along with the regular stack.
> 
> Unfortunately plain clone() is not extensible and existing clone3()
> users will not specify a stack so all existing code would be broken if
> we mandated specifying the stack explicitly.  For compatibility with
> these cases and also x86 (which did not initially implement clone3()
> support for shadow stacks) if no GCS is specified we will allocate one
> so when a thread is created which has GCS enabled allocate one for it.
> We follow the extensively discussed x86 implementation and allocate
> min(RLIMIT_STACK, 2G).  Since the GCS only stores the call stack and not
> any variables this should be more than sufficient for most applications.
> 
> GCSs allocated via this mechanism will be freed when the thread exits.
> 
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> Signed-off-by: Mark Brown <broonie@kernel.org>

Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
Szabolcs Nagy Sept. 6, 2024, 11:07 a.m. UTC | #2 
The 08/29/2024 00:27, Mark Brown wrote:
> Unfortunately plain clone() is not extensible and existing clone3()
> users will not specify a stack so all existing code would be broken if
> we mandated specifying the stack explicitly.  For compatibility with
> these cases and also x86 (which did not initially implement clone3()
> support for shadow stacks) if no GCS is specified we will allocate one
> so when a thread is created which has GCS enabled allocate one for it.
> We follow the extensively discussed x86 implementation and allocate
> min(RLIMIT_STACK, 2G).  Since the GCS only stores the call stack and not
> any variables this should be more than sufficient for most applications.

the code has RLIMIT_STACK/2

(which is what i expect on arm64, since gcs entry size
is min stack frame / 2 if the stack is correctly aligned)

> 
> GCSs allocated via this mechanism will be freed when the thread exits.

i see gcs still mapped after thread exit when testing.

> +static unsigned long gcs_size(unsigned long size)
> +{
> +	if (size)
> +		return PAGE_ALIGN(size);

no /2

> +
> +	/* Allocate RLIMIT_STACK/2 with limits of PAGE_SIZE..2G */
> +	size = PAGE_ALIGN(min_t(unsigned long long,
> +				rlimit(RLIMIT_STACK) / 2, SZ_2G));

has /2

> +	return max(PAGE_SIZE, size);
> +}
> +
> +unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
> +				     const struct kernel_clone_args *args)
> +{
> +	unsigned long addr, size;
> +
> +	if (!system_supports_gcs())
> +		return 0;
> +
> +	if (!task_gcs_el0_enabled(tsk))
> +		return 0;
> +
> +	if ((args->flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM) {
> +		tsk->thread.gcspr_el0 = read_sysreg_s(SYS_GCSPR_EL0);
> +		return 0;
> +	}
> +
> +	size = args->stack_size;

no /2 (i think this should be divided)

> +
> +	size = gcs_size(size);
> +	addr = alloc_gcs(0, size);
> +	if (IS_ERR_VALUE(addr))
> +		return addr;
> +
> +	tsk->thread.gcs_base = addr;
> +	tsk->thread.gcs_size = size;
> +	tsk->thread.gcspr_el0 = addr + size - sizeof(u64);
> +
> +	return addr;
> +}
...
>  void gcs_free(struct task_struct *task)
>  {
> +
> +	/*
> +	 * When fork() with CLONE_VM fails, the child (tsk) already
> +	 * has a GCS allocated, and exit_thread() calls this function
> +	 * to free it.  In this case the parent (current) and the
> +	 * child share the same mm struct.
> +	 */
> +	if (!task->mm || task->mm != current->mm)
> +		return;
> +
>  	if (task->thread.gcs_base)
>  		vm_munmap(task->thread.gcs_base, task->thread.gcs_size);

not sure why this logic fails to free thread gcs
(created with clone3 in glibc)

other the gcs leak, my tests pass.
diff mbox series

Patch

diff --git a/arch/arm64/include/asm/gcs.h b/arch/arm64/include/asm/gcs.h
index 04594ef59dad..c1f274fdb9c0 100644
--- a/arch/arm64/include/asm/gcs.h
+++ b/arch/arm64/include/asm/gcs.h
@@ -8,6 +8,8 @@ 
 #include <asm/types.h>
 #include <asm/uaccess.h>
 
+struct kernel_clone_args;
+
 static inline void gcsb_dsync(void)
 {
 	asm volatile(".inst 0xd503227f" : : : "memory");
@@ -58,6 +60,8 @@  static inline bool task_gcs_el0_enabled(struct task_struct *task)
 void gcs_set_el0_mode(struct task_struct *task);
 void gcs_free(struct task_struct *task);
 void gcs_preserve_current_state(void);
+unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
+				     const struct kernel_clone_args *args);
 
 #else
 
@@ -69,6 +73,11 @@  static inline bool task_gcs_el0_enabled(struct task_struct *task)
 static inline void gcs_set_el0_mode(struct task_struct *task) { }
 static inline void gcs_free(struct task_struct *task) { }
 static inline void gcs_preserve_current_state(void) { }
+static inline unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
+						   const struct kernel_clone_args *args)
+{
+	return -ENOTSUPP;
+}
 
 #endif
 
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 3622956b6515..de59aa16919c 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -285,9 +285,29 @@  static void flush_gcs(void)
 	write_sysreg_s(0, SYS_GCSPR_EL0);
 }
 
+static int copy_thread_gcs(struct task_struct *p,
+			   const struct kernel_clone_args *args)
+{
+	unsigned long gcs;
+
+	gcs = gcs_alloc_thread_stack(p, args);
+	if (IS_ERR_VALUE(gcs))
+		return PTR_ERR((void *)gcs);
+
+	p->thread.gcs_el0_mode = current->thread.gcs_el0_mode;
+	p->thread.gcs_el0_locked = current->thread.gcs_el0_locked;
+
+	return 0;
+}
+
 #else
 
 static void flush_gcs(void) { }
+static int copy_thread_gcs(struct task_struct *p,
+			   const struct kernel_clone_args *args)
+{
+	return 0;
+}
 
 #endif
 
@@ -303,6 +323,7 @@  void flush_thread(void)
 void arch_release_task_struct(struct task_struct *tsk)
 {
 	fpsimd_release_task(tsk);
+	gcs_free(tsk);
 }
 
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
@@ -366,6 +387,7 @@  int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	unsigned long stack_start = args->stack;
 	unsigned long tls = args->tls;
 	struct pt_regs *childregs = task_pt_regs(p);
+	int ret;
 
 	memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context));
 
@@ -407,6 +429,10 @@  int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 			p->thread.uw.tp_value = tls;
 			p->thread.tpidr2_el0 = 0;
 		}
+
+		ret = copy_thread_gcs(p, args);
+		if (ret != 0)
+			return ret;
 	} else {
 		/*
 		 * A kthread has no context to ERET to, so ensure any buggy
diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index b0a67efc522b..6e8a5e14fff1 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c
@@ -5,9 +5,69 @@ 
 #include <linux/syscalls.h>
 #include <linux/types.h>
 
+#include <asm/cmpxchg.h>
 #include <asm/cpufeature.h>
+#include <asm/gcs.h>
 #include <asm/page.h>
 
+static unsigned long alloc_gcs(unsigned long addr, unsigned long size)
+{
+	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
+	struct mm_struct *mm = current->mm;
+	unsigned long mapped_addr, unused;
+
+	if (addr)
+		flags |= MAP_FIXED_NOREPLACE;
+
+	mmap_write_lock(mm);
+	mapped_addr = do_mmap(NULL, addr, size, PROT_READ, flags,
+			      VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
+	mmap_write_unlock(mm);
+
+	return mapped_addr;
+}
+
+static unsigned long gcs_size(unsigned long size)
+{
+	if (size)
+		return PAGE_ALIGN(size);
+
+	/* Allocate RLIMIT_STACK/2 with limits of PAGE_SIZE..2G */
+	size = PAGE_ALIGN(min_t(unsigned long long,
+				rlimit(RLIMIT_STACK) / 2, SZ_2G));
+	return max(PAGE_SIZE, size);
+}
+
+unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
+				     const struct kernel_clone_args *args)
+{
+	unsigned long addr, size;
+
+	if (!system_supports_gcs())
+		return 0;
+
+	if (!task_gcs_el0_enabled(tsk))
+		return 0;
+
+	if ((args->flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM) {
+		tsk->thread.gcspr_el0 = read_sysreg_s(SYS_GCSPR_EL0);
+		return 0;
+	}
+
+	size = args->stack_size;
+
+	size = gcs_size(size);
+	addr = alloc_gcs(0, size);
+	if (IS_ERR_VALUE(addr))
+		return addr;
+
+	tsk->thread.gcs_base = addr;
+	tsk->thread.gcs_size = size;
+	tsk->thread.gcspr_el0 = addr + size - sizeof(u64);
+
+	return addr;
+}
+
 /*
  * Apply the GCS mode configured for the specified task to the
  * hardware.
@@ -30,6 +90,16 @@  void gcs_set_el0_mode(struct task_struct *task)
 
 void gcs_free(struct task_struct *task)
 {
+
+	/*
+	 * When fork() with CLONE_VM fails, the child (tsk) already
+	 * has a GCS allocated, and exit_thread() calls this function
+	 * to free it.  In this case the parent (current) and the
+	 * child share the same mm struct.
+	 */
+	if (!task->mm || task->mm != current->mm)
+		return;
+
 	if (task->thread.gcs_base)
 		vm_munmap(task->thread.gcs_base, task->thread.gcs_size);