arm64/gcs: Ensure that new threads have a GCS

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/2, 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: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-22-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/include/asm/gcs.h b/arch/arm64/include/asm/gcs.h
index 04594ef59dadd271f1a394ea0f85c43b67842e7b..c1f274fdb9c02f2565f9fed0761cfd8d791459e0 100644 (file)
--- 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/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index 7c09d47e09cb54a1b439e09ce65becfecac3bd05..48b3d9553b675c3c0b8bbe1b0030ed089afd3990 100644 (file)
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -20,6 +20,7 @@
 #include <asm/cacheflush.h>
 #include <asm/cpufeature.h>
 #include <asm/daifflags.h>
+#include <asm/gcs.h>
 #include <asm/proc-fns.h>
 #include <asm/cputype.h>
 #include <asm/sysreg.h>
@@ -311,6 +312,14 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
        return por_el0_allows_pkey(vma_pkey(vma), write, execute);
 }
 
+#define deactivate_mm deactivate_mm
+static inline void deactivate_mm(struct task_struct *tsk,
+                       struct mm_struct *mm)
+{
+       gcs_free(tsk);
+}
+
+
 #include <asm-generic/mmu_context.h>
 
 #endif /* !__ASSEMBLY__ */

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index aedcf332f42270a2a85761706c498da526f1d86c..fdd095480c3ffb8c13fd4e7c9abc79e88143e08b 100644 (file)
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -294,9 +294,35 @@ 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;
+
+       if (!system_supports_gcs())
+               return 0;
+
+       p->thread.gcs_base = 0;
+       p->thread.gcs_size = 0;
+
+       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
 
@@ -313,6 +339,7 @@ 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)
@@ -376,6 +403,7 @@ 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));
 
@@ -420,6 +448,10 @@ 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 f8f4f984a24719c2f555e543347a2c5d7de4f6bd..3c7a18f57ea90f9ee476672d94b83acf7396fd54 100644 (file)
--- 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 / 2;
+
+       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.
@@ -33,6 +93,15 @@ void gcs_free(struct task_struct *task)
        if (!system_supports_gcs())
                return;
 
+       /*
+        * 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);