On Wed, Nov 22, 2023 at 1:43 AM Mark Brown broonie@kernel.org wrote:
Three architectures (x86, aarch64, riscv) have announced support for shadow stacks with fairly similar functionality. While x86 is using arch_prctl() to control the functionality neither arm64 nor riscv uses that interface so this patch adds arch-agnostic prctl() support to get and set status of shadow stacks and lock the current configuation to prevent further changes, with support for turning on and off individual subfeatures so applications can limit their exposure to features that they do not need. The features are:
- PR_SHADOW_STACK_ENABLE: Tracking and enforcement of shadow stacks, including allocation of a shadow stack if one is not already allocated.
- PR_SHADOW_STACK_WRITE: Writes to specific addresses in the shadow stack.
- PR_SHADOW_STACK_PUSH: Push additional values onto the shadow stack.
These features are expected to be inherited by new threads and cleared on exec(), unknown features should be rejected for enable but accepted for locking (in order to allow for future proofing).
This is based on a patch originally written by Deepak Gupta but modified fairly heavily, support for indirect landing pads is removed, additional modes added and the locking interface reworked. The set status prctl() is also reworked to just set flags, if setting/reading the shadow stack pointer is required this could be a separate prctl.
Signed-off-by: Mark Brown broonie@kernel.org
include/linux/mm.h | 4 ++++ include/uapi/linux/prctl.h | 22 ++++++++++++++++++++++ kernel/sys.c | 30 ++++++++++++++++++++++++++++++ 3 files changed, 56 insertions(+)
diff --git a/include/linux/mm.h b/include/linux/mm.h index 10462f354614..8b28483b4afa 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -4143,4 +4143,8 @@ static inline bool pfn_is_unaccepted_memory(unsigned long pfn) return range_contains_unaccepted_memory(paddr, paddr + PAGE_SIZE); }
+int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *status); +int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status); +int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
#endif /* _LINUX_MM_H */ diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 370ed14b1ae0..3c66ed8f46d8 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -306,4 +306,26 @@ struct prctl_mm_map { # define PR_RISCV_V_VSTATE_CTRL_NEXT_MASK 0xc # define PR_RISCV_V_VSTATE_CTRL_MASK 0x1f
+/*
- Get the current shadow stack configuration for the current thread,
- this will be the value configured via PR_SET_SHADOW_STACK_STATUS.
- */
+#define PR_GET_SHADOW_STACK_STATUS 71
+/*
- Set the current shadow stack configuration. Enabling the shadow
- stack will cause a shadow stack to be allocated for the thread.
- */
+#define PR_SET_SHADOW_STACK_STATUS 72 +# define PR_SHADOW_STACK_ENABLE (1UL << 0)
Other architecture may require disabling shadow stack if glibc tunables is set to permissive mode. In permissive mode, if glibc encounters `dlopen` on an object which doesn't support shadow stack, glibc should be able to issue PR_SHADOW_STACK_DISABLE.
Architectures can choose to implement or not but I think arch agnostic code should enumerate this.
+# define PR_SHADOW_STACK_WRITE (1UL << 1) +# define PR_SHADOW_STACK_PUSH (1UL << 2)
+/*
- Prevent further changes to the specified shadow stack
- configuration. All bits may be locked via this call, including
- undefined bits.
- */