Historically, managing the user-space shadow stack state on x86 has been handled exclusively through the arch_prctl() interface via the ARCH_SHSTK_* operations. However, other architectures (such as arm64 and riscv) do not implement arch_prctl() and instead utilize the newer, arch-agnostic, prctl() interface (i.e. PR_GET_SHADOW_STACK_STATUS and PR_SET_SHADOW_STACK_STATUS).
To provide language runtimes, toolchains, and libc implementations with a consistent, cross-architecture interface for managing control-flow integrity, wire up the generic shadow stack prctl handlers for x86. Map the generic PR_SHADOW_STACK_ENABLE, PR_SHADOW_STACK_DISABLE, and PR_SHADOW_STACK_LOCK operations onto the underlying x86 internal CET helper routines. This allows portable userspace applications to toggle or query shadow stack states without relying on architecture-specific system calls, while maintaining backward compatibility with existing arch_prctl() calls. Signed-off-by: Bill Roberts <[email protected]> --- arch/x86/kernel/shstk.c | 56 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index 0ca64900192f..2c2a96b2d936 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -18,6 +18,7 @@ #include <linux/sizes.h> #include <linux/user.h> #include <linux/syscalls.h> +#include <linux/prctl.h> #include <asm/msr.h> #include <asm/fpu/xstate.h> #include <asm/fpu/types.h> @@ -630,3 +631,58 @@ bool shstk_is_enabled(void) { return features_enabled(ARCH_SHSTK_SHSTK); } + +#define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK \ + (PR_SHADOW_STACK_ENABLE | PR_SHADOW_STACK_WRITE) + +/* Handles the generic prctl interface for PR_SET_SHADOW_STACK_STATUS and its feature bits */ +int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status) +{ + int rc; + int tmp_rc; + + if (status & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK) + return -EINVAL; + + /* + * x86 arch_prctl is single bit at a time, so handle these one at time + * If anything fails, rollback state. + */ + if (!(status & PR_SHADOW_STACK_ENABLE)) + return shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK); + + rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK); + if (rc) + return rc; + + if (status & PR_SHADOW_STACK_WRITE) { + rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_WRSS); + /* rollback, best attempt, if we actually enabled shadow stack in feature bits */ + if (rc && (status & PR_SHADOW_STACK_ENABLE)) { + tmp_rc = shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK); + WARN(tmp_rc, "Could not rollback shadow stack enabled: %d\n", tmp_rc); + } + + return rc; + + } + + return shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_WRSS); +} + +/* Handles the generic prctl interface for PR_LOCK_SHADOW_STACK_STATUS and its feature bits */ +int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status) +{ + return shstk_prctl(t, ARCH_SHSTK_LOCK, status); +} + +/* + * We assume the prctl() feature bits line up with the arch_prctl() specific ones. If not, + * the flags returned via arch_get_shadow_stack_status will be mapped wrong. + */ +static_assert(PR_SHADOW_STACK_ENABLE == ARCH_SHSTK_SHSTK); +static_assert(PR_SHADOW_STACK_WRITE == ARCH_SHSTK_WRSS); +int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *status) +{ + return shstk_prctl(t, ARCH_SHSTK_STATUS, (unsigned long)status); +} -- 2.55.0
