From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id C8C774C7555; Tue, 15 Sep 2026 19:27:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789500452; cv=none; b=KFsOVYOhwSrsowP98w2woadPAxQBIQrR32YSh6eoYIn3qgrO4vToTd0BEkUw/SvEbeUtm/RGmasoAvMu9gMw+zf8nbqRX+hukcb8/WngJHIjloxQFYHQXLJEfZDdJ9vesbz5L3V9n2IhJexNdWSjh9m7OfsoQnkV4z28HVHltFQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789500452; c=relaxed/simple; bh=G1vVaQod4yPGe4rv5ZkoOe7qjpErYp0ggnq2+ZVR5WM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=pUtmuubsn3J+soSr9iZOl5XRt5q70GwnEhfXkUtpKR/xtjUubFZPoTw5DekZ/fi9XQq6BGQtPo3r6HHqiU+HsXVktW6Lpm2EGFs8bw5VsTLuL3/L0LzolvebWuPu3BbfPH8PZQ5XJO8++wzvZJUA98HEodLppu3Mk/IAN04K01s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=hd9kxPBT; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="hd9kxPBT" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C3BD71DB5; Tue, 15 Sep 2026 12:27:25 -0700 (PDT) Received: from fedora (LJ9QCPV96V.austin.arm.com [10.118.150.127]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CB8743F882; Tue, 15 Sep 2026 12:27:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1789500449; bh=G1vVaQod4yPGe4rv5ZkoOe7qjpErYp0ggnq2+ZVR5WM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=hd9kxPBTGtc0xptHY4Gms+93ep1oEbAtsfXAiendWitFYVyfKOgd3BeeQHSM+Z8np GDEB14OWNG8oLwkOaryeFIWuP85SFgkbGgKxGVRm5xJTrLwj8+KBxF2CnSVTNngYQp IzpdA91OWT3fcpWpIZvDuBSHX6TgK4pYputRYqVw= From: Bill Roberts To: "H. Peter Anvin" , Albert Ou , Alexandre Ghiti , Borislav Petkov , Dave Hansen , Ingo Molnar , Palmer Dabbelt , Paul Walmsley , rick.p.edgecombe@intel.com, Shuah Khan , Thomas Gleixner , x86@kernel.org Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-riscv@lists.infradead.org, Bill Roberts Subject: [PATCH v4 4/6] x86/shstk: support via prctl Date: Wed, 16 Sep 2026 04:52:22 -0500 Message-ID: <20260916095224.1533971-5-bill.roberts@arm.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260916095224.1533971-1-bill.roberts@arm.com> References: <20260916095224.1533971-1-bill.roberts@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- arch/x86/kernel/shstk.c | 83 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 83 insertions(+) diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index 0ca64900192f..249c4998732a 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -18,6 +18,7 @@ #include #include #include +#include #include #include #include @@ -630,3 +631,85 @@ bool shstk_is_enabled(void) { return features_enabled(ARCH_SHSTK_SHSTK); } + +/* 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; + bool shstk_was_enabled = features_enabled(ARCH_SHSTK_SHSTK); + + /* + * One must explicitly perform enable/disable calls for the whole feature set so + * that the locked bits are checked. It is also important to capture entry state + * for roll-back if needed. However, we must NOT try to enable or disable a feature + * unless there is a change in feature status from old to new state. This is + * because shstk_prctl(), correctly, checks locked bits before feature enabled/disabled + * short circuit. + */ + switch (status) { + case 0: + /* + * We do NOT explicitly disable WRSS here, disabling shadow stack will disable WRSS, + * and we don't want a locked WRSS to prevent disable. + */ + return shstk_was_enabled ? + shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK) : 0; + + case PR_SHADOW_STACK_ENABLE: + if (!shstk_was_enabled) { + rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK); + if (rc) + return rc; + } + + if (features_enabled(ARCH_SHSTK_WRSS)) { + rc = shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_WRSS); + if (rc && !shstk_was_enabled) + shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK); + return rc; + } + + return 0; + + case PR_SHADOW_STACK_ENABLE|PR_SHADOW_STACK_WRITE: + if (!shstk_was_enabled) { + rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK); + if (rc) + return rc; + } + + if (!features_enabled(ARCH_SHSTK_WRSS)) { + rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_WRSS); + if (rc && !shstk_was_enabled) + shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK); + + return rc; + } + + return 0; + + /* + * Unknown bit mapping, unsupported PR_SHADOW_STACK_PUSH or *only* + * PR_SHADOW_STACK_WRITE. + */ + default: + return -EINVAL; + } +} + +/* 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