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X-CSE-ConnectionGUID: XPE5PP98RBqVYIZV7VR86w== X-CSE-MsgGUID: 1vYWlk+2REa7tndeRZ5T+g== X-IronPort-AV: E=McAfee;i="6700,10204,11308"; a="36166482" X-IronPort-AV: E=Sophos;i="6.12,296,1728975600"; d="scan'208";a="36166482" Received: from orviesa001.jf.intel.com ([10.64.159.141]) by orvoesa112.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Jan 2025 10:01:20 -0800 X-CSE-ConnectionGUID: Dq+x41nORoWR0koNWhx+rg== X-CSE-MsgGUID: HuV32W67R0GpruAo35kGuQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.12,224,1728975600"; d="scan'208";a="140185619" Received: from daliomra-mobl3.amr.corp.intel.com (HELO [10.124.221.86]) ([10.124.221.86]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Jan 2025 10:01:16 -0800 Message-ID: <81d3af8f-bad8-4559-8a0f-3271dd7f0abc@intel.com> Date: Tue, 7 Jan 2025 10:01:14 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86: Check if shadow stack is active for ssp_get() To: Rick Edgecombe , x86@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, luto@kernel.org, peterz@infradead.org, linux-kernel@vger.kernel.org Cc: christina.schimpe@intel.com References: <20231204190709.3907254-1-rick.p.edgecombe@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Some missing background: The x86 shadow stack support has its own set of registers. Those registers are XSAVE-managed, but they are "supervisor state components" which means that userspace can't touch them with XSAVE/XRSTOR. It also means that they are not accessible from the existing ptrace ABI like the FPU register or GPRs. Thus, there is a new ptrace get/set interface for it. The ptrace code is also provided an ->active() handler in addition to the get/set ones. But this ->active() handler is _not_ checked before the get/set handlers are called. This was not understood when shadow stack support was put in place. I think I'd also phrase the problem like this: As a result, both the set/get handlers can be called with XFEATURE_CET_USER in its init state, which would cause get_xsave_addr() to return NULL and trigger a WARN_ON(). The ssp_set() handler luckily has an ssp_active() check to avoid surprising the kernel with shadow stack behavior when the kernel is not read for it (ARCH_SHSTK_SHSTK==0). That check just happened to avoid the warning. But the ->get() side wasn't so lucky. It can be called with shadow stacks disabled, triggering the warning in practice, as reported by Christina Schimpe: ... Ensure that shadow stacks are active in a thread before looking them up in the XSAVE buffer. Since ARCH_SHSTK_SHSTK and user_ssp[SHSTK_EN] are set at the same time, the active check ensures that there will be something to find in the XSAVE buffer. --------------- We also want this to cc:stable, right?