[PATCH v1 7/8] sbm: documentation of the x86-64 SandBox Mode implementation

From: Petr Tesarik <petrtesarik@huaweicloud.com>
To: Jonathan Corbet <corbet@lwn.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
	"H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>, Xin Li <xin3.li@intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Kees Cook <keescook@chromium.org>,
	"Masami Hiramatsu (Google)" <mhiramat@kernel.org>,
	Pengfei Xu <pengfei.xu@intel.com>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Ze Gao <zegao2021@gmail.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Kai Huang <kai.huang@intel.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Brian Gerst <brgerst@gmail.com>, Jason Gunthorpe <jgg@ziepe.ca>,
	Joerg Roedel <jroedel@suse.de>,
	"Mike Rapoport (IBM)" <rppt@kernel.org>,
	Tina Zhang <tina.zhang@intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	linux-doc@vger.kernel.org (open list:DOCUMENTATION),
	linux-kernel@vger.kernel.org (open list)
Cc: Roberto Sassu <roberto.sassu@huaweicloud.com>,
	petr@tesarici.cz,
	Petr Tesarik <petr.tesarik1@huawei-partners.com>
Subject: [PATCH v1 7/8] sbm: documentation of the x86-64 SandBox Mode implementation
Date: Wed, 14 Feb 2024 12:35:15 +0100	[thread overview]
Message-ID: <20240214113516.2307-8-petrtesarik@huaweicloud.com> (raw)
In-Reply-To: <20240214113516.2307-1-petrtesarik@huaweicloud.com>

From: Petr Tesarik <petr.tesarik1@huawei-partners.com>

Add a section about the x86-64 implementation.

Signed-off-by: Petr Tesarik <petr.tesarik1@huawei-partners.com>
---
 Documentation/security/sandbox-mode.rst | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/Documentation/security/sandbox-mode.rst b/Documentation/security/sandbox-mode.rst
index 4405b8858c4a..84816b6b68de 100644
--- a/Documentation/security/sandbox-mode.rst
+++ b/Documentation/security/sandbox-mode.rst
@@ -111,6 +111,31 @@ These hooks must be implemented to select HAVE_ARCH_SBM.
 		:identifiers: arch_sbm_init arch_sbm_destroy arch_sbm_exec
 			      arch_sbm_map_readonly arch_sbm_map_writable
 
+X86_64 Implementation
+=====================
+
+The x86_64 implementation provides strong isolation and recovery from CPU
+exceptions.
+
+Sandbox mode runs in protection ring 3 (same as user mode). This means that:
+
+* sandbox code cannot execute privileged CPU instructions,
+* memory accesses are treated as user accesses.
+
+The thread stack is readable in sandbox mode, because an on-stack data
+structure is used by call helpers and thunks to pass target function
+arguments. However, it is not writable, and sandbox code runs on its own
+stack. The thread stack is not used by interrupt handlers either. Non-IST
+interrupt handlers run on a separate sandbox exception stack.
+
+The interrupt entry path modifies the saved pt_regs to make it appear as
+coming from kernel mode. The CR3 register is then switched to kernel mode.
+The interrupt exit path is modified to restore actual pt_regs and switch the
+CR3 register back to its sandbox mode value, overriding CR3 changes for page
+table isolation.
+
+Support for paravirtualized kernels is not (yet) provided.
+
 Current Limitations
 ===================
 
-- 
2.34.1

