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X-IronPort-AV: E=McAfee;i="6600,9927,10763"; a="429811671" X-IronPort-AV: E=Sophos;i="6.01,187,1684825200"; d="scan'208";a="429811671" Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Jul 2023 16:33:12 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10763"; a="789741239" X-IronPort-AV: E=Sophos;i="6.01,187,1684825200"; d="scan'208";a="789741239" Received: from wangmei-mobl.amr.corp.intel.com (HELO rpedgeco-desk4.amr.corp.intel.com) ([10.212.211.11]) by fmsmga004-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Jul 2023 16:33:09 -0700 From: Rick Edgecombe To: broonie@kernel.org Cc: akpm@linux-foundation.org, andrew.cooper3@citrix.com, arnd@arndb.de, bp@alien8.de, bsingharora@gmail.com, christina.schimpe@intel.com, corbet@lwn.net, dave.hansen@linux.intel.com, david@redhat.com, debug@rivosinc.com, dethoma@microsoft.com, eranian@google.com, esyr@redhat.com, fweimer@redhat.com, gorcunov@gmail.com, hjl.tools@gmail.com, hpa@zytor.com, jamorris@linux.microsoft.com, jannh@google.com, john.allen@amd.com, kcc@google.com, keescook@chromium.org, kirill.shutemov@linux.intel.com, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, luto@kernel.org, mike.kravetz@oracle.com, mingo@redhat.com, nadav.amit@gmail.com, oleg@redhat.com, pavel@ucw.cz, pengfei.xu@intel.com, peterz@infradead.org, rdunlap@infradead.org, rick.p.edgecombe@intel.com, rppt@kernel.org, szabolcs.nagy@arm.com, tglx@linutronix.de, torvalds@linux-foundation.org, weijiang.yang@intel.com, willy@infradead.org, x86@kernel.org, yu-cheng.yu@intel.com Subject: [PATCH] x86/shstk: Move arch detail comment out of core mm Date: Thu, 6 Jul 2023 16:32:48 -0700 Message-Id: <20230706233248.445713-1-rick.p.edgecombe@intel.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: BA6798000C X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: qj3smwu64xshfc3hc99g755uf3e385uy X-HE-Tag: 1688686394-458010 X-HE-Meta: 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 /sRikadQ f6njpGwFmu/sI0VRKQdBzBkoxiTBgQYJ0oqpZINX3henwxe+lwuULxUrRBfPJ/+LcKPaWZPSlpng/6hRHyWI7T/yxGyWJKwVwuQnrtbAUkNNxSnNbaZsnacNYh/l/PO2Hrhm7DDMC+ZKohfW2cQRN/WaoJku8PZRz0qu+PXSvGfafCijLd4v5gEGi0tWn6l4LbaoqAJiSoWP7dxrLwjsogBrUfN0hCquStwKbTJyfVV2nJos= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: The comment around VM_SHADOW_STACK in mm.h refers to a lot of x86 specific details that don't belong in a cross arch file. Remove these out of core mm, and just leave the non-arch details. Since the comment includes some useful details that would be good to retain in the source somewhere, put the arch specifics parts in arch/x86/shstk.c near alloc_shstk(), where memory of this type is allocated. Include a reference to the existence of the x86 details near the VM_SHADOW_STACK definition mm.h. Signed-off-by: Rick Edgecombe --- arch/x86/kernel/shstk.c | 25 +++++++++++++++++++++++++ include/linux/mm.h | 32 ++++++-------------------------- 2 files changed, 31 insertions(+), 26 deletions(-) diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index b26810c7cd1c..47f5204b0fa9 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -72,6 +72,31 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr) return 0; } +/* + * VM_SHADOW_STACK will have a guard page. This helps userspace protect + * itself from attacks. The reasoning is as follows: + * + * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The + * INCSSP instruction can increment the shadow stack pointer. It is the + * shadow stack analog of an instruction like: + * + * addq $0x80, %rsp + * + * However, there is one important difference between an ADD on %rsp + * and INCSSP. In addition to modifying SSP, INCSSP also reads from the + * memory of the first and last elements that were "popped". It can be + * thought of as acting like this: + * + * READ_ONCE(ssp); // read+discard top element on stack + * ssp += nr_to_pop * 8; // move the shadow stack + * READ_ONCE(ssp-8); // read+discard last popped stack element + * + * The maximum distance INCSSP can move the SSP is 2040 bytes, before + * it would read the memory. Therefore a single page gap will be enough + * to prevent any operation from shifting the SSP to an adjacent stack, + * since it would have to land in the gap at least once, causing a + * fault. + */ static unsigned long alloc_shstk(unsigned long addr, unsigned long size, unsigned long token_offset, bool set_res_tok) { diff --git a/include/linux/mm.h b/include/linux/mm.h index 535c58d3b2e4..b647cf2e94ea 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -343,33 +343,13 @@ extern unsigned int kobjsize(const void *objp); #ifdef CONFIG_X86_USER_SHADOW_STACK /* - * This flag should not be set with VM_SHARED because of lack of support - * core mm. It will also get a guard page. This helps userspace protect - * itself from attacks. The reasoning is as follows: + * VM_SHADOW_STACK should not be set with VM_SHARED because of lack of + * support core mm. * - * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The - * INCSSP instruction can increment the shadow stack pointer. It is the - * shadow stack analog of an instruction like: - * - * addq $0x80, %rsp - * - * However, there is one important difference between an ADD on %rsp - * and INCSSP. In addition to modifying SSP, INCSSP also reads from the - * memory of the first and last elements that were "popped". It can be - * thought of as acting like this: - * - * READ_ONCE(ssp); // read+discard top element on stack - * ssp += nr_to_pop * 8; // move the shadow stack - * READ_ONCE(ssp-8); // read+discard last popped stack element - * - * The maximum distance INCSSP can move the SSP is 2040 bytes, before - * it would read the memory. Therefore a single page gap will be enough - * to prevent any operation from shifting the SSP to an adjacent stack, - * since it would have to land in the gap at least once, causing a - * fault. - * - * Prevent using INCSSP to move the SSP between shadow stacks by - * having a PAGE_SIZE guard gap. + * These VMAs will get a single end guard page. This helps userspace protect + * itself from attacks. A single page is enough for current shadow stack archs + * (x86). See the comments near alloc_shstk() in arch/x86/kernel/shstk.c + * for more details on the guard size. */ # define VM_SHADOW_STACK VM_HIGH_ARCH_5 #else -- 2.34.1