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[92.26.98.202]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43961811315sm54926895e9.20.2025.02.14.14.40.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 Feb 2025 14:40:30 -0800 (PST) Message-ID: <7ae6ee84-b5ae-479b-b963-9e9aefcd3bfa@citrix.com> Date: Fri, 14 Feb 2025 22:40:28 +0000 Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/kcfi: Require FRED for FineIBT To: Kees Cook Cc: Peter Zijlstra , Jennifer Miller , Sami Tolvanen , Jann Horn , Nathan Chancellor , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Alexei Starovoitov , Josh Poimboeuf , "Mike Rapoport (Microsoft)" , Rik van Riel , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org References: <20250214192210.work.253-kees@kernel.org> <5d26bcdf-0e72-45b6-98c5-f90481650f09@citrix.com> <202502141345.C78A253D@keescook> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <202502141345.C78A253D@keescook> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 14/02/2025 9:54 pm, Kees Cook wrote: > On Fri, Feb 14, 2025 at 07:39:20PM +0000, Andrew Cooper wrote: >> On 14/02/2025 7:22 pm, Kees Cook wrote: >>> diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h >>> index ef5e0a698253..dfa2ba4cceca 100644 >>> --- a/arch/x86/include/asm/cfi.h >>> +++ b/arch/x86/include/asm/cfi.h >>> @@ -93,7 +93,7 @@ >>> * >>> */ >>> enum cfi_mode { >>> - CFI_AUTO, /* FineIBT if hardware has IBT, otherwise kCFI */ >>> + CFI_AUTO, /* FineIBT if hardware has IBT, FRED, and XOM */ >> You discuss XOM in the commit message, but there's no check ... >> >>> CFI_OFF, /* Taditional / IBT depending on .config */ >>> CFI_KCFI, /* Optionally CALL_PADDING, IBT, RETPOLINE */ >>> CFI_FINEIBT, /* see arch/x86/kernel/alternative.c */ >>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c >>> index 97422292b609..acc12a6efc18 100644 >>> --- a/arch/x86/kernel/alternative.c >>> +++ b/arch/x86/kernel/alternative.c >>> @@ -1323,7 +1323,9 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline, >>> >>> if (cfi_mode == CFI_AUTO) { >>> cfi_mode = CFI_KCFI; >>> - if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT)) >>> + /* FineIBT requires IBT and will only be safe with FRED */ >>> + if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT) && >>> + cpu_feature_enabled(X86_FEATURE_FRED)) >> ... here. >> >> Is this meant to be "/* TODO: wire up XOM */" or is that accounted for >> somewhere else? > Yeah, I wasn't sure how to best capture that in here. XOM doesn't exist > yet for x86... I could add a TODO like that? I get the feeling that the PKS series would have an easier time starting with XOM (even if it hard-codes pkey1=xom and avoids the allocator in the short term, seeing as Linux does have a good grasp of where it's executable pages are, and how they're accessed) rather than trying to do general page hiding.  The capability is in 3 generations of Intel CPU now. > >> Also, while I hate to come back and contradict myself from earlier... >> >> Architecturally, FineIBT without FRED seems to be no improvement over >> simple IBT.  (I'd love to find some way of hardening the entrypoints, >> but I can't see a robust way of doing so.) > If you're just looking at IBT, yes. But kCFI (with or without IBT, > but without FineIBT) will do hash checking at the call site, which > should make it impossible to reach the entrypoints from an indirect call > in the first place, as they have no hash preceding them. > >> However, micro-architecturally, FineIBT is still far better than simple >> IBT for speculation issue, seeing as Intel keep on staunchly refusing to >> turn off the indirect predictors by default like AMD do. >> >> A security conscious user ought to be using FineIBT for this, given a >> choice, even if it's not perfect in other regards. > A security conscious user should use kCFI without FineIBT. :) But I > think we might be thinking about different elements of security. I am > focusing on control flow, and I think you're considering speculation? True.  The security realist knows they're dammed either way, and gets a stiff drink instead. ~Andrew