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Date: Mon, 1 Nov 2021 09:36:32 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Peter Zijlstra' <peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org> CC: Sami Tolvanen <samitolvanen@...gle.com>, Mark Rutland <mark.rutland@....com>, X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>, "Nathan Chancellor" <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Sedat Dilek <sedat.dilek@...il.com>, Steven Rostedt <rostedt@...dmis.org>, "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>, "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>, "llvm@...ts.linux.dev" <llvm@...ts.linux.dev> Subject: RE: [PATCH] static_call,x86: Robustify trampoline patching From: Peter Zijlstra > Sent: 01 November 2021 09:02 .. > In any case, I really want the discussion to start at square one, and > show/explain why any chosen CFI scheme is actually good for the kernel. > Just because clang happened to have implemented it, doesn't make it the > most suitable scheme for the kernel. How much overhead does it add to write("/dev/null", "", 1) ? You've two large jump tables. One for the syscall entry - (all the syscalls have the same prototype), and a second for selecting the correct device driver's 'write' entry point. You really don't want to be doing any kind of search. Hardware that supported a (say) 16-bit constant in both the 'landing pad' and call indirect instruction and trapped if they differed would be useful - but I doubt any hardware that checks landing pads is anywhere near that useful. David. - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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