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Date: Wed, 3 Nov 2021 10:01:40 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Peter Zijlstra' <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org> CC: Kees Cook <keescook@...omium.org>, Ard Biesheuvel <ardb@...nel.org>, "Sami Tolvanen" <samitolvanen@...gle.com>, Mark Rutland <mark.rutland@....com>, the arch/x86 maintainers <x86@...nel.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: 03 November 2021 08:36 > > On Tue, Nov 02, 2021 at 05:20:05PM -0700, Andy Lutomirski wrote: > > I think that's a big mistake -- any sane ENDBR-using scheme would > > really prefer that ENDBR to be right next to the actual function body, > > and really any scheme would benefit due to better cache locality. > > Agreed, IBT/BTI want the landing pad in front of the actual function. > > > But, more importantly, IMO any sane ENDBR-using scheme wants to > > generate the indirect stub as part of code gen for the actual > > function. > > Sorta, I really want to be able to not have a landing pad for functions > whose address is never taken. At that point it doesn't matter if it gets > generated along with the function and then stripped/poisoned later, or > generated later. > > As such, the landing pad should not be part of the function proper, > direct calls should never observe it. > > Less landing pads is more better. One problem is when a direct call is 'too far' for a call instruction. IIRC this can happen in arm64 with modules (all 64bit except x86?). So an indirect call has to be used instead - which needs the landing pad. Although it may actually be better to put a trampoline (landing pad + near jump) elsewhere and have the module loader do the correct fixup. (Is the loader already generating a trampoline in the module code?) The function body can then be cache-line aligned - with its benefits. Can't anything that can write instructions always use a retpoline to implement a jump indirect to an arbitrary address? (Not to mention just generating the required code rather than a call.) AFAICT CFI is all about detecting invalid values in function pointer tables. It doesn't really protect in any way from JIT code doing incorrect things. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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