Date: Thu, 25 Jan 2018 15:04:27 -0500 From: Brian Gerst <brgerst@...il.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Andy Lutomirski <luto@...nel.org>, "the arch/x86 maintainers" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Alan Cox <alan@...ux.intel.com>, Jann Horn <jannh@...gle.com>, Samuel Neves <samuel.c.p.neves@...il.com>, Dan Williams <dan.j.williams@...el.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Borislav Petkov <bp@...en8.de> Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on In Thu, Jan 25, 2018 at 2:16 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Thu, Jan 25, 2018 at 10:48 AM, Linus Torvalds > <torvalds@...ux-foundation.org> wrote: >> >> So the biggest impact of this is the extra register saves > > Actually, the other noticeable part is the reloading of the argument > registers from ptregs. Together with just the extra level of > 'call/ret' and the stack setup, I'm guessing we're talking maybe 20 > cycles or so. > > So there's the extra register saves, and simply the fact that the > fastpath had a flatter calling structure. > > It still feels worth it. And if we do decide that we want to do the > register clearing on kernel entry for some paranoid mode, we'd pretty > much have to do this anyway. > > Linus Another extra step the slow path does is checking to see if ptregs is safe for SYSRET. I think that can be mitigated by moving the check to the places that do modify ptregs (ptrace, sigreturn, and exec) which would set a flag to force return with IRET if the modified regs do not satisfy the criteria for SYSRET. -- Brian Gerst
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