Date: Thu, 25 Jan 2018 12:54:49 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Brian Gerst <brgerst@...il.com> 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 On Thu, Jan 25, 2018 at 12:04 PM, Brian Gerst <brgerst@...il.com> wrote: > > 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. I tried to do some profiling, and none of that shows up for me. That said, what _also_ doesn't show up is the actual page table switch on entry. And that seems to be because the per-pcu trampoline code isn't captures by perf (or at least not shown). Oh well. What _does_ show up a bit is this in prepare_exit_to_usermode(): #ifdef CONFIG_COMPAT /* * Compat syscalls set TS_COMPAT. Make sure we clear it before * returning to user mode. We need to clear it *after* signal * handling, because syscall restart has a fixup for compat * syscalls. The fixup is exercised by the ptrace_syscall_32 * selftest. * * We also need to clear TS_REGS_POKED_I386: the 32-bit tracer * special case only applies after poking regs and before the * very next return to user mode. */ current->thread.status &= ~(TS_COMPAT|TS_I386_REGS_POKED); #endif and I think the problem there is that it is unnecessarily dirtying that cacheline. Afaik, those bits are already clear 99.999% of the time. So things would be better if that 'status' would be in the thread-info (to keep cachelines close to the other stuff we already touch) and the code should have something like if (unlikely(ti->status & (TS_COMPAT|TS_I386_REGS_POKED))) or whatever. There might be other similar small tuning issues going on. So there is room for improvement there in the slow path. Linus
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