kernel-hardening - Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

Follow @Openwall on Twitter for new release announcements and other news

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

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

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.