linux-kernel - Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

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Message-ID: <CALCETrWA8r6_zbfrYV-E-iykoEj-Fw=HeRQ5mAM4iSPrfU8jKw@mail.gmail.com>
Date:   Wed, 10 Apr 2019 07:52:17 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

On Wed, Apr 10, 2019 at 3:24 AM Reshetova, Elena
<elena.reshetova@...el.com> wrote:
>
>
> > > > On Mon, Apr 08, 2019 at 09:13:58AM +0300, Elena Reshetova wrote:
> > > > > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > > > > index 7bc105f47d21..38ddc213a5e9 100644
> > > > > --- a/arch/x86/entry/common.c
> > > > > +++ b/arch/x86/entry/common.c
> > > > > @@ -35,6 +35,12 @@
> > > > >  #define CREATE_TRACE_POINTS
> > > > >  #include <trace/events/syscalls.h>
> > > > >
> > > > > +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> > > > > +#include <linux/random.h>
> > > > > +
> > > > > +void *alloca(size_t size);
> > > > > +#endif
> > > > > +
> > > > >  #ifdef CONFIG_CONTEXT_TRACKING
> > > > >  /* Called on entry from user mode with IRQs off. */
> > > > >  __visible inline void enter_from_user_mode(void)
> > > > > @@ -273,6 +279,13 @@ __visible void do_syscall_64(unsigned long nr, struct
> > > pt_regs *regs)
> > > > >  {
> > > > >         struct thread_info *ti;
> > > > >
> > > > > +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> > > > > +       size_t offset = ((size_t)prandom_u32()) % 256;
> > > > > +       char *ptr = alloca(offset);
> > > > > +
> > > > > +       asm volatile("":"=m"(*ptr));
> > > > > +#endif
> > > > > +
> > > > >         enter_from_user_mode();
> > > > >         local_irq_enable();
> > > > >         ti = current_thread_info();
> > > >
> > > > Would it make sense to also do this for the compat syscalls
> > > > (do_fast_syscall_32, do_int80_syscall_32)?
> > >
> > > Could someone please include the full patch, with justification and
> > > performance impact analysis etc.? Can only find the code part of the
> > > thread on lkml, which leaves out this context.
> > >
> >
> > Sorry, this is very weird, I cannot find it either from lkml, but it was sent there
> > to begin with (and as visible from reply-to headers).
> >
> > Do you want me to resent original version or with "do_fast_syscall_32,
> > do_int80_syscall_32" additions (I am finishing testing them now).
>
> I will resend the original x86_64 now since this is the one I tested and
> measured properly. The 32 bit changes seem to work fine inside my 32 bit VM,
> but since I don't have any real 32 bit HW, I am hesitant to send them out without
> real HW testing and measuring.
>
> This is the asm code for 32 bits (note it requires __builtin_alloca definition and not just alloca,
> so I will change the 64 bit version to use it also):
>
> #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
>     size_t offset = ((size_t)prandom_u32()) % 256;
> 0xc10025b6 call   0xc146f7d0 <prandom_u32>
> 0xc10025bb movzbl %al,%eax
>     char *ptr = __builtin_alloca(offset);
> 0xc10025be add    $0x12,%eax
> 0xc10025c1 and    $0x1fc,%eax
> 0xc10025c6 sub    %eax,%esp
> 0xc10025c8 lea    0x27(%esp),%eax
> 0xc10025cc and    $0xfffffff0,%eax
>
> Also, the result is 47 different random offsets produced,
> which is slightly better than 33 offsets for x86_64.
>

I would suggest that you macro-ify this thing:

#ifdef WHATEVER
#define add_random_stack_offset() do { void *addr = ... } while (0)
#else
#define add_random_stack_offset() do {} while (0)
#endif

since you'll end up with more than one call site.