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

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Date:   Wed, 10 Apr 2019 10:26:42 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Elena Reshetova <elena.reshetova@...el.com>, luto@...nel.org,
        linux-kernel@...r.kernel.org, luto@...capital.net,
        keescook@...omium.org, jannh@...gle.com, enrico.perla@...el.com,
        mingo@...hat.com, bp@...en8.de, tglx@...utronix.de,
        peterz@...radead.org, gregkh@...uxfoundation.org
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall


* Josh Poimboeuf <jpoimboe@...hat.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.

Thanks,

	Ingo