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

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Date:   Wed, 10 Apr 2019 12:17:33 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "luto@...capital.net" <luto@...capital.net>,
        "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


* Reshetova, Elena <elena.reshetova@...el.com> wrote:

> > * 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.
> > 
> 
> 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).

The original patch would be fine to me, was missing the context.

Thanks,

	Ingo