RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

[Reshetova, Elena]

> > > 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.

Best Regards,
Elena