Re: [PATCHv4 4/5] x86/mm: check in_compat_syscall() instead TIF_ADDR32 for mmap(MAP_32BIT)

[Dmitry Safonov]

On 02/14/2017 07:11 PM, Dmitry Safonov wrote:
> On 02/11/2017 11:13 PM, Thomas Gleixner wrote:
> > On Mon, 30 Jan 2017, Dmitry Safonov wrote:
> >
> > > At this momet, logic in arch_get_unmapped_area{,_topdown} for mmaps with
> > > MAP_32BIT flag checks TIF_ADDR32 which means:
> > > o if 32-bit ELF changes mode to 64-bit on x86_64 and then tries to
> > >   mmap() with MAP_32BIT it'll result in addr over 4Gb (as default is
> > >   top-down allocation)
> > > o if 64-bit ELF changes mode to 32-bit and tries mmap() with MAP_32BIT,
> > >   it'll allocate only memory in 1GB space: [0x40000000, 0x80000000).
> > >
> > > Fix it by handeling MAP_32BIT in 64-bit syscalls only.
> >
> > I really have a hard time to understand what is fixed and how that is
> > related to the $subject.
> >
> > Again. Please explain the problem first properly so one can understand
> > the
> > issue immediately.
>
> Ok, rewrote the changes log.
>
> > > As a little bonus it'll make thread flag a little less used.
> >
> > I really do not understand the bonus part here. You replace the thread
> > flag
> > check with a different one and AFAICT this looks like oart of the 'fix'.
>
> It's a part of the fix, right.
> What I meant here is that after those patches TIF_ADDR32 is no more
> used after exec() time. That's bonus as if we manage to change exec()
> code in some way (i.e., pass address restriction as a parameter), we'll
> have additional free thread info flag.

Oh, I lied here: it'll be still used for TASK_SIZE macro and etc.
But, I don't see why in generic code we can't use TASK_SIZE_MAX,
at least at this moment.

> > Thanks,
> >
> >     tglx
> >
> > > @@ -101,7 +101,7 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr,
> > > unsigned long, len,
> > >  static void find_start_end(unsigned long flags, unsigned long *begin,
> > >                 unsigned long *end)
> > >  {
> > > -    if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT)) {
> > > +    if (!in_compat_syscall() && (flags & MAP_32BIT)) {
> > >          /* This is usually used needed to map code in small
> > >             model, so it needs to be in the first 31bit. Limit
> > >             it to that.  This means we need to move the
> > > @@ -195,7 +195,7 @@ arch_get_unmapped_area_topdown(struct file *filp,
> > > const unsigned long addr0,
> > >          return addr;
> > >
> > >      /* for MAP_32BIT mappings we force the legacy mmap base */
> > > -    if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT))
> > > +    if (!in_compat_syscall() && (flags & MAP_32BIT))
> > >          goto bottomup;
> > >
> > >      /* requesting a specific address */
> > > --
> > > 2.11.0


--
             Dmitry