Re: [PATCH] prctl: allow to setup brk for et_dyn executables

[Cyrill Gorcunov]

On Tue, Jul 20, 2021 at 02:51:48PM -0700, Andrew Morton wrote:
> > 
> > This of course prevent criu from restoring such programs.
> > Looking into how kernel operates with brk/start_brk inside
> > brk() syscall I don't see any problem if we allow to setup
> > brk/start_brk without checking for end_data. Even if someone
> > pass some weird address here on a purpose then the worst
> > possible result will be an unexpected unmapping of existing
> > vma (own vma, since prctl works with the callers memory) but
> > test for RLIMIT_DATA is still valid and a user won't be able
> > to gain more memory in case of expanding VMAs via new values
> > shipped with prctl call.
> 
> So...  do you recall why you added that test originally?

To be honest, when I added this test in first place I simply forgot
about et_dyn executables because we usually run executables via
traditional exec call (where brk map sits before end_data VMA),
not via loader and that's the reason why I didn't hit this problem
before and why this get revealed only after a couple of years.
This is simply rarely used.

> 
> This is under prctl(CAP_SET_MM), yes?  What capabilities does this
> require?

Yes, it is for prctl(PR_SET_MM_MAP) and requires no additional
caps. The most important thing here is check_data_rlimit() function
which called at the end of memory map verification -- we make sure
the user won't get more memory than been granted by RLIMIT_DATA limit
even if he passes some bad brk value here on a purpose.

	/*
	 * Neither we should allow to override limits if they set.
	 */
	if (check_data_rlimit(rlimit(RLIMIT_DATA), prctl_map->brk,
			      prctl_map->start_brk, prctl_map->end_data,
			      prctl_map->start_data))
			goto out;

which expands to (I wrapped code to make it a bit more readable)

static inline int check_data_rlimit(unsigned long rlim,
				    unsigned long new,
				    unsigned long start,
				    unsigned long end_data,
				    unsigned long start_data)
{
	if (rlimit(RLIMIT_DATA) < RLIM_INFINITY) {
		if (((prctl_map->brk - prctl_map->start_brk) +
		     (prctl_map->end_data - prctl_map->start_data)) > rlimit(RLIMIT_DATA))
			return -ENOSPC;
	}

	return 0;
}