linux-kernel - Re: [PATCH] prctl: allow to setup brk for et

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Date:   Tue, 20 Jul 2021 14:51:48 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Keno Fischer <keno@...iacomputing.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrey Vagin <avagin@...il.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] prctl: allow to setup brk for et_dyn executables

On Fri, 22 Jan 2021 01:12:07 +0300 Cyrill Gorcunov <gorcunov@...il.com> wrote:

> Keno Fischer reported that when a binray loaded via
> ld-linux-x the prctl(PR_SET_MM_MAP) doesn't allow to
> setup brk value because it lays before mm:end_data.
> 
> For example a test program shows
> 
>  | # ~/t
>  |
>  | start_code      401000
>  | end_code        401a15
>  | start_stack     7ffce4577dd0
>  | start_data	   403e10
>  | end_data        40408c
>  | start_brk	   b5b000
>  | sbrk(0)         b5b000
> 
> and when executed via ld-linux
> 
>  | # /lib64/ld-linux-x86-64.so.2 ~/t
>  |
>  | start_code      7fc25b0a4000
>  | end_code        7fc25b0c4524
>  | start_stack     7fffcc6b2400
>  | start_data	   7fc25b0ce4c0
>  | end_data        7fc25b0cff98
>  | start_brk	   55555710c000
>  | sbrk(0)         55555710c000
> 
> 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?

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