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Date:   Fri, 22 Jan 2021 01:12:07 +0300
From:   Cyrill Gorcunov <gorcunov@gmail.com>
To:     LKML <linux-kernel@vger.kernel.org>
Cc:     Keno Fischer <keno@juliacomputing.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Andrey Vagin <avagin@gmail.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Subject: [PATCH] prctl: allow to setup brk for et_dyn executables
Message-ID: <20210121221207.GB2174@grain>
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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.

Reported-by: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Dmitry Safonov <0x7f454c46@gmail.com>
CC: Andrey Vagin <avagin@gmail.com>
CC: Kirill Tkhai <ktkhai@virtuozzo.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
---
Guys, take a look please once time permit. Hopefully I didn't
miss something 'cause made this patch via code reading only.

Andrey, do we still have a criu container which tests new kernels,
right? Would be great to run criu tests with this patch applied
to make sure everything is intact.

 kernel/sys.c |    7 -------
 1 file changed, 7 deletions(-)

Index: linux-tip.git/kernel/sys.c
===================================================================
--- linux-tip.git.orig/kernel/sys.c
+++ linux-tip.git/kernel/sys.c
@@ -1943,13 +1943,6 @@ static int validate_prctl_map_addr(struc
 	error = -EINVAL;
 
 	/*
-	 * @brk should be after @end_data in traditional maps.
-	 */
-	if (prctl_map->start_brk <= prctl_map->end_data ||
-	    prctl_map->brk <= prctl_map->end_data)
-		goto out;
-
-	/*
 	 * Neither we should allow to override limits if they set.
 	 */
 	if (check_data_rlimit(rlimit(RLIMIT_DATA), prctl_map->brk,