[Devel] [PATCH RH7] prctl: allow to setup brk for et_dyn executables

[Alexander Mikhalitsyn]

From: Cyrill Gorcunov <gorcunov at gmail.com>

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.

Link: https://lkml.kernel.org/r/20210121221207.GB2174@grain
Fixes: bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing direct loader exec")
Signed-off-by: Cyrill Gorcunov <gorcunov at gmail.com>
Reported-by: Keno Fischer <keno at juliacomputing.com>
Acked-by: Andrey Vagin <avagin at gmail.com>
Tested-by: Andrey Vagin <avagin at gmail.com>
Cc: Dmitry Safonov <0x7f454c46 at gmail.com>
Cc: Kirill Tkhai <ktkhai at virtuozzo.com>
Cc: Eric W. Biederman <ebiederm at xmission.com>
Cc: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Cc: Alexander Mikhalitsyn <alexander.mikhalitsyn at virtuozzo.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>

This patch comes from LKML
https://lore.kernel.org/all/20210121221207.GB2174@grain/T/#u
It's still not in ms tree but will be commited soon.

https://jira.sw.ru/browse/PSBM-125448

Signed-off-by: Alexander Mikhalitsyn <alexander.mikhalitsyn at virtuozzo.com>
---
 kernel/sys.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 1ec78ffa8f98..c286ecc60ad5 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2163,13 +2163,6 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
 
 	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.
 	 */
-- 
2.28.0