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

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

[Cyrill Gorcunov]

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,

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

[Kirill Tkhai]

CC: Pavel and Alexander

On 22.01.2021 01:12, Cyrill Gorcunov 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.
> 
> 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,
> 

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

[Andrei Vagin]

On Thu, Jan 21, 2021 at 2:12 PM Cyrill Gorcunov <gorcunov@gmail.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.
>
> 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>

Acked-by: Andrey Vagin <avagin@gmail.com>
Fixes: bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing
direct loader exec")

> 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.

Sorry for the delay. I run tests and everything works as expected.

Thanks,
Andrei

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

[Cyrill Gorcunov]

On Tue, Jul 20, 2021 at 12:33:11AM -0700, Andrei Vagin wrote:
> >
> > 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>
> 
> Acked-by: Andrey Vagin <avagin@gmail.com>
> Fixes: bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing
> direct loader exec")

Thanks for review, Andrew! I reviseted this patch recently again and
indeed we still need it.

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

[Andrew Morton]

On Fri, 22 Jan 2021 01:12:07 +0300 Cyrill Gorcunov <gorcunov@gmail.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?

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;
}