linux-kernel - [regression v2.6.38] Re: [PATCH v2] brk: fix min_brk lower bound computation for COMPAT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 24 Mar 2011 22:49:45 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	"Linux/m68k" <linux-m68k@...r.kernel.org>
Subject: [regression v2.6.38] Re: [PATCH v2] brk: fix min_brk lower bound
 computation for COMPAT_BRK

On Mon, Jan 3, 2011 at 14:24, Jiri Kosina <jkosina@...e.cz> wrote:
>> > diff --git a/mm/mmap.c b/mm/mmap.c
>> > index 50a4aa0..35d9f9c 100644
>> > --- a/mm/mmap.c
>> > +++ b/mm/mmap.c
>> > @@ -253,7 +253,15 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
>> >     down_write(&mm->mmap_sem);
>> >
>> >  #ifdef CONFIG_COMPAT_BRK
>> > -   min_brk = mm->end_code;
>> > +   /*
>> > +    * CONFIG_COMPAT_BRK can still be overridden by setting
>> > +    * randomize_va_space to 2, which will still make mm->start_brk
>> > +    * to be arbitrarily shifted
>> > +    */
>> > +   if (mm->start_brk > mm->end_code)
>> > +           min_brk = mm->start_brk;
>> > +   else
>> > +           min_brk = mm->end_code;
>> >  #else
>> >     min_brk = mm->start_brk;
>> >  #endif
>>
>> Hmm, actually no, that will bring us back into pre-a5b4592c era, please
>> disregard that.
>>
>> I will have to come up with something more clever (we can't just check
>> 'randomize_va_space == 2', as that would be racy for already running
>> programs).
>
> The patch below handles all the cases properly. Ingo, Andrew, please
> consider applying. Thanks.
>
>
>
> From: Jiri Kosina <jkosina@...e.cz>
> Subject: [PATCH] brk: fix min_brk lower bound computation for COMPAT_BRK
>
> Even if CONFIG_COMPAT_BRK is set in the kernel configuration, it can still
> be overriden by randomize_va_space sysctl.
>
> If this is the case, the min_brk computation in sys_brk() implementation
> is wrong, as it solely takes into account COMPAT_BRK setting, assuming
> that brk start is not randomized. But that might not be the case if
> randomize_va_space sysctl has been set to '2' at the time the binary has
> been loaded from disk.
>
> In such case, the check has to be done in a same way as in
> !CONFIG_COMPAT_BRK case.
>
> In addition to that, the check for the COMPAT_BRK case introduced back in
> a5b4592c ("brk: make sys_brk() honor COMPAT_BRK when computing lower
> bound") is slightly wrong -- the lower bound shouldn't be mm->end_code,
> but mm->end_data instead, as that's where the legacy applications expect
> brk section to start (i.e. immediately after last global variable).
>
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> ---
>  mm/mmap.c |   10 +++++++++-
>  1 files changed, 9 insertions(+), 1 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 50a4aa0..ca2f164 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -253,7 +253,15 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
>        down_write(&mm->mmap_sem);
>
>  #ifdef CONFIG_COMPAT_BRK
> -       min_brk = mm->end_code;
> +       /*
> +        * CONFIG_COMPAT_BRK can still be overridden by setting
> +        * randomize_va_space to 2, which will still make mm->start_brk
> +        * to be arbitrarily shifted
> +        */
> +       if (mm->start_brk > PAGE_ALIGN(mm->end_data))
> +               min_brk = mm->start_brk;
> +       else
> +               min_brk = mm->end_data;
>  #else
>        min_brk = mm->start_brk;
>  #endif
> --
> 1.7.3.1

Sorry for chiming in this late, but I've just bisected a problem in
2.6.38 to commit
5520e89485252c759ee60d313e9422447659947b ("brk: fix min_brk lower bound
computation for COMPAT_BRK").

When booting my very old test ramdisk on Amiga/m68k, it fails like this:

| RAMDISK: gzip image found at block 0
| VFS: Mounted root (ext2 filesystem) readonly on device 1:0.
| warning: process `update' used the obsolete bdflush system call
| Fix your initscripts?
| init: cannot open inittab
| Kernel panic - not syncing: Attempted to kill init!

Sorry for not noticing earlier, I usually boot full Debians under ARAnyM,
instead of booting old ramdisks with libc5-based binaries that once were
considered new.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/