linux-kernel - Re: brk randomization breaks columns

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Message-ID: <20080205220342.GB1630@elf.ucw.cz>
Date:	Tue, 5 Feb 2008 23:03:42 +0100
From:	Pavel Machek <pavel@....cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Hugh Dickins <hugh@...itas.com>, Jiri Kosina <jkosina@...e.cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Abel Bernabeu <abel.bernabeu@...il.com>
Subject: Re: brk randomization breaks columns

Hi!

> > . Yes, setarch i386 -R /usr/local/bin/uemacs (etc) fixes them, too.
> > 	
> > What about this?
> > 
> > Heap randomization breaks /lib/libc.so.5.4.33, make it possible to 
> > randomize normal stuff but leave the heap alone.
> 
> certainly looks fine to me, but please also add a .config to make it 
> default to 2. The reason is that a good portions of the overflows happen 
> on the heap and 99.9% of the Linux users do not run 1996-era glibc 
> anymore.
> 
> something like CONFIG_COMPAT_BRK=y, which would cause randomize_va_space 
> to default to 1, and if a user or distro disables it, it will default to 
> 2.

I named it slightly differently, but idea is same. Heap randomization
breaks ancient binaries, so disable it by default.

Signed-off-by: Pavel Machek <pavel@...e.cz>

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 18ed6dd..4b099ea 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1077,7 +1077,7 @@ #endif /* ARCH_HAS_SETUP_ADDITIONAL_PAGE
 	current->mm->start_stack = bprm->p;
 
 #ifdef arch_randomize_brk
-	if (current->flags & PF_RANDOMIZE)
+	if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1))
 		current->mm->brk = current->mm->start_brk =
 			arch_randomize_brk(current->mm);
 #endif
diff --git a/init/Kconfig b/init/Kconfig
index dcc96a8..37c7852 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -541,6 +541,16 @@ config ELF_CORE
 	help
 	  Enable support for generating core dumps. Disabling saves about 4k.
 
+config RANDOMIZE_HEAP
+	bool "Randomize heap placement"
+	help
+	  Randomizing heap placement makes heap exploits harder, but it also
+	  breaks old programs (including anything libc5 based). This only
+	  changes default, and can be overriden in runtime by tweaking
+	  /proc/sys/kernel/randomize_va_space.
+
+	  N is safe choice here.	  
+
 config BASE_FULL
 	default y
 	bool "Enable full-sized data structures for core" if EMBEDDED
diff --git a/mm/memory.c b/mm/memory.c
index 4706af1..5134014 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -82,7 +82,12 @@ void * high_memory;
 EXPORT_SYMBOL(num_physpages);
 EXPORT_SYMBOL(high_memory);
 
-int randomize_va_space __read_mostly = 1;
+int randomize_va_space __read_mostly = 
+#ifdef CONFIG_RANDOMIZE_HEAP
+					2;
+#else
+					1;
+#endif
 
 static int __init disable_randmaps(char *s)
 {


-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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