linux-kernel - [49/70] brk: COMPAT_BRK: fix detection of randomized brk

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Date:	Tue, 19 Apr 2011 13:08:58 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Jiri Kosina <jkosina@...e.cz>
Subject: [49/70] brk: COMPAT_BRK: fix detection of randomized brk

2.6.38-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Jiri Kosina <jkosina@...e.cz>

commit 4471a675dfc7ca676c165079e91c712b09dc9ce4 upstream.

5520e89 ("brk: fix min_brk lower bound computation for COMPAT_BRK")
tried to get the whole logic of brk randomization for legacy
(libc5-based) applications finally right.

It turns out that the way to detect whether brk has actually been
randomized in the end or not introduced by that patch still doesn't work
for those binaries, as reported by Geert:

: /sbin/init from my old m68k ramdisk exists prematurely.
:
: Before the patch:
:
: | brk(0x80005c8e)                         = 0x80006000
:
: After the patch:
:
: | brk(0x80005c8e)                         = 0x80005c8e
:
: Old libc5 considers brk() to have failed if the return value is not
: identical to the requested value.

I don't like it, but currently see no better option than a bit flag in
task_struct to catch the CONFIG_COMPAT_BRK && randomize_va_space == 2
case.

Signed-off-by: Jiri Kosina <jkosina@...e.cz>
Tested-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 fs/binfmt_elf.c       |    6 +++++-
 include/linux/sched.h |    3 +++
 mm/mmap.c             |    2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)

--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -941,9 +941,13 @@ static int load_elf_binary(struct linux_
 	current->mm->start_stack = bprm->p;
 
 #ifdef arch_randomize_brk
-	if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1))
+	if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1)) {
 		current->mm->brk = current->mm->start_brk =
 			arch_randomize_brk(current->mm);
+#ifdef CONFIG_COMPAT_BRK
+		current->brk_randomized = 1;
+#endif
+	}
 #endif
 
 	if (current->personality & MMAP_PAGE_ZERO) {
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1254,6 +1254,9 @@ struct task_struct {
 #endif
 
 	struct mm_struct *mm, *active_mm;
+#ifdef CONFIG_COMPAT_BRK
+	unsigned brk_randomized:1;
+#endif
 #if defined(SPLIT_RSS_COUNTING)
 	struct task_rss_stat	rss_stat;
 #endif
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -259,7 +259,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
 	 * randomize_va_space to 2, which will still cause mm->start_brk
 	 * to be arbitrarily shifted
 	 */
-	if (mm->start_brk > PAGE_ALIGN(mm->end_data))
+	if (current->brk_randomized)
 		min_brk = mm->start_brk;
 	else
 		min_brk = mm->end_data;


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