[PATCH] brk: fix min_brk lower bound computation for COMPAT_BRK

[Jiri Kosina]

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.

Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
---
 mm/mmap.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

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
-- 
1.7.3.1

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