[PATCH] arm64: Make arch_randomize_brk avoid stack area

[Jon Medhurst (Tixy)]

When a process is created, various address randomisations could end up
colluding to place the address of brk in the stack memory. This would
mean processes making small heap based memory allocations are in danger
of having them overwriting, or overwritten by, the stack.

Another consequence, is that even for processes that make no use of
brk, the output of /proc/*/maps may show the stack area listed as
'[heap]' rather than '[stack]'. Apart from being misleading this causes
fatal errors with the Android run-time like:
"No [stack] line found in /proc/self/task/*/maps"

To prevent this problem pick a limit for brk that allows for the stack's
memory. At the same time we remove randomize_base() as that was only
used by arch_randomize_brk().

Note, in practice, since commit d1fd836dcf00 ("mm: split ET_DYN ASLR
from mmap ASLR") this problem shouldn't occur because the address chosen
for loading binaries is well clear of the stack, however, prior to that
the problem does occur because of the following...

The memory layout of a task is determined by arch_pick_mmap_layout. If
address randomisation is enabled (task has flag PF_RANDOMIZE) this sets
mmap_base to a random address at the top of task memory just below a
region calculated to allow for a stack which itself may have a random
base address. Any mmap operations that then happen which require an
address allocating will use the topdown allocation method, i.e. the
first allocated memory will be at the top of memory, just below the
area set aside for the stack.

When a relocatable binary is loaded into a new process by
load_elf_binary and randomised address are enabled, it uses a
'load_bias' of zero, so that when mmap is called to create a memory
region for it, a new address is picked (address zero not being
available). As this is the first memory region in the task, it gets the
region just below the stack, as described previously.

The loader then set's brk to the end of the elf data section, which will
be near the end of the loaded binary and then it calls
arch_randomize_brk. As this currently stands, this adds a random amount
to brk, which unfortunately may take it into the address range where the
stack lies.

Testing:

These changes have been tested on Linux v4.6-rc4 using 100000
invocations of a program [1] that can display the offset of a process's
brk...

$for i in $(seq 100000); do ./aslr --report brk ; done

This shows values of brk are evenly distributed over a 1GB range, both
before and after this change.

With Linux version 3.18 (where the collision of brk and stack can happen
and this change limits brk to avoid that) the distribution of brk values
after the change shows a slope where lower values for brk are more
common and upper values have about half the frequency of those.

[1] http://bazaar.launchpad.net/~ubuntu-bugcontrol/qa-regression-testing/master/files/2499/scripts/kernel-security/aslr/

Signed-off-by: Jon Medhurst <tixy at linaro.org>
Cc: <stable at vger.kernel.org> # 4.0 and earlier

---

Changes since RFC.
- Fixed compilation errors (finger trouble preparing original email)
- Updated commit message to included notes on testing
- Added CC stable for Linux '4.0 and earlier'

 arch/arm64/kernel/process.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 8062482..26429a0 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -382,13 +382,25 @@ unsigned long arch_align_stack(unsigned long sp)
 	return sp & ~0xf;
 }
 
-static unsigned long randomize_base(unsigned long base)
+unsigned long arch_randomize_brk(struct mm_struct *mm)
 {
+	unsigned long base = mm->brk;
 	unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1;
-	return randomize_range(base, range_end, 0) ? : base;
-}
+	unsigned long max_stack, range_limit;
 
-unsigned long arch_randomize_brk(struct mm_struct *mm)
-{
-	return randomize_base(mm->brk);
+	/*
+	 * Determine how much room we need to leave available for the stack.
+	 * We limit this to a reasonable value, because extremely large or
+	 * unlimited stacks are always going to bump up against brk at some
+	 * point and we don't want to fail to randomise brk in those cases.
+	 */
+	max_stack = rlimit(RLIMIT_STACK);
+	if (max_stack > SZ_128M)
+		max_stack = SZ_128M;
+
+	range_limit = mm->start_stack - max_stack - 1;
+	if (range_end > range_limit)
+		range_end = range_limit;
+
+	return randomize_range(base, range_end, 0) ? : base;
 }
-- 
2.1.4