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Subject[PATCH] ASLR: fix stack randomization on 64-bit systems
From: Hector Marco-Gisbert <hecmargi@upv.es>

The issue is that the stack for processes is not properly randomized on 64 bit
architectures due to an integer overflow.

The affected function is randomize_stack_top() in file "fs/binfmt_elf.c":

static unsigned long randomize_stack_top(unsigned long stack_top)
{
unsigned int random_variable = 0;

if ((current->flags & PF_RANDOMIZE) &&
!(current->personality & ADDR_NO_RANDOMIZE)) {
random_variable = get_random_int() & STACK_RND_MASK;
random_variable <<= PAGE_SHIFT;
}
return PAGE_ALIGN(stack_top) + random_variable;
return PAGE_ALIGN(stack_top) - random_variable;
}

Note that, it declares the "random_variable" variable as "unsigned int". Since
the result of the shifting operation between STACK_RND_MASK (which is
0x3fffff on x86_64, 22 bits) and PAGE_SHIFT (which is 12 on x86_64):

random_variable <<= PAGE_SHIFT;

then the two leftmost bits are dropped when storing the result in the
"random_variable". This variable shall be at least 34 bits long to hold the
(22+12) result.

These two dropped bits have an impact on the entropy of process stack.
Concretely, the total stack entropy is reduced by four: from 2^28 to 2^30 (One
fourth of expected entropy).

This patch restores back the entropy by correcting the types involved in the
operations in the functions randomize_stack_top() and stack_maxrandom_size().

The successful fix can be tested with:
$ for i in `seq 1 10`; do cat /proc/self/maps | grep stack; done
7ffeda566000-7ffeda587000 rw-p 00000000 00:00 0 [stack]
7fff5a332000-7fff5a353000 rw-p 00000000 00:00 0 [stack]
7ffcdb7a1000-7ffcdb7c2000 rw-p 00000000 00:00 0 [stack]
7ffd5e2c4000-7ffd5e2e5000 rw-p 00000000 00:00 0 [stack]
...

Once corrected, the leading bytes should be between 7ffc and 7fff, rather
than always being 7fff.

CVE-2015-1593

Signed-off-by: Hector Marco-Gisbert <hecmargi@upv.es>
Signed-off-by: Ismael Ripoll <iripoll@upv.es>
[kees: rebase, fix 80 char, clean up commit message, add test example, cve]
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
---
arch/x86/mm/mmap.c | 6 +++---
fs/binfmt_elf.c | 5 +++--
2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 919b91205cd4..df4552bd239e 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -35,12 +35,12 @@ struct va_alignment __read_mostly va_align = {
.flags = -1,
};

-static unsigned int stack_maxrandom_size(void)
+static unsigned long stack_maxrandom_size(void)
{
- unsigned int max = 0;
+ unsigned long max = 0;
if ((current->flags & PF_RANDOMIZE) &&
!(current->personality & ADDR_NO_RANDOMIZE)) {
- max = ((-1U) & STACK_RND_MASK) << PAGE_SHIFT;
+ max = ((-1UL) & STACK_RND_MASK) << PAGE_SHIFT;
}

return max;
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 02b16910f4c9..995986b8e36b 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -645,11 +645,12 @@ out:

static unsigned long randomize_stack_top(unsigned long stack_top)
{
- unsigned int random_variable = 0;
+ unsigned long random_variable = 0;

if ((current->flags & PF_RANDOMIZE) &&
!(current->personality & ADDR_NO_RANDOMIZE)) {
- random_variable = get_random_int() & STACK_RND_MASK;
+ random_variable = (unsigned long) get_random_int();
+ random_variable &= STACK_RND_MASK;
random_variable <<= PAGE_SHIFT;
}
#ifdef CONFIG_STACK_GROWSUP
--
1.9.1

--
Kees Cook
Chrome OS Security


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