The patch titled
Subject: ASLR: fix stack randomization on 64-bit systems
has been added to the -mm tree. Its filename is
aslr-fix-stack-randomization-on-64-bit-systems.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/aslr-fix-stack-randomization-on-64-bit-systems.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/aslr-fix-stack-randomization-on-64-bit-systems.patch
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------------------------------------------------------
From: Hector Marco-Gisbert <hecmargi@xxxxxx>
Subject: ASLR: fix stack randomization on 64-bit systems
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
[keescook@xxxxxxxxxxxx: rebase, fix 80 char, clean up commit message, add test example, cve]
Signed-off-by: Hector Marco-Gisbert <hecmargi@xxxxxx>
Signed-off-by: Ismael Ripoll <iripoll@xxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/x86/mm/mmap.c | 6 +++---
fs/binfmt_elf.c | 5 +++--
2 files changed, 6 insertions(+), 5 deletions(-)
diff -puN arch/x86/mm/mmap.c~aslr-fix-stack-randomization-on-64-bit-systems arch/x86/mm/mmap.c
--- a/arch/x86/mm/mmap.c~aslr-fix-stack-randomization-on-64-bit-systems
+++ a/arch/x86/mm/mmap.c
@@ -35,12 +35,12 @@ struct va_alignment __read_mostly va_ali
.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 -puN fs/binfmt_elf.c~aslr-fix-stack-randomization-on-64-bit-systems fs/binfmt_elf.c
--- a/fs/binfmt_elf.c~aslr-fix-stack-randomization-on-64-bit-systems
+++ a/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
_
Patches currently in -mm which might be from hecmargi@xxxxxx are
aslr-fix-stack-randomization-on-64-bit-systems.patch
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