[PATCH] Fix crashes due to stack randomization on stack-grows-upwards architectures

From: Helge Deller
Date: Mon May 11 2015 - 16:01:49 EST

On architectures where the stack grows upwards (CONFIG_STACK_GROWSUP=y,
currently parisc and metag only) stack randomization sometimes leads to crashes
when the stack ulimit is set to lower values than STACK_RND_MASK (which is 8 MB
by default if not defined in arch-specific headers).

The problem is, that when the stack vm_area_struct is set up in fs/exec.c, the
additional space needed for the stack randomization (as defined by the value of
STACK_RND_MASK) was not taken into account yet and as such, when the stack
randomization code added a random offset to the stack start, the stack
effectively got smaller than what the user defined via rlimit_max(RLIMIT_STACK)
which then sometimes leads to out-of-stack situations and crashes.

This patch fixes it by adding the maximum possible amount of memory (based on
STACK_RND_MASK) which theoretically could be added by the stack randomization
code to the initial stack size. That way, the user-defined stack size is always
guaranteed to be at minimum what is defined via rlimit_max(RLIMIT_STACK).

This bug is currently not visible on the metag architecture, because on metag
STACK_RND_MASK is defined to 0 which effectively disables stack randomization.

The changes to fs/exec.c are inside an "#ifdef CONFIG_STACK_GROWSUP"
section, so it does not affect other platformws beside those where the
stack grows upwards (parisc and metag).

Signed-off-by: Helge Deller <deller@xxxxxx>
Cc: linux-parisc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: James Hogan <james.hogan@xxxxxxxxxx>
Cc: linux-metag@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx

arch/parisc/include/asm/elf.h | 4 ++++
arch/parisc/kernel/sys_parisc.c | 3 +++
fs/exec.c | 3 +++
3 files changed, 10 insertions(+)

diff --git a/arch/parisc/include/asm/elf.h b/arch/parisc/include/asm/elf.h
index 3391d06..78c9fd3 100644
--- a/arch/parisc/include/asm/elf.h
+++ b/arch/parisc/include/asm/elf.h
@@ -348,6 +348,10 @@ struct pt_regs; /* forward declaration... */

#define ELF_HWCAP 0

+#define STACK_RND_MASK (is_32bit_task() ? \
+ 0x7ff >> (PAGE_SHIFT - 12) : \
+ 0x3ffff >> (PAGE_SHIFT - 12))
struct mm_struct;
extern unsigned long arch_randomize_brk(struct mm_struct *);
#define arch_randomize_brk arch_randomize_brk
diff --git a/arch/parisc/kernel/sys_parisc.c b/arch/parisc/kernel/sys_parisc.c
index e1ffea2..5aba01a 100644
--- a/arch/parisc/kernel/sys_parisc.c
+++ b/arch/parisc/kernel/sys_parisc.c
@@ -77,6 +77,9 @@ static unsigned long mmap_upper_limit(void)
if (stack_base > STACK_SIZE_MAX)
stack_base = STACK_SIZE_MAX;

+ /* Add space for stack randomization. */
+ stack_base += (STACK_RND_MASK << PAGE_SHIFT);
return PAGE_ALIGN(STACK_TOP - stack_base);

diff --git a/fs/exec.c b/fs/exec.c
index 49a1c61..1977c2a 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -659,6 +659,9 @@ int setup_arg_pages(struct linux_binprm *bprm,
if (stack_base > STACK_SIZE_MAX)
stack_base = STACK_SIZE_MAX;

+ /* Add space for stack randomization. */
+ stack_base += (STACK_RND_MASK << PAGE_SHIFT);
/* Make sure we didn't let the argument array grow too large. */
if (vma->vm_end - vma->vm_start > stack_base)
return -ENOMEM;
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