Fix crashes due to stack randomization on stack-grows-upwards architectures

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).

diffstat:

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

index 3391d06..78c9fd3 100644
@@ -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

index e1ffea2..5aba01a 100644
@@ -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);
 }
 

index 49a1c61..1977c2a 100644
@@ -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;