uml: fix mm_context memory leak

[ Spotted by Miklos ]

Fix a memory leak in init_new_context.  The struct page ** buffer allocated
for install_special_mapping was never recorded, and thus leaked when the
mm_struct was freed.  Fix it by saving the pointer in mm_context_t and freeing
it in arch_exit_mmap.
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/include/um_mmu.h

@@ -13,6 +13,7 @@
 typedef struct mm_context {
 	struct mm_id id;
 	struct uml_ldt ldt;
+	struct page **stub_pages;
 } mm_context_t;
 
 extern void __switch_mm(struct mm_id * mm_idp);

arch/um/kernel/skas/mmu.c

@@ -91,6 +91,8 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
 		goto out_free;
 	}
 
+	to_mm->stub_pages = NULL;
+
 	return 0;
 
  out_free:
@@ -126,6 +128,7 @@ void arch_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
 
 	pages[0] = virt_to_page(&__syscall_stub_start);
 	pages[1] = virt_to_page(mm->context.id.stack);
+	mm->context.stub_pages = pages;
 
 	/* dup_mmap already holds mmap_sem */
 	err = install_special_mapping(mm, STUB_START, STUB_END - STUB_START,
@@ -147,6 +150,8 @@ void arch_exit_mmap(struct mm_struct *mm)
 {
 	pte_t *pte;
 
+	if (mm->context.stub_pages != NULL)
+		kfree(mm->context.stub_pages);
 	pte = virt_to_pte(mm, STUB_CODE);
 	if (pte != NULL)
 		pte_clear(mm, STUB_CODE, pte);