[PATCH -mm 2/3] uprobes: introduce the global struct vm_special_mapping xol_mapping

[Oleg Nesterov <oleg@redhat.com>]

Currently each xol_area has its own instance of vm_special_mapping, this
is suboptimal and ugly. Kill xol_area->xol_mapping and add a single global
instance of vm_special_mapping, the ->fault() method can use area->pages
rather than xol_mapping->pages.

As a side effect this fixes the problem introduced by the recent commit
223febc6e557 ("mm: add optional close() to struct vm_special_mapping"), if
special_mapping_close() is called from the __mmput() paths, it will use
vma->vm_private_data = &area->xol_mapping freed by uprobe_clear_state().

Reported-by: Sven Schnelle <svens@linux.ibm.com>
Closes: https://lore.kernel.org/all/yt9dy149vprr.fsf@linux.ibm.com/
Fixes: 223febc6e557 ("mm: add optional close() to struct vm_special_mapping")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/events/uprobes.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 73cc47708679..a478e028043f 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -99,7 +99,6 @@ struct xol_area {
 	atomic_t 			slot_count;	/* number of in-use slots */
 	unsigned long 			*bitmap;	/* 0 = free slot */
 
-	struct vm_special_mapping	xol_mapping;
 	struct page 			*pages[2];
 	/*
 	 * We keep the vma's vm_start rather than a pointer to the vma
@@ -1433,6 +1432,21 @@ void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned lon
 		set_bit(MMF_RECALC_UPROBES, &vma->vm_mm->flags);
 }
 
+static vm_fault_t xol_fault(const struct vm_special_mapping *sm,
+			    struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	struct xol_area *area = vma->vm_mm->uprobes_state.xol_area;
+
+	vmf->page = area->pages[0];
+	get_page(vmf->page);
+	return 0;
+}
+
+static const struct vm_special_mapping xol_mapping = {
+	.name = "[uprobes]",
+	.fault = xol_fault,
+};
+
 /* Slot allocation for XOL */
 static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
 {
@@ -1459,7 +1473,7 @@ static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
 
 	vma = _install_special_mapping(mm, area->vaddr, PAGE_SIZE,
 				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO,
-				&area->xol_mapping);
+				&xol_mapping);
 	if (IS_ERR(vma)) {
 		ret = PTR_ERR(vma);
 		goto fail;
@@ -1498,9 +1512,6 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
 	if (!area->bitmap)
 		goto free_area;
 
-	area->xol_mapping.name = "[uprobes]";
-	area->xol_mapping.fault = NULL;
-	area->xol_mapping.pages = area->pages;
 	area->pages[0] = alloc_page(GFP_HIGHUSER);
 	if (!area->pages[0])
 		goto free_bitmap;
-- 
2.25.1.362.g51ebf55