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Subject[PATCH 5/X] uprobes: xol_alloc_area() needs memory barriers
If xol_get_insn_slot() or xol_alloc_area() races with another thread
doing xol_add_vma() it is not safe to dereference ->uprobes_xol_area.

Add the necessary wmb/read_barrier_depends pair, this ensures that
xol_get_insn_slot() always sees the properly initialized memory.

Other users of ->uprobes_xol_area look fine, they can't race with
xol_add_vma() this way. xol_free_insn_slot() checks utask->xol_vaddr,
and free_uprobes_xol_area() is calles by mmput().

Except: valid_vma() is racy but it should not use ->uprobes_xol_area
as we discussed.
---
kernel/uprobes.c | 15 ++++++++++++---
1 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/uprobes.c b/kernel/uprobes.c
index 5c2554c..b59af3b 100644
--- a/kernel/uprobes.c
+++ b/kernel/uprobes.c
@@ -1087,6 +1087,7 @@ static int xol_add_vma(struct uprobes_xol_area *area)
}

area->vaddr = addr;
+ smp_wmb(); /* pairs with get_uprobes_xol_area() */
mm->uprobes_xol_area = area;
ret = 0;
fail:
@@ -1094,6 +1095,14 @@ fail:
return ret;
}

+static inline
+struct uprobes_xol_area *get_uprobes_xol_area(struct mm_struct *mm)
+{
+ struct uprobes_xol_area *area = mm->uprobes_xol_area;
+ smp_read_barrier_depends(); /* pairs with wmb in xol_add_vma() */
+ return area;
+}
+
/*
* xol_alloc_area - Allocate process's uprobes_xol_area.
* This area will be used for storing instructions for execution out of
@@ -1124,7 +1133,7 @@ static struct uprobes_xol_area *xol_alloc_area(void)
fail:
kfree(area->bitmap);
kfree(area);
- return current->mm->uprobes_xol_area;
+ return get_uprobes_xol_area(current->mm);
}

/*
@@ -1183,17 +1192,17 @@ static unsigned long xol_take_insn_slot(struct uprobes_xol_area *area)
static unsigned long xol_get_insn_slot(struct uprobe *uprobe,
unsigned long slot_addr)
{
- struct uprobes_xol_area *area = current->mm->uprobes_xol_area;
+ struct uprobes_xol_area *area;
unsigned long offset;
void *vaddr;

+ area = get_uprobes_xol_area(current->mm);
if (!area) {
area = xol_alloc_area();
if (!area)
return 0;
}
current->utask->xol_vaddr = xol_take_insn_slot(area);
-
/*
* Initialize the slot if xol_vaddr points to valid
* instruction slot.
--
1.5.5.1



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