[PATCH 1/6] ksm: fix rmap_item->anon_vma memory corruption and vma user after free

[Andrea Arcangeli <aarcange@redhat.com>]

The ksm_test_exit() run after down_read(&mm->mmap_sem) assumed it
could serialize against ksm_exit() and prevent exit_mmap() to run
until the up_read(&mm->mmap_sem). That is true when the rmap_item->mm
is the one associated with the ksm_scan.mm_slot, as ksm_exit() would
take the !easy_to_free_path.

The problem is that when merging the current rmap_item (the one whose
->mm pointer is always associated ksm_scan.mm_slot) with a
tree_rmap_item in the unstable tree, the unstable tree
tree_rmap_item->mm can be any random mm. The locking technique
described above is a noop if the rmap_item->mm is not the one
associated with the ksm_scan.mm_slot. In turn the tree_rmap_item when
converted to a stable tree rmap_item and added to the
stable_node->hlist, can have a &rmap_item->anon_vma that points to
already freed memory. The find_vma and other vma operations to reach
the anon_vma also run on potentially already freed memory. The
get_anon_vma atomic_inc itself could corrupt memory randomly in
already re-used memory.

The result are oopses like below:

general protection fault: 0000 [#1] SMP
last sysfs file: /sys/kernel/mm/ksm/sleep_millisecs
CPU 14
Modules linked in: netconsole nfs nfs_acl auth_rpcgss fscache lockd sunrpc msr binfmt_misc sr_mod

Pid: 904, comm: ksmd Not tainted 2.6.32 #21 Supermicro X8DTN/X8DTN
RIP: 0010:[<ffffffff81138300>]  [<ffffffff81138300>] drop_anon_vma+0x0/0xe0
RSP: 0000:ffff88023b94bd28  EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff880231e64d50 RCX: 0000000000000017
RDX: ffff88023b94bfd8 RSI: 0000000000000216 RDI: 80000002192c6067
RBP: ffff88023b94bd40 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffff880217fa4198
R13: ffff880217fa4198 R14: 000000000021916f R15: ffff880217fa419b
FS:  0000000000000000(0000) GS:ffff880030600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 0000000000d116a0 CR3: 000000023aa35000 CR4: 00000000000007e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process ksmd (pid: 904, threadinfo ffff88023b948000, task ffff88023b946500)
Stack:
 ffffffff8114d049 ffffea000757d048 ffffea0000000000 ffff88023b94bda0
<d> ffffffff8114d153 ffff88023b946500 ffff88023afaab40 ffffffff8114e85d
<d> 0100000000000038 ffff88023b94bdb0 ffff880231d33560 ffff880217fa4198
Call Trace:
 [<ffffffff8114d049>] ? remove_node_from_stable_tree+0x29/0x80
 [<ffffffff8114d153>] get_ksm_page+0xb3/0x1e0
 [<ffffffff8114e85d>] ? ksm_scan_thread+0x60d/0x1130
 [<ffffffff8114d499>] remove_rmap_item_from_tree+0x99/0x130
 [<ffffffff8114ed19>] ksm_scan_thread+0xac9/0x1130
 [<ffffffff81095ce0>] ? autoremove_wake_function+0x0/0x40
 [<ffffffff8114e250>] ? ksm_scan_thread+0x0/0x1130
 [<ffffffff8109508b>] kthread+0x8b/0xb0
 [<ffffffff8100c0ea>] child_rip+0xa/0x20
 [<ffffffff8100b910>] ? restore_args+0x0/0x30
 [<ffffffff81095000>] ? kthread+0x0/0xb0
 [<ffffffff8100c0e0>] ? child_rip+0x0/0x20
Code: 01 75 10 e8 d3 f7 ff ff 5d c3 90 0f 0b eb fe 0f 1f 40 00 e8 73 f6 ff ff 5d c3 e8 4c 77 01 00 5d c3 66 2e 0f 1f 84 00 00 00 00 00 <8b> 47 48 85 c0 0f 8e c5 00 00 00 55 48 89 e5 41 56 41 55 41 54
RIP  [<ffffffff81138300>] drop_anon_vma+0x0/0xe0
 RSP <ffff88023b94bd28>
---[ end trace b1f69fd4c12ce1ce ]---

rmap_item->anon_vma was set to the RDI value 0x80000002192c6067.

   0xffffffff81138300 <+0>:     mov    0x48(%rdi),%eax
   0xffffffff81138303 <+3>:     test   %eax,%eax
   0xffffffff81138305 <+5>:     jle    0xffffffff811383d0       <drop_anon_vma+208>
   0xffffffff8113830b <+11>:    push   %rbp

Other oopses are more random and harder to debug side effects of
memory corruption. In this case the anon_vma was a dangling pointer
because when try_to_merge_with_ksm_page did rmap_item->anon_vma =
vma->anon_vma, the vma already was already freed and reused memory. At
other times the oopses materialize with an vma->anon_vma pointer that
looks legit but it points to an already freed and reused anon_vma.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 mm/ksm.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 51 insertions(+), 4 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 7ee101e..8fc6793 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -350,6 +350,24 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
 }
 
 /*
+ * If the mm isn't the one associated with the current
+ * ksm_scan.mm_slot ksm_exit() will not down_write();up_write() and in
+ * turn the ksm_test_exit() check run inside a mm->mmap_sem critical
+ * section, will not prevent exit_mmap() to run from under us. In
+ * turn, in those cases where we could work with an "mm" that isn't
+ * guaranteed to be associated with the current ksm_scan.mm_slot,
+ * ksm_get_mm() is needed instead of the ksm_test_exit() run inside
+ * the mmap_sem. Return true if the mm_users was incremented or false
+ * if it we failed at taking the mm because it was freed from under
+ * us. If it returns 1, the caller must take care of calling mmput()
+ * after it finishes using the mm.
+ */
+static __always_inline bool ksm_get_mm(struct mm_struct *mm)
+{
+	return likely(atomic_inc_not_zero(&mm->mm_users));
+}
+
+/*
  * We use break_ksm to break COW on a ksm page: it's a stripped down
  *
  *	if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1)
@@ -412,8 +430,6 @@ static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
 		unsigned long addr)
 {
 	struct vm_area_struct *vma;
-	if (ksm_test_exit(mm))
-		return NULL;
 	vma = find_vma(mm, addr);
 	if (!vma || vma->vm_start > addr)
 		return NULL;
@@ -434,11 +450,21 @@ static void break_cow(struct rmap_item *rmap_item)
 	 */
 	put_anon_vma(rmap_item->anon_vma);
 
+	/*
+	 * The "mm" of the unstable tree rmap_item isn't necessairly
+	 * associated with the current ksm_scan.mm_slot, it could be
+	 * any random mm. So we need ksm_get_mm here to prevent the
+	 * exit_mmap to run from under us in mmput().
+	 */
+	if (!ksm_get_mm(mm))
+		return;
+
 	down_read(&mm->mmap_sem);
 	vma = find_mergeable_vma(mm, addr);
 	if (vma)
 		break_ksm(vma, addr);
 	up_read(&mm->mmap_sem);
+	mmput(mm);
 }
 
 static struct page *page_trans_compound_anon(struct page *page)
@@ -462,6 +488,15 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item)
 	struct vm_area_struct *vma;
 	struct page *page;
 
+	/*
+	 * The "mm" of the unstable tree rmap_item isn't necessairly
+	 * associated with the current ksm_scan.mm_slot, it could be
+	 * any random mm. So we need ksm_get_mm here to prevent the
+	 * exit_mmap to run from under us in mmput().
+	 */
+	if (!ksm_get_mm(mm))
+		return NULL;
+
 	down_read(&mm->mmap_sem);
 	vma = find_mergeable_vma(mm, addr);
 	if (!vma)
@@ -478,6 +513,7 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item)
 out:		page = NULL;
 	}
 	up_read(&mm->mmap_sem);
+	mmput(mm);
 	return page;
 }
 
@@ -1086,9 +1122,19 @@ static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
 	struct vm_area_struct *vma;
 	int err = -EFAULT;
 
+	/*
+	 * The "mm" of the unstable tree rmap_item isn't necessairly
+	 * associated with the current ksm_scan.mm_slot, it could be
+	 * any random mm. So we need ksm_get_mm() here to prevent the
+	 * exit_mmap to run from under us in mmput(). Otherwise
+	 * rmap_item->anon_vma could point to an anon_vma that has
+	 * already been freed (i.e. get_anon_vma() below would run too
+	 * late).
+	 */
+	if (!ksm_get_mm(mm))
+		return err;
+
 	down_read(&mm->mmap_sem);
-	if (ksm_test_exit(mm))
-		goto out;
 	vma = find_vma(mm, rmap_item->address);
 	if (!vma || vma->vm_start > rmap_item->address)
 		goto out;
@@ -1105,6 +1151,7 @@ static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
 	get_anon_vma(vma->anon_vma);
 out:
 	up_read(&mm->mmap_sem);
+	mmput(mm);
 	return err;
 }
 

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