[PATCH v4 3/3] mm: fix use-after-free when anon vma name is used after vma is freed

[Suren Baghdasaryan <surenb@google.com>]

When adjacent vmas are being merged it can result in the vma that was
originally passed to madvise_update_vma being destroyed.  In the current
implementation, the name parameter passed to madvise_update_vma points
directly to vma->anon_name->name and it is used after the call to
vma_merge.  In the cases when vma_merge merges the original vma and
destroys it, this will result in use-after-free bug as shown below:

madvise_vma_behavior << passes vma->anon_name->name as name param
  madvise_update_vma(name)
    vma_merge
      __vma_adjust
        vm_area_free <-- frees the vma
    replace_vma_anon_name(name) <-- UAF

Fix this by raising the name refcount and stabilizing it.

Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: syzbot+aa7b3d4b35f9dc46a366@syzkaller.appspotmail.com
---
changes in v3:
- Reapplied the fix after code refactoring, per Michal Hocko

 mm/madvise.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index a395884aeecb..00e8105430e9 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -140,6 +140,8 @@ static int replace_vma_anon_name(struct vm_area_struct *vma,
 /*
  * Update the vm_flags on region of a vma, splitting it or merging it as
  * necessary.  Must be called with mmap_sem held for writing;
+ * Caller should ensure anon_name stability by raising its refcount even when
+ * anon_name belongs to a valid vma because this function might free that vma.
  */
 static int madvise_update_vma(struct vm_area_struct *vma,
 			      struct vm_area_struct **prev, unsigned long start,
@@ -1021,8 +1023,10 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
 	}
 
 	anon_name = vma_anon_name(vma);
+	anon_vma_name_get(anon_name);
 	error = madvise_update_vma(vma, prev, start, end, new_flags,
 				   anon_name);
+	anon_vma_name_put(anon_name);
 
 out:
 	/*
-- 
2.35.1.473.g83b2b277ed-goog