[PATCH v6 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 and it is used after the call to vma_merge.
In the cases when vma_merge merges the original vma and destroys it,
this might result in UAF. For that the original vma would have to hold
the anon_vma_name with the last reference. The following vma would need
to contain a different anon_vma_name object with the same string. Such
scenario is shown below:

madvise_vma_behavior(vma)
  madvise_update_vma(vma, ..., anon_name == vma->anon_name)
    vma_merge(vma)
      __vma_adjust(vma) <-- merges vma with adjacent one
        vm_area_free(vma) <-- frees the original vma
    replace_vma_anon_name(anon_name) <-- UAF of vma->anon_name

Fix this by raising the name refcount and stabilizing it.

Link: https://lkml.kernel.org/r/20220223153613.835563-3-surenb@google.com
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
Acked-by: Michal Hocko <mhocko@suse.com>
---
changes in v6:
- Updated description clarifying the last reference requirement,
per Michal Hocko
- Added Ack, per Michal Hocko

 mm/madvise.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 1f2693dccf7b..38d0f515d548 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -131,6 +131,8 @@ static int replace_anon_vma_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,
@@ -945,6 +947,7 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
 				unsigned long behavior)
 {
 	int error;
+	struct anon_vma_name *anon_name;
 	unsigned long new_flags = vma->vm_flags;
 
 	switch (behavior) {
@@ -1010,8 +1013,11 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
 		break;
 	}
 
+	anon_name = anon_vma_name(vma);
+	anon_vma_name_get(anon_name);
 	error = madvise_update_vma(vma, prev, start, end, new_flags,
-				   anon_vma_name(vma));
+				   anon_name);
+	anon_vma_name_put(anon_name);
 
 out:
 	/*
-- 
2.35.1.574.g5d30c73bfb-goog