[PATCH 1/1] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped

[Suren Baghdasaryan <surenb@google.com>]

By inducing delays in the right places, Jann Horn created a reproducer
for a hard to hit UAF issue that became possible after VMAs were allowed
to be recycled by adding SLAB_TYPESAFE_BY_RCU to their cache.

Race description is borrowed from Jann's discovery report:
lock_vma_under_rcu() looks up a VMA locklessly with mas_walk() under
rcu_read_lock(). At that point, the VMA may be concurrently freed, and
it can be recycled by another process. vma_start_read() then
increments the vma->vm_refcnt (if it is in an acceptable range), and
if this succeeds, vma_start_read() can return a recycled VMA.

In this scenario where the VMA has been recycled, lock_vma_under_rcu()
will then detect the mismatching ->vm_mm pointer and drop the VMA
through vma_end_read(), which calls vma_refcount_put().
vma_refcount_put() drops the refcount and then calls rcuwait_wake_up()
using a copy of vma->vm_mm. This is wrong: It implicitly assumes that
the caller is keeping the VMA's mm alive, but in this scenario the caller
has no relation to the VMA's mm, so the rcuwait_wake_up() can cause UAF.

The diagram depicting the race:
T1         T2         T3
==         ==         ==
lock_vma_under_rcu
  mas_walk
          <VMA gets removed from mm>
                      mmap
                        <the same VMA is reallocated>
  vma_start_read
    __refcount_inc_not_zero_limited_acquire
                      munmap
                        __vma_enter_locked
                          refcount_add_not_zero
  vma_end_read
    vma_refcount_put
      __refcount_dec_and_test
                          rcuwait_wait_event
                            <finish operation>
      rcuwait_wake_up [UAF]

Note that rcuwait_wait_event() in T3 does not block because refcount
was already dropped by T1. At this point T3 can exit and free the mm
causing UAF in T1.
To avoid this we move vma->vm_mm verification into vma_start_read() and
grab vma->vm_mm to stabilize it before vma_refcount_put() operation.

Fixes: 3104138517fc ("mm: make vma cache SLAB_TYPESAFE_BY_RCU")
Reported-by: Jann Horn <jannh@google.com>
Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: <stable@vger.kernel.org>
---
- Applies cleanly over mm-unstable.
- Should be applied to 6.15 and 6.16 but these branches do not
have lock_next_vma() function, so the change in lock_next_vma() should be
skipped when applying to those branches.

 include/linux/mmap_lock.h | 21 +++++++++++++++++++++
 mm/mmap_lock.c            | 10 +++-------
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 1f4f44951abe..4ee4ab835c41 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -12,6 +12,7 @@ extern int rcuwait_wake_up(struct rcuwait *w);
 #include <linux/tracepoint-defs.h>
 #include <linux/types.h>
 #include <linux/cleanup.h>
+#include <linux/sched/mm.h>
 
 #define MMAP_LOCK_INITIALIZER(name) \
 	.mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock),
@@ -183,6 +184,26 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
 	}
 
 	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
+
+	/*
+	 * If vma got attached to another mm from under us, that mm is not
+	 * stable and can be freed in the narrow window after vma->vm_refcnt
+	 * is dropped and before rcuwait_wake_up(mm) is called. Grab it before
+	 * releasing vma->vm_refcnt.
+	 */
+	if (unlikely(vma->vm_mm != mm)) {
+		/*
+		 * __mmdrop() is a heavy operation and we don't need RCU
+		 * protection here. Release RCU lock during these operations.
+		 */
+		rcu_read_unlock();
+		mmgrab(vma->vm_mm);
+		vma_refcount_put(vma);
+		mmdrop(vma->vm_mm);
+		rcu_read_lock();
+		return NULL;
+	}
+
 	/*
 	 * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
 	 * False unlocked result is impossible because we modify and check
diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 729fb7d0dd59..aa3bc42ecde0 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -164,8 +164,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	 */
 
 	/* Check if the vma we locked is the right one. */
-	if (unlikely(vma->vm_mm != mm ||
-		     address < vma->vm_start || address >= vma->vm_end))
+	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
 		goto inval_end_read;
 
 	rcu_read_unlock();
@@ -236,11 +235,8 @@ struct vm_area_struct *lock_next_vma(struct mm_struct *mm,
 		goto fallback;
 	}
 
-	/*
-	 * Verify the vma we locked belongs to the same address space and it's
-	 * not behind of the last search position.
-	 */
-	if (unlikely(vma->vm_mm != mm || from_addr >= vma->vm_end))
+	/* Verify the vma is not behind of the last search position. */
+	if (unlikely(from_addr >= vma->vm_end))
 		goto fallback_unlock;
 
 	/*

base-commit: c617a4dd7102e691fa0fb2bc4f6b369e37d7f509
-- 
2.50.1.487.gc89ff58d15-goog