linux-kernel - [PATCH 1/2] mm: lock_vma_under_rcu() must check vma->anon

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20230726214103.3261108-3-jannh@google.com>
Date:   Wed, 26 Jul 2023 23:41:03 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linus Torvalds <torvalds@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 1/2] mm: lock_vma_under_rcu() must check vma->anon_vma under vma lock

lock_vma_under_rcu() tries to guarantee that __anon_vma_prepare() can't
be called in the VMA-locked page fault path by ensuring that
vma->anon_vma is set.
However, this check happens before the VMA is locked, which means a
concurrent move_vma() can concurrently call unlink_anon_vmas(), which
disassociates the VMA's anon_vma.

This means we can get UAF in the following scenario:

  THREAD 1                   THREAD 2
  ========                   ========
  <page fault>
    lock_vma_under_rcu()
      rcu_read_lock()
      mas_walk()
      check vma->anon_vma

                             mremap() syscall
                               move_vma()
                                vma_start_write()
                                 unlink_anon_vmas()
                             <syscall end>

    handle_mm_fault()
      __handle_mm_fault()
        handle_pte_fault()
          do_pte_missing()
            do_anonymous_page()
              anon_vma_prepare()
                __anon_vma_prepare()
                  find_mergeable_anon_vma()
                    mas_walk() [looks up VMA X]

                             munmap() syscall (deletes VMA X)

                    reusable_anon_vma() [called on freed VMA X]

This is a security bug if you can hit it, although an attacker would
have to win two races at once where the first race window is only a
few instructions wide.

This patch is based on some previous discussion with Linus Torvalds on
the security list.

Cc: stable@...r.kernel.org
Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control it")
Signed-off-by: Jann Horn <jannh@...gle.com>
---
 mm/memory.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 01f39e8144ef..603b2f419948 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5393,27 +5393,28 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma_is_anonymous(vma) && !vma_is_tcp(vma))
 		goto inval;
 
-	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
-	if (!vma->anon_vma && !vma_is_tcp(vma))
-		goto inval;
-
 	if (!vma_start_read(vma))
 		goto inval;
 
+	/*
+	 * find_mergeable_anon_vma uses adjacent vmas which are not locked.
+	 * This check must happen after vma_start_read(); otherwise, a
+	 * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
+	 * from its anon_vma.
+	 */
+	if (unlikely(!vma->anon_vma && !vma_is_tcp(vma)))
+		goto inval_end_read;
+
 	/*
 	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
 	 * it for now and fall back to page fault handling under mmap_lock.
 	 */
-	if (userfaultfd_armed(vma)) {
-		vma_end_read(vma);
-		goto inval;
-	}
+	if (userfaultfd_armed(vma))
+		goto inval_end_read;
 
 	/* Check since vm_start/vm_end might change before we lock the VMA */
-	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
-		vma_end_read(vma);
-		goto inval;
-	}
+	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
+		goto inval_end_read;
 
 	/* Check if the VMA got isolated after we found it */
 	if (vma->detached) {
@@ -5425,6 +5426,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 
 	rcu_read_unlock();
 	return vma;
+
+inval_end_read:
+	vma_end_read(vma);
 inval:
 	rcu_read_unlock();
 	count_vm_vma_lock_event(VMA_LOCK_ABORT);
-- 
2.41.0.487.g6d72f3e995-goog