From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f74.google.com (mail-pj1-f74.google.com [209.85.216.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D43B42877E9 for ; Mon, 28 Jul 2025 17:53:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753725242; cv=none; b=Uo4Cr1NU5kfMxo967DJ4Q+Qa3Bi8RQVfV/idBaS3JmpW2GsLY/UTg0SV7zh3YuhvdvNL9a7WlS/nvH3r8lgBwbwPgtP+OzKqj013zFCK4FAaH4v7mSXvUbcCnQ5/bG4NMEbrpc4LLhrW+dMKqVykCcsSypGcsAHLIpmIswhgqn8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753725242; c=relaxed/simple; bh=2Cwe+MGmj/I+AL0nZ7h4AzKfFmp+A2BonqkMwz7EQOk=; h=Date:Mime-Version:Message-ID:Subject:From:To:Cc:Content-Type; b=tUgrpeoLjbuYS9TNlVH6+RtZub/XRcKgFOcOpjx3giEfzS+1MU2yg7Uz/24BXPu+NP0CTnJmpA8/frN7CoUfKhUpeUktLctMtAS72oQJkg1IUIW7veZpGJS0L2a4fr1rdNREKv1jlPaVvwlCujREJiisz/L49x35TeEj07H7Cnw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=rrovw2XU; arc=none smtp.client-ip=209.85.216.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="rrovw2XU" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-313d6d671ffso4833333a91.2 for ; Mon, 28 Jul 2025 10:53:59 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1753725239; x=1754330039; darn=vger.kernel.org; h=cc:to:from:subject:message-id:mime-version:date:from:to:cc:subject :date:message-id:reply-to; bh=CsmkSz7oeq7HMJDzlu8bMB8qretQ/dwlis7q/g4uBKU=; b=rrovw2XU5Auhqrq09h3Nw+Tb4+Mp0u3DgCBONswcAKyqPg6VocrpRvqesGIrlqHBAD hrHyeb03grMfnoYKrpU0NZl+IPU+9S52yDBXy+0XsdfSPaTBC1++QSEDOzGiT9zv2emP vQpogbWvhHhNyYVdnVhu841s2nDA5NFyaxVkefB7PuxUbiDVZKBtw0SwMugiXXN5dNMW QROJUrtaKdgmqWp6tU6FFbqHETe9ZDhmU0lR5drIEy/TnM9B6PsD5dF3HrUx+XchdDOa iwUmNB/TU8o7lhtvqhH1ngJF0QdbuKuTIkm+AnDNXT6lI0ldp2VwSC180a5Pdfp82b51 pAsg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1753725239; x=1754330039; h=cc:to:from:subject:message-id:mime-version:date:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=CsmkSz7oeq7HMJDzlu8bMB8qretQ/dwlis7q/g4uBKU=; b=wnza985UghPCU5K+jR04rVLSDLixDgqVWRrdWJW7fFqwUIFQ0kdKyIBcE0DTVHbxQA Uv2lqeyvRjDB2ShBK2UiTXh+ulH9r6oUyhdbh5V2tiP3fl5jqZvkVcIcAvyWADaW76hx wsBgE8XyKNDzwQ1bf1KEEDJHkqrmA0oH5+8YloOBB5nSXgtpvgR2iKO/RfKpj+smlfYY wYRyMrZ/mBBFP9R40lVzi/Ott5BdJA+rrLJoTwugOexolZnazJ0c0LHx9SKZWnUu4dAh AJOqA36VluZH3ZIw9+JqvJxd5GMX3OgRvLh5ADAwON8/AD4/CCDlCiQEJMzoB7J3Syum lDpQ== X-Forwarded-Encrypted: i=1; AJvYcCWEU4mIhivrw9J71VFPpL9gjeTAGLqudauNno4L+WcmApCGLGwy5PGljAb0veP1H+K85sh20pg=@vger.kernel.org X-Gm-Message-State: AOJu0YwX2AjtyjIwCLLqVleLPQLSI42Y0/STX0TTQ2uC+HsBou8hTY9G P0wf1/STErGYD3ULTA0XPiEdG1yRhr3eIpj7a7/SZVwSjS6+obH7r/1dYMH2DD8FdbbnMfOUQDq 7jWY8Ug== X-Google-Smtp-Source: AGHT+IGWBAtmMVPRkSv2vbxg+lJ0GqdvX81gg47Uv9TyhPMSsOBhXdi9Q/f4gBG5YR7+TJysH+MScLJ/WmU= X-Received: from pjtu8.prod.google.com ([2002:a17:90a:c888:b0:31c:2fe4:33b7]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:514c:b0:31c:c434:dec8 with SMTP id 98e67ed59e1d1-31e77af1e9amr18091051a91.20.1753725238711; Mon, 28 Jul 2025 10:53:58 -0700 (PDT) Date: Mon, 28 Jul 2025 10:53:55 -0700 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 X-Mailer: git-send-email 2.50.1.487.gc89ff58d15-goog Message-ID: <20250728175355.2282375-1-surenb@google.com> Subject: [PATCH v2 1/1] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped From: Suren Baghdasaryan To: akpm@linux-foundation.org Cc: jannh@google.com, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, vbabka@suse.cz, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, surenb@google.com, stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" 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 mmap 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 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 Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/ Signed-off-by: Suren Baghdasaryan Cc: --- Changes since v1 [1] - Made a copy of vma->mm before using it in vma_start_read(), per Vlastimil Babka Notes: - 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. [1] https://lore.kernel.org/all/20250728170950.2216966-1-surenb@google.com/ include/linux/mmap_lock.h | 23 +++++++++++++++++++++++ mm/mmap_lock.c | 10 +++------- 2 files changed, 26 insertions(+), 7 deletions(-) diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index 1f4f44951abe..da34afa2f8ef 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 #include #include +#include #define MMAP_LOCK_INITIALIZER(name) \ .mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock), @@ -183,6 +184,28 @@ 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)) { + /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */ + struct mm_struct *other_mm = vma->vm_mm; + /* + * __mmdrop() is a heavy operation and we don't need RCU + * protection here. Release RCU lock during these operations. + */ + rcu_read_unlock(); + mmgrab(other_mm); + vma_refcount_put(vma); + mmdrop(other_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