From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3EADE1E47AD; Tue, 12 Aug 2025 19:29:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755026944; cv=none; b=NTM29ONn48wqQal9NKyFxZjzqsduIuzJAmN+UMUY2Q26//Kngb7RK1p4s7RGJhO64d07X3aEfy8aNFkk+6Xn7bwug72B3S2R2hgB5GkrAXsPAA2D+dfIE89jw7MN2yr+JdSSSUldn18SwgBvpIFuaErxWWH0qvqIiI+AB61cvAg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755026944; c=relaxed/simple; bh=3ZJo11Aw3tj/49Z1pjljWQPFmfIk4RTToaiXfl+frlE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=K8A/fjQ+s4Q0LC+yjDwV++vyq8uc9RbZ8xJP/SJ61n7fc90m+rY1jg8jWDXUlbl7smc/1v+KK5CR5vX+Ug69Zs+yqiMOW8T5J9ytnj/KphsLLUMEESPk/Txbx/uC3OHZmKcKhMdJyzjDtGA89DV+zF6JDH29dQi8YlwuWt7b1rc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=FXuco9rC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="FXuco9rC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BB17AC4CEF0; Tue, 12 Aug 2025 19:29:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1755026944; bh=3ZJo11Aw3tj/49Z1pjljWQPFmfIk4RTToaiXfl+frlE=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=FXuco9rC+oj6uA4UmN59tEbhRe9hrjw0xrx24ZaOertA0GXr5Pegz/rNUN7ZGOP5N 8uaPfNOoxQ2qzt+vYXY0Hd9EDy4h571WsxRNaLLKWaMLzdz8C+YuTjKpvGjxcGWCtW tPhRuY1Wp1rLpvMni4xHA1JiX3Plcljjd1Zi8iiA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Suren Baghdasaryan , Jann Horn , Vlastimil Babka , Lorenzo Stoakes , Liam Howlett , Andrew Morton Subject: [PATCH 6.15 480/480] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped Date: Tue, 12 Aug 2025 19:51:28 +0200 Message-ID: <20250812174417.167901301@linuxfoundation.org> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20250812174357.281828096@linuxfoundation.org> References: <20250812174357.281828096@linuxfoundation.org> User-Agent: quilt/0.68 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Suren Baghdasaryan commit 9bbffee67ffd16360179327b57f3b1245579ef08 upstream. 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. [surenb@google.com: v3] Link: https://lkml.kernel.org/r/20250729145709.2731370-1-surenb@google.com Link: https://lkml.kernel.org/r/20250728175355.2282375-1-surenb@google.com Fixes: 3104138517fc ("mm: make vma cache SLAB_TYPESAFE_BY_RCU") Signed-off-by: Suren Baghdasaryan Reported-by: Jann Horn Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/ Reviewed-by: Vlastimil Babka Acked-by: Lorenzo Stoakes Cc: Jann Horn Cc: Liam Howlett Cc: Signed-off-by: Andrew Morton Signed-off-by: Suren Baghdasaryan --- include/linux/mm.h | 30 ++++++++++++++++++++++++++++++ mm/memory.c | 3 +-- 2 files changed, 31 insertions(+), 2 deletions(-) --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -33,6 +33,7 @@ #include #include #include +#include struct mempolicy; struct anon_vma; @@ -716,6 +717,10 @@ static inline void vma_refcount_put(stru * reused and attached to a different mm before we lock it. * Returns the vma on success, NULL on failure to lock and EAGAIN if vma got * detached. + * + * WARNING! The vma passed to this function cannot be used if the function + * fails to lock it because in certain cases RCU lock is dropped and then + * reacquired. Once RCU lock is dropped the vma can be concurently freed. */ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma) @@ -745,6 +750,31 @@ static inline struct vm_area_struct *vma } 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. + * We reinstate the RCU read lock as the caller expects it to + * be held when this function returns even on error. + */ + 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 --- a/mm/memory.c +++ b/mm/memory.c @@ -6554,8 +6554,7 @@ retry: */ /* 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();