From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f73.google.com (mail-pj1-f73.google.com [209.85.216.73]) (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 CD4621DE4D2 for ; Thu, 31 Jul 2025 15:19:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753975167; cv=none; b=uKjLxmcK1E8GHrcFmJ9zsssu+nBXj4jGnIOs19qHgCVSQ6BZ8L9ySN+KOoD0tmPMITRQ1AQFOHg5dX5AVduN7wm9ro9W9204FFLKgPPqI3ZWt3C7dX6vA0Ol+G69BpLCBTI1R3CDWccSAcJ39q8pu+EOesUULBwQP8b0LTo/1XM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753975167; c=relaxed/simple; bh=QPAZ78hFT+nhefwTtIn8MUOVzgEXOS7hUjuUKewsH0s=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=B0rA8QJKBj46eG4Ynv8JTQM3WMERrB3UpckZ7M1gQeeVnwEW0Ikp/0l3ZwvO71iVYukGpozZk/GQVDDD+OpDgqePQzKce4RuGcwiLP1hUi9qT2g+1dOdk2Bfzv4o21N5GV4vOE5KLjOt2X24AFNbCVwMdfjbeXWLdyhlbDvVRyc= 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=dAYhqvdW; arc=none smtp.client-ip=209.85.216.73 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="dAYhqvdW" Received: by mail-pj1-f73.google.com with SMTP id 98e67ed59e1d1-31f030b1cb9so1685100a91.2 for ; Thu, 31 Jul 2025 08:19:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1753975165; x=1754579965; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=tRIt8AKcHnKINXyvEkxYf94Yh7IhkF311FvqvgzwnyM=; b=dAYhqvdWF7bRqA9zV9mbbJJZFTzh8POlcwsJ4kCvi1WEFyH2BQ/isWwYlnwkoufeXx Qfr2uGhdENbo3dsuyMRmwZanNgkSpGhEjDBtvxKNkQqLbG66ZsTsXRxZ5R7RnVujROQr gbtupOq3relYpyfamfLeSnTCbYUpn4zqPSU/qTYMNKhuErI5k3gB83p6w6wlBGy43Pax ZDqHeiA4Ij1HJgrWTIK8TZh1htn0RNZ+kgal0c6z4DjbQLCsstps5EblAaZJKPRaLeLO G/5EonCL3wzAcVo9QBe3YRGx1efOO+W9TlbCbxJkUEj4tYqcNNU9T8lMXSe569tzNKug 4meQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1753975165; x=1754579965; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=tRIt8AKcHnKINXyvEkxYf94Yh7IhkF311FvqvgzwnyM=; b=fFJCxsRJiKKYYRHDQEWfueP0A7bmbRGPXdcpGMxv/9taXA0Lrm5iXT7jctICWpAMtz amK4sA5vPPxG16TTig98lcM8GdHXaDmSXUb36Gese4h175L7bOBm4hRFg5vEgYlolNqJ 6rTXb34wjXxNnTXIbC4EJUXAwdqLEkMFwgUnmitwFdpC04AmStiuimW27E+/W4QxP2Dn H7vgG4K5Ev0bjMGJ4DYiHk5yYCFNQ2vyXAqcfkSOKC2oqLo12aBqTKt58ksKTVlwhLTn oteONa9NVIxLRLTKzWdMlZATUKYLgrQCuKnNK03Q7v7NVsJI3x1pr607DgOwHq/vuZRW Tt8g== X-Forwarded-Encrypted: i=1; AJvYcCXoxkfWZRcG40BMBGbrHT02p5OzF+LBAtRzIV3z2ZoWX380SG2Trpcx7NdzvxvcHkr9aD7uX+LfSTquF3Q=@vger.kernel.org X-Gm-Message-State: AOJu0Yy4Gxe2/mp/Fi6U7tlBCsIMYVRL76GFbg3xk9+bJEpD7SGiAhUY G3Iif4Rvn/7M3KsPU2gZ6ukcSIo8/O9AzWGzEqowBmnibRadd9dRecXObcgpUHpxvl0Kk2PDot6 V3gHVhg== X-Google-Smtp-Source: AGHT+IG+hqz8k3yQGzHNMFz+PXNcZKFUGe9Z5W3kyHUNP/PcUJ2Ofl9W0o8GvY+qmKvXki2uaR6Oh1vSMeI= X-Received: from pjk16.prod.google.com ([2002:a17:90b:5590:b0:312:151d:c818]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:2888:b0:312:959:dc3f with SMTP id 98e67ed59e1d1-31f5dd8ca12mr10609509a91.3.1753975165105; Thu, 31 Jul 2025 08:19:25 -0700 (PDT) Date: Thu, 31 Jul 2025 08:19:19 -0700 In-Reply-To: <20250731151919.212829-1-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250731151919.212829-1-surenb@google.com> X-Mailer: git-send-email 2.50.1.552.g942d659e1b-goog Message-ID: <20250731151919.212829-2-surenb@google.com> Subject: [PATCH v2 2/2] mm: change vma_start_read() to drop RCU lock on failure 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 Content-Type: text/plain; charset="UTF-8" vma_start_read() can drop and reacquire RCU lock in certain failure cases. It's not apparent that the RCU session started by the caller of this function might be interrupted when vma_start_read() fails to lock the vma. This might become a source of subtle bugs and to prevent that we change the locking rules for vma_start_read() to drop RCU read lock upon failure. This way it's more obvious that RCU-protected objects are unsafe after vma locking fails. Suggested-by: Vlastimil Babka Signed-off-by: Suren Baghdasaryan --- Changes since v1 [1]: - Fixed missing RCU unlock in lock_vma_under_rcu(), per Lorenzo Stoakes - Modified comments, per Lorenzo Stoakes [1] https://lore.kernel.org/all/20250731013405.4066346-2-surenb@google.com/ mm/mmap_lock.c | 86 +++++++++++++++++++++++++++----------------------- 1 file changed, 47 insertions(+), 39 deletions(-) diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c index 10826f347a9f..7ea603f26975 100644 --- a/mm/mmap_lock.c +++ b/mm/mmap_lock.c @@ -136,15 +136,16 @@ void vma_mark_detached(struct vm_area_struct *vma) * 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. + * IMPORTANT: RCU lock must be held upon entering the function, but upon error + * IT IS RELEASED. The caller must handle this correctly. */ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma) { + struct mm_struct *other_mm; int oldcnt; + RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "no rcu lock held"); /* * Check before locking. A race might cause false locked result. * We can use READ_ONCE() for the mm_lock_seq here, and don't need @@ -152,8 +153,10 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, * we don't rely on for anything - the mm_lock_seq read against which we * need ordering is below. */ - if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence)) - return NULL; + if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence)) { + vma = NULL; + goto err; + } /* * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited_acquire() @@ -164,34 +167,14 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, if (unlikely(!__refcount_inc_not_zero_limited_acquire(&vma->vm_refcnt, &oldcnt, VMA_REF_LIMIT))) { /* return EAGAIN if vma got detached from under us */ - return oldcnt ? NULL : ERR_PTR(-EAGAIN); + vma = oldcnt ? NULL : ERR_PTR(-EAGAIN); + goto err; } 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; - } + if (unlikely(vma->vm_mm != mm)) + goto err_unstable; /* * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result. @@ -206,10 +189,31 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, */ if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) { vma_refcount_put(vma); - return NULL; + vma = NULL; + goto err; } return vma; +err: + rcu_read_unlock(); + + return vma; +err_unstable: + /* + * 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. + */ + other_mm = vma->vm_mm; /* use a copy as vma can be freed after we drop vm_refcnt */ + + /* __mmdrop() is a heavy operation, do it after dropping RCU lock. */ + rcu_read_unlock(); + mmgrab(other_mm); + vma_refcount_put(vma); + mmdrop(other_mm); + + return NULL; } /* @@ -223,11 +227,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, MA_STATE(mas, &mm->mm_mt, address, address); struct vm_area_struct *vma; - rcu_read_lock(); retry: + rcu_read_lock(); vma = mas_walk(&mas); - if (!vma) + if (!vma) { + rcu_read_unlock(); goto inval; + } vma = vma_start_read(mm, vma); if (IS_ERR_OR_NULL(vma)) { @@ -241,6 +247,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, /* Failed to lock the VMA */ goto inval; } + + rcu_read_unlock(); + /* * At this point, we have a stable reference to a VMA: The VMA is * locked and we know it hasn't already been isolated. @@ -249,16 +258,14 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, */ /* Check if the vma we locked is the right one. */ - if (unlikely(address < vma->vm_start || address >= vma->vm_end)) - goto inval_end_read; + if (unlikely(address < vma->vm_start || address >= vma->vm_end)) { + vma_end_read(vma); + goto inval; + } - rcu_read_unlock(); return vma; -inval_end_read: - vma_end_read(vma); inval: - rcu_read_unlock(); count_vm_vma_lock_event(VMA_LOCK_ABORT); return NULL; } @@ -313,6 +320,7 @@ struct vm_area_struct *lock_next_vma(struct mm_struct *mm, */ if (PTR_ERR(vma) == -EAGAIN) { /* reset to search from the last address */ + rcu_read_lock(); vma_iter_set(vmi, from_addr); goto retry; } @@ -342,9 +350,9 @@ struct vm_area_struct *lock_next_vma(struct mm_struct *mm, return vma; fallback_unlock: + rcu_read_unlock(); vma_end_read(vma); fallback: - rcu_read_unlock(); vma = lock_next_vma_under_mmap_lock(mm, vmi, from_addr); rcu_read_lock(); /* Reinitialize the iterator after re-entering rcu read section */ -- 2.50.1.552.g942d659e1b-goog