From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=209.85.218.51 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794563701; b=KJLe0UYkSg06QXQEaPEocrnXT013aFK3iqAM+cb+uyoHpi4QNpVNJ+CJayR91d2IN8nr sS7IZyapdnHTa1To7xDjKEPvQQYZXYMYoLKvbtlCs/zAxUaTuBfiSrIMT6g20F4wnMK3f E4XiEJDcxGrNK1ju2DqZw2FZ6aL8oAirQLovTZzsct4a8L/75t1noYIBFloIfWxEg4r78 P88M4fkceuKkjcg1ypdKBmciqe+lRNCtv0rM7VkEl7Ly8io9SisvNlrey3RXK1VDT2PWb OIgVtlKrKQNmgnytVC8nBS2jBNkqH4bCr2UYKEFeyK9nXKbOUNPcqi48dudTGvvxTEw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794563701; h=DMARC-Filter:Received:DKIM-Signature:X-Google-DKIM-Signature: X-Forwarded-Encrypted:X-Gm-Message-State:X-Gm-Gg:X-Received:Received: Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:Content-Transfer-Encoding:User-Agent:MIME-Version; bh=G8DmFU4pJwAPCJlqHBF8SKasEEThxtJNlC1sg/CSkLw=; b=mI/znB0jxi0jcCaDS9lb37rowOAGUu+eE5lLtBG3IyyL6/9I1JYUvE/fMYc9XjeD7FU5 VPcqTd/biwtQxpT9AG8nJwy3YLKoQAKr9hHhQWjkGZoG87la/KbALo1pUU2XfMbB7cfnP iB7LUmcZDceOXmH5sfA7EWnCQ6q62Yd3zmtuDXTmpjK0x/sTr6i/Hq1SK/XObWDAw9Ew4 JOPPyhtETaC34sazrhEuZneR5Y3hTOT5rkwOF3lkzr4ZXGmHpxme4p1xgO3LD13wAN9YV FVvBQ38Fh/Z5eec6gAgcOExFhNTsouQatKi1pVHnmZ9C60L/CUIxsnH3gVeJZCc5REA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none header.from=amutable.com; spf=pass smtp.mailfrom=amutable.com; dkim=pass header.d=amutable-com.20260707.gappssmtp.com header.i=@amutable-com.20260707.gappssmtp.com header.a=rsa-sha256 header.s=20260707 header.b="Pp/iLDW+"; arc=none smtp.remote-ip=209.85.218.51 Received: from mail-ej1-f51.google.com (mail-ej1-f51.google.com [209.85.218.51]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3D8B860A2602 for ; Fri, 13 Nov 2026 09:55:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 3D8B860A2602 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=amutable.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=amutable.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=amutable-com.20260707.gappssmtp.com header.i=@amutable-com.20260707.gappssmtp.com header.a=rsa-sha256 header.s=20260707 header.b=Pp/iLDW+ Received: by mail-ej1-f51.google.com with SMTP id a640c23a62f3a-c5083bc0e28so142296966b.0 for ; Fri, 13 Nov 2026 01:55:00 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amutable-com.20260707.gappssmtp.com; s=20260707; t=1794563698; x=1795168498; darn=lists.linux.dev; h=mime-version:user-agent:content-transfer-encoding:content-type :references:in-reply-to:date:cc:to:from:subject:message-id:from:to :cc:subject:date:message-id:reply-to:content-type; bh=G8DmFU4pJwAPCJlqHBF8SKasEEThxtJNlC1sg/CSkLw=; b=Pp/iLDW+KULLKBMDnLmVBWDK+QDaUtq+Niif6l1EqKwNsNYtPcwvnQTLzN0LEJUP6L PGJDotaWKmardus4LLZNBqKDLeC5C760DHoSHYCBjeu9GOQ9nu0DUeM1r3TgFlXbdPcy 2b4QgvYrhndUcOdRAv48ft5UlYrCX20BkFPnU8mdn/J4gpAqRelBDEhKm+G47Wdc21Um dB60lx8fBejOlfSgstrd6pVmDWmsrtkJd4EP1M/XlEbkcm3/IPBAsBPnQ62VuwbnBfxQ oxHGU4ZPSKpSsjROdfa5xGV+prmKLZ0Q3cGRMHeCeMEfFndorbVIsFzp5VGEsLT7vky3 ANiw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1794563698; x=1795168498; h=mime-version:user-agent:content-transfer-encoding:content-type :references:in-reply-to:date:cc:to:from:subject:message-id:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=G8DmFU4pJwAPCJlqHBF8SKasEEThxtJNlC1sg/CSkLw=; b=jfkA+2SCWr4AaXC8mgoL2WNFebzmt7PjtYfA7moGKNy9HK/+DbZp8UsRI819i7WQlQ YUEw0R7RndqnDid5MplsBcPrIAKFr4ANzJJ2W0226dXnir7VN917osM/NiRMCI0BVmrM y+dE9OyhncBZnpDyTwJVnG2o5QXqqr0BzKdN79kB1fOJNbG+TCk3lgPUmvr8rWVjjgIN ed6Zr++27XDr4w7Z5tnr6ecRYaasbFzCYRQhjoV7ret6+1wEPe3o0dytqpU4rEEt3K1g tfuvBuI2raTmlo5OOVQLCqArEcchYjRIpIx4rVaQDViplgg98jd3eRF6tZlwTj4/3KtG Uy2Q== X-Forwarded-Encrypted: i=1; AL+hErGyJlsbBxKqaK0aW848uwF8CfM1W9w53b9tAO5Z+ImeIl9mtEy4NPhWz2pEtFo3t85aJo+eIoTIzxj4eg==@lists.linux.dev X-Gm-Message-State: APuU4Z81Y9bmu2TiVXYo5XEHO6Nfkk7nTAqw6NPeR2zTb+kutpjqSctd 3zPsjdTpjZ9H/9V7JuJA0t8KfbAwaqcBfqw8Gk4PUy0rogXMSwkYl8veJvIAVPSJwKSU X-Gm-Gg: AYBFou0NWxzdYEa+Kw1sz5wMi3DbE8pNsqMOCfRXG5Rzin1STcWBB1omMsu7fxJH8ZG I/jPbL4H5Fil4XGIcgTOZmc4/zRVdpd669K3/zqvRL/nhX/zlwCZcXGYPzpgjJprFPuyWWbXg9C 7uQqNQMos1AXiMTynk7URssIbAHliEIfpIafN2uITHW3vL3XkIb2bFcXNpIiABpqBHDwaZZNcjc 8m9LvQGka07Vrd+QFJxAoVlxcq1G3Zo2VKH/rq3LVnI8BOTeFmYGyTvYOha1TsEbZBDrWoXW7in 5Aj39r0RKqBDamXOY/T5XIVUdXTRFreSDOeu4tICNcgw+wYvyBfBh5LfcYsTwhoYvYjl8zgVbu+ xQqiBMQi4zVmTp+/vGQGB8ujPWdv00+TvwZJGaETTrQPH17jcdQgoZHCyCrOLN6xqAIgnS9XjSJ sr2wH1rXFV3QTwFk93PefgSPm783H1NMp50bS/WcqAwPiEylql5uCkh4kYeRzWDWKj98eVOSng4 f3/+il1SEmnnEU8HelHR1QbK0ULV9ehRf6gesWq2nUdVlBFoyEdOxWaxcpndpHOPVGlmS4P32An DXckrWt7IwrcgEiWUyibLGFqMH1MIT6P1/g9DSXiLei9fCzrtKNxvdklIjYRwEM= X-Received: by 2002:a17:907:ea93:b0:c41:4c20:5a80 with SMTP id a640c23a62f3a-c50963bdb9bmr198299566b.44.1794563698514; Fri, 13 Nov 2026 01:54:58 -0800 (PST) Received: from [10.0.0.233] (tmo-086-54.customers.d1-online.com. [80.187.86.54]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c50957c6c1fsm75151566b.0.2026.11.13.01.54.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Nov 2026 01:54:58 -0800 (PST) Message-ID: Subject: Re: [PATCH RFC 01/41] loop: refuse backing files that aren't open for reading From: Alexander Mikhalitsyn To: Christian Brauner , linux-fsdevel@vger.kernel.org Cc: Jann Horn , Chris Mason , Josef Bacik , Christoph Hellwig , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, Alexander Viro , Jan Kara , linux-scsi@vger.kernel.org, target-devel@vger.kernel.org, driver-core@lists.linux.dev, linux-mm@kvack.org, bpf@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, apparmor@lists.ubuntu.com, linux-nfs@vger.kernel.org, netdev@vger.kernel.org, fuse-devel@lists.linux.dev, linux-efi@vger.kernel.org, xen-devel@lists.xenproject.org, ocfs2-devel@lists.linux.dev, linux-s390@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-usb@vger.kernel.org, Amir Goldstein , linux-erofs@lists.ozlabs.org, linux-kselftest@vger.kernel.org, stable@vger.kernel.org Date: Fri, 13 Nov 2026 10:54:56 +0100 In-Reply-To: <20261110-work-loop-pseudo-v1-1-4c998976c6c7@kernel.org> References: <20261110-work-loop-pseudo-v1-0-4c998976c6c7@kernel.org> <20261110-work-loop-pseudo-v1-1-4c998976c6c7@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2-0+deb13u1 X-Mailing-List: driver-core@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Tue, 2026-11-10 at 10:14 +0100, Christian Brauner wrote: > loop_configure() takes any file as long as it implements - > >read_iter(). > Loop itself never checked whether the file was actually opened for > reading. Buffered I/O went through vfs_iter_read() which refuses > files > without FMODE_READ. Direct I/O has called ->read_iter() of the > backing > file directly ever since loop grew support for it. >=20 > Since commit f2fed441c69b ("loop: stop using vfs_iter_{read,write} > for > buffered I/O") buffered I/O calls ->read_iter() directly as well. > That > commit was about O_DIRECT handling and the FMODE_READ check went away > as a side effect. >=20 > So whoever can set up a loop device can read a file it was only > allowed > to open for writing. Open the file O_WRONLY, configure a loop device > with it and read the contents through the loop device. >=20 > Refuse backing files that aren't open for reading. Loop always reads > from its backing file so a write-only backing file never made sense > in > the first place. >=20 > Fixes: bc07c10a3603 ("block: loop: support DIO & AIO") > Fixes: f2fed441c69b ("loop: stop using vfs_iter_{read,write} for > buffered I/O") > Cc: stable@vger.kernel.org > Signed-off-by: Christian Brauner (Amutable) Reviewed-by: Alexander Mikhalitsyn > --- > =C2=A0drivers/block/loop.c | 3 +++ > =C2=A01 file changed, 3 insertions(+) >=20 > diff --git a/drivers/block/loop.c b/drivers/block/loop.c > index a3325ceab484..c96e35c3ef42 100644 > --- a/drivers/block/loop.c > +++ b/drivers/block/loop.c > @@ -528,6 +528,9 @@ static void loop_assign_backing_file(struct > loop_device *lo, struct file *file) > =C2=A0 > =C2=A0static int loop_check_backing_file(struct file *file) > =C2=A0{ > + if (!(file->f_mode & FMODE_READ)) > + return -EBADF; > + > =C2=A0 if (!file->f_op->read_iter) > =C2=A0 return -EINVAL; > =C2=A0