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=1794564960; b=XQOsVG9wJqRSPSProuEDneyFQrbBpn5C7DREbluxBZZkHNnhabNM4l375RwzhfE5kKxP OPw1Wdgtrqjoe1z2ob69gHc5ouxKw6tD2QT0sRVRyGkmM23cjXo/BOGEi3yugHKxlzQOT 1xQS3sMBWaHJZaD0+xUle0makAr9uDTOcrodRc+ccoN1Ol4ISWrSPLL6Z8dxY9gCAfGan as5MqSk9p02QnMv5UNX85Hovmjp0dS2TDyOFFq0mIggfhiJGefgxSdO38+ryHMkbUd2z+ Xvuj7x8R+E4l5hvV+Q52xFcqCMGfDDJnmWIHOXNS8YTGCoTwx6aFh7CUSAwPPctYoQA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794564960; 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=PfCz/38ILdxPmJBY1NDzDaIsiECcLTMyKpoKPJStEV0=; b=ZL74livzHPDaizBQrTfboUECUxkXMVuW95yVIDR3LhTUgZIQW2e7sXBNffisdWsYL444 +MGm9XZxuguGrEj4TR+GwbHA+n38UDcDJaqmRken7rvoYzYN/dMfTE5ku5B3AcgQRjs34 Ihi+MQaf4qajQXQUpxTG9HDorzYITSNk50zmEAqRlf4X4RwKBrPJrjpDIQFec0saz2BnK LUYTlHhfdPepC1l58EfYOLn+8G7DU1MjtHF4L9dt08for3UsFiLXs9KT2TnyWiaZPoVBJ TTMp+HsMYrUpHhxPnKjOKsRY8Z7RlTNXBmu/lrG5IMgo/pCa3ceUPXi0BKk16SDQdgg== 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=lkMNxDps; 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 B610B606A8BE for ; Fri, 13 Nov 2026 10:15:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org B610B606A8BE 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=lkMNxDps Received: by mail-ej1-f51.google.com with SMTP id a640c23a62f3a-c506094fd93so151136366b.1 for ; Fri, 13 Nov 2026 02:15:59 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amutable-com.20260707.gappssmtp.com; s=20260707; t=1794564958; x=1795169758; darn=vger.kernel.org; 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=PfCz/38ILdxPmJBY1NDzDaIsiECcLTMyKpoKPJStEV0=; b=lkMNxDpsxePjk96S0ywzwnWUXxYio8wx2cNNe0nC6j26qPYCKZTFlNU+9IrTfafYUJ vVJYUkmMgC1UbEefAlU1yHOjVK+ZyaavdAq5vDpOrqfNUQTfZSYOp+z5MtAN0fGM0C/g WKoVDnfAgRrvHWrOgLujvGV7IRsdQm2D1xIiTvFC1PPHHAhefB4+1EzNQGHJZGVEqqG3 mRj5HPfDo3Do87jK8wkSwQeESRRW3NHA44wwS7GSfUWi+U+GxhbfTaI9vHs9ExXVB/2E aEkBTT5WDqLpZOCj3LhS4i2xxQF422KmjG442iJAtFRLE9RsLg4/ErcRXtOWPDB+ahLm /22g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1794564958; x=1795169758; 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=PfCz/38ILdxPmJBY1NDzDaIsiECcLTMyKpoKPJStEV0=; b=hDIzz2iPMgIiJUpwN/iNsbBq1m7l0KXvLvDFfO/bUnpTBODrnbPFQJ2wXoknDVW+CN BtpDVAbg85mxtO2Lb0VwTXj7E0yhybdAZrfVsp5ezqgk/qm29kmQiUPlGNnHMtdC2TXu 6AdUVhjI3jjeEZRVg4d9rCOQi4niAOKYybfxTLpKgJANU4WbxALrVUKgwbSoPs6lxd/R 51BAdQGAWR2OafJFV5aOwDaLJgxgG2Gd4EC1ap5EM9jgx7taBTh6mVChCHkWkT7gHSmg ru5d2fZGAjCGPRZH2/WvP5l+qVWkQMI7mAvz04qcUdzC1+ULCTea+ePFfGrdNfuYCe6M JGSA== X-Forwarded-Encrypted: i=1; AL+hErHs2WnCSwgP1xeDmQyswI7eRBOIYenGGMnppD+JX2V/2dN6g+4t3XPqpGOWv4wEkBPUlb8rhrkcCt+Ftw==@vger.kernel.org X-Gm-Message-State: APuU4Z/EX6t9v7LQMGSOt8rtjXYKwMI+QlME9O6N5sct+gXW64wTrh6T r7N3ENB0Jx3pFrgqSIExpdWcksmFw6doK7Xv/SmlW7NYJwrmQu4iMfAiuQUTIz8V6NAP X-Gm-Gg: AYBFou3vEOQnJjIazysIKCLFu+ArDOFxAB/6A4Sl+AR0lwwr6XCa6FC0YyyTR/2v9z6 aU4PeuXlwDTkW36ZQRuhxYJfiCw7QJEGyuKxOz5wklI3jnyzUVlk8T8R4+T2ee8ZyePF3rIDz42 iAEuqezoLCN242RqX5QpMW2qeM17Tftj9LoBVVELxGfLl7SO6BGQksGi0Pgov4Qzx1UhDcTLJBg 9gtJy1odTfjV7TvEvh3cI1PJcY9iIARRrqanMI8KkUAEdrDNNdO/qvaDs8RoPAVI5HCZOXigcB6 mRzRI4qCP5FHSJCdaVgdQ7icikRMXuySH9tq3w7Cj+H75+qHoAWCmKdRyenEYf5p2eRz/QrIXoz p65NwDfcw1MItsR+uIdPOaNepNj2Vd7ClZP9HBdMN8UuSqGYfwlrULcljQwFHwsIGGAm39/5sFW NKcfRKDserjGNxNodK9uvLIVSgbhsODMbJIf6W14Ra+1E5u+pqllv3mHTnbJkLrEUaoHfhisZBu MkAOVhYHPtvIPM2G1iiCFDnXPih9MTnkSAH9X4yxpM9WOvycL+bJmOHjTQaUW0un8DO5EnG+coW 887a1J0AmTkd7sgH+Ic4DMHV6XFtiyb+II5mqgNLbxo5tmUmcGmPVk8cA8KAxNY= X-Received: by 2002:a17:906:f59c:b0:c42:102f:15f0 with SMTP id a640c23a62f3a-c509614d4c1mr171873666b.3.1794564958075; Fri, 13 Nov 2026 02:15:58 -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-c5095b7d1b7sm75606066b.74.2026.11.13.02.15.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Nov 2026 02:15:57 -0800 (PST) Message-ID: Subject: Re: [PATCH RFC 34/41] loop: refuse files on pseudo filesystems as backing files 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 Date: Fri, 13 Nov 2026 11:15:56 +0100 In-Reply-To: <20261110-work-loop-pseudo-v1-34-4c998976c6c7@kernel.org> References: <20261110-work-loop-pseudo-v1-0-4c998976c6c7@kernel.org> <20261110-work-loop-pseudo-v1-34-4c998976c6c7@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: linux-block@vger.kernel.org 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: > Currently loop accepts any regular file that implements ->read_iter() > as > a backing file and any that also implements ->write_iter() as a > writable > one. That includes files on procfs, sysfs and other pseudo > filesystems. > For example, /proc/kcore attaches read-only as a loop device the size > of > the kernel's virtual address space, 64 PiB with 5-level paging. sysfs > attributes attach read-write as 4096 byte loop devices. Writing a > block > to the loop device of /sys/kernel/mm/transparent_hugepage/enabled > changes the THP setting. >=20 > That's not just nonsensical, it can be abused. Loop calls - > >read_iter() > and ->write_iter() of the backing file directly from its worker which > runs with kernel credentials. But pseudo files enforce part of their > access rules at read and write time against current. Dozens of sysfs > ->show() and ->store() methods call capable(). Behind a loop device > all > of these checks are done against the loop worker instead of the task > that opened the file. And setting up a loop device requires no > capabilities, only access to the device nodes. So on v7.4-rc1: >=20 > - uid 0 without any capabilities can't write to > /sys/kernel/reboot/mode > =C2=A0 as mode_store() requires CAP_SYS_BOOT. Writing "warm" through a > loop > =C2=A0 device works. >=20 > - uid 0 without any capabilities can't read the files in > =C2=A0 /sys/firmware/dmi/entries/*/ as dmi_sysfs_attr_show() requires > =C2=A0 CAP_SYS_ADMIN. Reading them through a loop device works. >=20 > - An unprivileged container that was handed a loop device can't > change > =C2=A0 gro_flush_timeout of its own network devices as > =C2=A0 gro_flush_timeout_store() requires CAP_NET_ADMIN in the initial > user > =C2=A0 namespace. Writing it through a loop device works. >=20 > The same goes for hard_offline_page, the md and sd attributes, rfkill > and others. >=20 > Refuse files on pseudo filesystems as loop backing files. Note that > file_is_pseudo() looks through overlayfs so stacking overlayfs on top > of > sysfs doesn't get around this. >=20 > 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 c96e35c3ef42..1482580a1979 100644 > --- a/drivers/block/loop.c > +++ b/drivers/block/loop.c > @@ -537,6 +537,9 @@ static int loop_check_backing_file(struct file > *file) > =C2=A0 if ((file->f_mode & FMODE_WRITE) && !file->f_op->write_iter) > =C2=A0 return -EINVAL; > =C2=A0 > + if (file_is_pseudo(file)) > + return -EINVAL; > + > =C2=A0 return 0; > =C2=A0} > =C2=A0