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[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-security-module@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