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Tue, 4 Apr 2023 07:58:49 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 50FB863163; Tue, 4 Apr 2023 14:58:49 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0F40CC433D2; Tue, 4 Apr 2023 14:58:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1680620328; bh=Sq9+Vb89Rx0M4Rx8FyPhF0pa8OcMmukvsyftvx5GcSI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=jTovAp2hPoe1O4WmMbBh35bRuNhX0mdCmAAavd3SueVmaYSooKwdpMywItaC27DE/ ekVKIzjmmYxMBABZqUV+9h42WZNEVilUc+IwrQGJ4OW+mnQEfI5w3tcgLQviGF92LB QMDvDA448ZeTf1Zm8F7ddV7EBzhPfNqMx0YWwRkhtE1qecRS5BF9huSs7/Nr423Rjn tUBtXd+YrNy51Gxp12x21NaXzCY0cSZnpVHkjf4MbWJhBY8dEQBvXYahfTBMq7ekcu twFQzPBGdKApzgzOdetJnKOPfm2gL3LkiFQT6RRPNL1BWOyeE/v4HvfwyhPsfNXYxU pmt7dX3B4SCcw== Date: Tue, 4 Apr 2023 16:58:34 +0200 From: Christian Brauner To: Ackerley Tng Cc: kvm@vger.kernel.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, qemu-devel@nongnu.org, aarcange@redhat.com, ak@linux.intel.com, akpm@linux-foundation.org, arnd@arndb.de, bfields@fieldses.org, bp@alien8.de, chao.p.peng@linux.intel.com, corbet@lwn.net, dave.hansen@intel.com, david@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, hpa@zytor.com, hughd@google.com, jlayton@kernel.org, jmattson@google.com, joro@8bytes.org, jun.nakajima@intel.com, kirill.shutemov@linux.intel.com, linmiaohe@huawei.com, luto@kernel.org, mail@maciej.szmigiero.name, mhocko@suse.com, michael.roth@amd.com, mingo@redhat.com, naoya.horiguchi@nec.com, pbonzini@redhat.com, qperret@google.com, rppt@kernel.org, seanjc@google.com, shuah@kernel.org, steven.price@arm.com, tabba@google.com, tglx@linutronix.de, vannapurve@google.com, vbabka@suse.cz, vkuznets@redhat.com, wanpengli@tencent.com, wei.w.wang@intel.com, x86@kernel.org, yu.c.zhang@linux.intel.com Subject: Re: [RFC PATCH v3 1/2] mm: restrictedmem: Allow userspace to specify mount for memfd_restricted Message-ID: <20230404-engraved-rumble-d871e0403f3b@brauner> References: <592ebd9e33a906ba026d56dc68f42d691706f865.1680306489.git.ackerleytng@google.com> <20230404-amnesty-untying-01de932d4945@brauner> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20230404-amnesty-untying-01de932d4945@brauner> X-Spam-Status: No, score=-5.2 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_HI,SPF_HELO_NONE, SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 04, 2023 at 03:53:13PM +0200, Christian Brauner wrote: > On Fri, Mar 31, 2023 at 11:50:39PM +0000, Ackerley Tng wrote: > > By default, the backing shmem file for a restrictedmem fd is created > > on shmem's kernel space mount. > > > > With this patch, an optional tmpfs mount can be specified via an fd, > > which will be used as the mountpoint for backing the shmem file > > associated with a restrictedmem fd. > > > > This will help restrictedmem fds inherit the properties of the > > provided tmpfs mounts, for example, hugepage allocation hints, NUMA > > binding hints, etc. > > > > Permissions for the fd passed to memfd_restricted() is modeled after > > the openat() syscall, since both of these allow creation of a file > > upon a mount/directory. > > > > Permission to reference the mount the fd represents is checked upon fd > > creation by other syscalls (e.g. fsmount(), open(), or open_tree(), > > etc) and any process that can present memfd_restricted() with a valid > > fd is expected to have obtained permission to use the mount > > represented by the fd. This behavior is intended to parallel that of > > the openat() syscall. > > > > memfd_restricted() will check that the tmpfs superblock is > > writable, and that the mount is also writable, before attempting to > > create a restrictedmem file on the mount. > > > > Signed-off-by: Ackerley Tng > > --- > > include/linux/syscalls.h | 2 +- > > include/uapi/linux/restrictedmem.h | 8 ++++ > > mm/restrictedmem.c | 74 +++++++++++++++++++++++++++--- > > 3 files changed, 77 insertions(+), 7 deletions(-) > > create mode 100644 include/uapi/linux/restrictedmem.h > > > > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > > index f9e9e0c820c5..a23c4c385cd3 100644 > > --- a/include/linux/syscalls.h > > +++ b/include/linux/syscalls.h > > @@ -1056,7 +1056,7 @@ asmlinkage long sys_memfd_secret(unsigned int flags); > > asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len, > > unsigned long home_node, > > unsigned long flags); > > -asmlinkage long sys_memfd_restricted(unsigned int flags); > > +asmlinkage long sys_memfd_restricted(unsigned int flags, int mount_fd); > > > > /* > > * Architecture-specific system calls > > diff --git a/include/uapi/linux/restrictedmem.h b/include/uapi/linux/restrictedmem.h > > new file mode 100644 > > index 000000000000..22d6f2285f6d > > --- /dev/null > > +++ b/include/uapi/linux/restrictedmem.h > > @@ -0,0 +1,8 @@ > > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ > > +#ifndef _UAPI_LINUX_RESTRICTEDMEM_H > > +#define _UAPI_LINUX_RESTRICTEDMEM_H > > + > > +/* flags for memfd_restricted */ > > +#define RMFD_USERMNT 0x0001U > > + > > +#endif /* _UAPI_LINUX_RESTRICTEDMEM_H */ > > diff --git a/mm/restrictedmem.c b/mm/restrictedmem.c > > index c5d869d8c2d8..f7b62364a31a 100644 > > --- a/mm/restrictedmem.c > > +++ b/mm/restrictedmem.c > > @@ -1,11 +1,12 @@ > > // SPDX-License-Identifier: GPL-2.0 > > -#include "linux/sbitmap.h" > > +#include > > #include > > #include > > #include > > #include > > #include > > #include > > +#include > > #include > > > > struct restrictedmem { > > @@ -189,19 +190,20 @@ static struct file *restrictedmem_file_create(struct file *memfd) > > return file; > > } > > > > -SYSCALL_DEFINE1(memfd_restricted, unsigned int, flags) > > +static int restrictedmem_create(struct vfsmount *mount) > > { > > struct file *file, *restricted_file; > > int fd, err; > > > > - if (flags) > > - return -EINVAL; > > - > > fd = get_unused_fd_flags(0); > > Any reasons the file descriptors aren't O_CLOEXEC by default? I don't > see any reasons why we should introduce new fdtypes that aren't > O_CLOEXEC by default. The "don't mix-and-match" train has already left > the station anyway as we do have seccomp noitifer fds and pidfds both of > which are O_CLOEXEC by default. > > > if (fd < 0) > > return fd; > > > > - file = shmem_file_setup("memfd:restrictedmem", 0, VM_NORESERVE); > > + if (mount) > > + file = shmem_file_setup_with_mnt(mount, "memfd:restrictedmem", 0, VM_NORESERVE); > > + else > > + file = shmem_file_setup("memfd:restrictedmem", 0, VM_NORESERVE); > > + > > if (IS_ERR(file)) { > > err = PTR_ERR(file); > > goto err_fd; > > @@ -223,6 +225,66 @@ SYSCALL_DEFINE1(memfd_restricted, unsigned int, flags) > > return err; > > } > > > > +static bool is_shmem_mount(struct vfsmount *mnt) > > +{ > > + return mnt && mnt->mnt_sb && mnt->mnt_sb->s_magic == TMPFS_MAGIC; > > This can just be if (mnt->mnt_sb->s_magic == TMPFS_MAGIC). > > > +} > > + > > +static bool is_mount_root(struct file *file) > > +{ > > + return file->f_path.dentry == file->f_path.mnt->mnt_root; > > mount -t tmpfs tmpfs /mnt > touch /mnt/bla > touch /mnt/ble > mount --bind /mnt/bla /mnt/ble > fd = open("/mnt/ble") > fd_restricted = memfd_restricted(fd) > > IOW, this doesn't restrict it to the tmpfs root. It only restricts it to > paths that refer to the root of any tmpfs mount. To exclude bind-mounts > that aren't bind-mounts of the whole filesystem you want: > > path->dentry == path->mnt->mnt_root && > path->mnt->mnt_root == path->mnt->mnt_sb->s_root > > > +} > > + > > +static int restrictedmem_create_on_user_mount(int mount_fd) > > +{ > > + int ret; > > + struct fd f; > > + struct vfsmount *mnt; > > + > > + f = fdget_raw(mount_fd); > > + if (!f.file) > > + return -EBADF; > > + > > + ret = -EINVAL; > > + if (!is_mount_root(f.file)) > > + goto out; > > + > > + mnt = f.file->f_path.mnt; > > + if (!is_shmem_mount(mnt)) > > + goto out; > > + > > + ret = file_permission(f.file, MAY_WRITE | MAY_EXEC); > > With the current semantics you're asking whether you have write > permissions on the /mnt/ble file in order to get answer to the question > whether you're allowed to create an unlinked restricted memory file. > That doesn't make much sense afaict. > > > + if (ret) > > + goto out; > > + > > + ret = mnt_want_write(mnt); > > + if (unlikely(ret)) > > + goto out; > > + > > + ret = restrictedmem_create(mnt); > > + > > + mnt_drop_write(mnt); > > +out: > > + fdput(f); > > + > > + return ret; > > +} > > + > > +SYSCALL_DEFINE2(memfd_restricted, unsigned int, flags, int, mount_fd) > > +{ > > + if (flags & ~RMFD_USERMNT) > > + return -EINVAL; > > + > > + if (flags == RMFD_USERMNT) { > > Why do you even need this flag? It seems that @mount_fd being < 0 is > sufficient to indicate that a new restricted memory fd is supposed to be > created in the system instance. > > > + if (mount_fd < 0) > > + return -EINVAL; > > + > > + return restrictedmem_create_on_user_mount(mount_fd); > > + } else { > > + return restrictedmem_create(NULL); > > + } > > +} > > I have to say that I'm very confused by all of this the more I look at it. > > Effectively memfd restricted functions as a wrapper filesystem around > the tmpfs filesystem. This is basically a weird overlay filesystem. > You're allocating tmpfs files that you stash in restrictedmem files. > I have to say that this seems very hacky. I didn't get this at all at > first. > > So what does the caller get if they call statx() on a restricted memfd? > Do they get the device number of the tmpfs mount and the inode numbers > of the tmpfs mount? Because it looks like they would: > > static int restrictedmem_getattr(struct user_namespace *mnt_userns, > const struct path *path, struct kstat *stat, > u32 request_mask, unsigned int query_flags) > { > struct inode *inode = d_inode(path->dentry); > struct restrictedmem *rm = inode->i_mapping->private_data; > struct file *memfd = rm->memfd; > > return memfd->f_inode->i_op->getattr(mnt_userns, path, stat, This is pretty broken btw, because @path refers to a restrictedmem path which you're passing to a tmpfs iop... I see that in return memfd->f_inode->i_op->getattr(mnt_userns, &memfd->f_path, stat, request_mask, query_flags); this if fixed but still, this is... not great. > request_mask, query_flags); > > That @memfd would be a struct file allocated in a tmpfs instance, no? So > you'd be calling the inode operation of the tmpfs file meaning that > struct kstat will be filled up with the info from the tmpfs instance. > > But then if I call statfs() and check the fstype I would get > RESTRICTEDMEM_MAGIC, no? This is... unorthodox? > > I'm honestly puzzled and this sounds really strange. There must be a > better way to implement all of this. > > Shouldn't you try and make this a part of tmpfs proper? Make a really > separate filesystem and add a memfs library that both tmpfs and > restrictedmemfs can use? Add a mount option to tmpfs that makes it a > restricted tmpfs?