Re: [RFC PATCH v3 1/2] mm: restrictedmem: Allow userspace to specify mount for memfd_restricted

Christian Brauner
2023-04-04T14:59:39Z

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 <ackerleytng@google.com>
> > ---
> > 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 <linux/namei.h>
> > #include <linux/pagemap.h>
> > #include <linux/pseudo_fs.h>
> > #include <linux/shmem_fs.h>
> > #include <linux/syscalls.h>
> > #include <uapi/linux/falloc.h>
> > #include <uapi/linux/magic.h>
> > +#include <uapi/linux/restrictedmem.h>
> > #include <linux/restrictedmem.h>
> >
> > 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?