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Message-ID: <87mvw2d2cn.fsf@x220.int.ebiederm.org>
Date: Thu, 01 Oct 2015 10:40:08 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Seth Forshee <seth.forshee@...onical.com>
Cc: Kent Overstreet <kent.overstreet@...il.com>,
Alasdair Kergon <agk@...hat.com>,
Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
Neil Brown <neilb@...e.com>,
David Woodhouse <dwmw2@...radead.org>,
Brian Norris <computersforpeace@...il.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Jan Kara <jack@...e.com>,
Jeff Layton <jlayton@...chiereds.net>,
"J. Bruce Fields" <bfields@...ldses.org>,
Serge Hallyn <serge.hallyn@...onical.com>,
Andy Lutomirski <luto@...capital.net>,
linux-fsdevel@...r.kernel.org,
linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
linux-bcache@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: [PATCH 1/5] fs: Verify access of user towards block device file when mounting
Seth Forshee <seth.forshee@...onical.com> writes:
> When mounting a filesystem on a block device there is currently
> no verification that the user has appropriate access to the
> device file passed to mount. This has not been an issue so far
> since the user in question has always been root, but this must
> be changed before allowing unprivileged users to mount in user
> namespaces.
>
> To fix this, add an argument to lookup_bdev() to specify the
> required permissions. If the mask of permissions is zero, or
> if the user has CAP_SYS_ADMIN, the permission check is skipped,
> otherwise the lookup fails if the user does not have the
> specified access rights for the inode at the supplied path.
>
> Callers associated with mounting are updated to pass permission
> masks to lookup_bdev() so that these mounts will fail for an
> unprivileged user who lacks permissions for the block device
> inode. All other callers pass 0 to maintain their current
> behaviors.
>
Seth can you split this patch?
One patch to add an argument to lookup_bdev,
and then for each kind of callsite a follow-on patch (if we are ready
for that).
That will separate the logical changes and make things easier to track
via bisect and more importantly easier to review things.
Eric
> Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
> ---
> drivers/md/bcache/super.c | 2 +-
> drivers/md/dm-table.c | 2 +-
> drivers/mtd/mtdsuper.c | 6 +++++-
> fs/block_dev.c | 18 +++++++++++++++---
> fs/quota/quota.c | 2 +-
> include/linux/fs.h | 2 +-
> 6 files changed, 24 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 679a093a3bf6..e8287b0d1dac 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1926,7 +1926,7 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
> sb);
> if (IS_ERR(bdev)) {
> if (bdev == ERR_PTR(-EBUSY)) {
> - bdev = lookup_bdev(strim(path));
> + bdev = lookup_bdev(strim(path), 0);
> mutex_lock(&bch_register_lock);
> if (!IS_ERR(bdev) && bch_is_open(bdev))
> err = "device already registered";
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index e76ed003769e..35bb3ea4cbe2 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -380,7 +380,7 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> BUG_ON(!t);
>
> /* convert the path to a device */
> - bdev = lookup_bdev(path);
> + bdev = lookup_bdev(path, 0);
> if (IS_ERR(bdev)) {
> dev = name_to_dev_t(path);
> if (!dev)
> diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
> index 20c02a3b7417..5d7e7705fed8 100644
> --- a/drivers/mtd/mtdsuper.c
> +++ b/drivers/mtd/mtdsuper.c
> @@ -125,6 +125,7 @@ struct dentry *mount_mtd(struct file_system_type *fs_type, int flags,
> #ifdef CONFIG_BLOCK
> struct block_device *bdev;
> int ret, major;
> + int perm;
> #endif
> int mtdnr;
>
> @@ -176,7 +177,10 @@ struct dentry *mount_mtd(struct file_system_type *fs_type, int flags,
> /* try the old way - the hack where we allowed users to mount
> * /dev/mtdblock$(n) but didn't actually _use_ the blockdev
> */
> - bdev = lookup_bdev(dev_name);
> + perm = MAY_READ;
> + if (!(flags & MS_RDONLY))
> + perm |= MAY_WRITE;
> + bdev = lookup_bdev(dev_name, perm);
> if (IS_ERR(bdev)) {
> ret = PTR_ERR(bdev);
> pr_debug("MTDSB: lookup_bdev() returned %d\n", ret);
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 26cee058dc02..54d94cd64577 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1394,9 +1394,14 @@ struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
> void *holder)
> {
> struct block_device *bdev;
> + int perm = 0;
> int err;
>
> - bdev = lookup_bdev(path);
> + if (mode & FMODE_READ)
> + perm |= MAY_READ;
> + if (mode & FMODE_WRITE)
> + perm |= MAY_WRITE;
> + bdev = lookup_bdev(path, perm);
> if (IS_ERR(bdev))
> return bdev;
>
> @@ -1706,12 +1711,14 @@ EXPORT_SYMBOL(ioctl_by_bdev);
> /**
> * lookup_bdev - lookup a struct block_device by name
> * @pathname: special file representing the block device
> + * @mask: rights to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
> *
> * Get a reference to the blockdevice at @pathname in the current
> * namespace if possible and return it. Return ERR_PTR(error)
> - * otherwise.
> + * otherwise. If @mask is non-zero, check for access rights to the
> + * inode at @pathname.
> */
> -struct block_device *lookup_bdev(const char *pathname)
> +struct block_device *lookup_bdev(const char *pathname, int mask)
> {
> struct block_device *bdev;
> struct inode *inode;
> @@ -1726,6 +1733,11 @@ struct block_device *lookup_bdev(const char *pathname)
> return ERR_PTR(error);
>
> inode = d_backing_inode(path.dentry);
> + if (mask != 0 && !capable(CAP_SYS_ADMIN)) {
> + error = __inode_permission(inode, mask);
> + if (error)
> + goto fail;
> + }
> error = -ENOTBLK;
> if (!S_ISBLK(inode->i_mode))
> goto fail;
> diff --git a/fs/quota/quota.c b/fs/quota/quota.c
> index 3746367098fd..a40eaecbd5cc 100644
> --- a/fs/quota/quota.c
> +++ b/fs/quota/quota.c
> @@ -733,7 +733,7 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
>
> if (IS_ERR(tmp))
> return ERR_CAST(tmp);
> - bdev = lookup_bdev(tmp->name);
> + bdev = lookup_bdev(tmp->name, 0);
> putname(tmp);
> if (IS_ERR(bdev))
> return ERR_CAST(bdev);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 458ee7b213be..cc18dfb0b98e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2388,7 +2388,7 @@ static inline void unregister_chrdev(unsigned int major, const char *name)
> #define BLKDEV_MAJOR_HASH_SIZE 255
> extern const char *__bdevname(dev_t, char *buffer);
> extern const char *bdevname(struct block_device *bdev, char *buffer);
> -extern struct block_device *lookup_bdev(const char *);
> +extern struct block_device *lookup_bdev(const char *, int mask);
> extern void blkdev_show(struct seq_file *,off_t);
>
> #else
--
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