[PATCH 01/11] block_dev: Support checking inode permissions in lookup_bdev()

[PATCH 01/11] block_dev: Support checking inode permissions in lookup_bdev()

[Dongsu Park]

From: Seth Forshee <seth.forshee@canonical.com>

When looking up a block device by path no permission check is
done to verify that the user has access to the block device inode
at the specified path. In some cases it may be necessary to
check permissions towards the inode, such as allowing
unprivileged users to mount block devices in user namespaces.

Add an argument to lookup_bdev() to optionally perform this
permission check. A value of 0 skips the permission check and
behaves the same as before. A non-zero value specifies the mask
of access rights required towards the inode at the specified
path. The check is always skipped if the user has CAP_SYS_ADMIN.

All callers of lookup_bdev() currently pass a mask of 0, so this
patch results in no functional change. Subsequent patches will
add permission checks where appropriate.

Patch v4 is available: https://patchwork.kernel.org/patch/8943601/

Cc: dm-devel@redhat.com
Cc: linux-bcache@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mtd@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.com>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
---
 drivers/md/bcache/super.c |  2 +-
 drivers/md/dm-table.c     |  2 +-
 drivers/mtd/mtdsuper.c    |  2 +-
 fs/block_dev.c            | 13 ++++++++++---
 fs/quota/quota.c          |  2 +-
 include/linux/fs.h        |  2 +-
 6 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index b4d28928..acc9d56c 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1967,7 +1967,7 @@ 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 88130b5d..bca5eaf4 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -410,7 +410,7 @@ dev_t dm_get_dev_t(const char *path)
 	dev_t dev;
 	struct block_device *bdev;
 
-	bdev = lookup_bdev(path);
+	bdev = lookup_bdev(path, 0);
 	if (IS_ERR(bdev))
 		dev = name_to_dev_t(path);
 	else {
diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
index e43fea89..4a4d40c0 100644
--- a/drivers/mtd/mtdsuper.c
+++ b/drivers/mtd/mtdsuper.c
@@ -180,7 +180,7 @@ 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);
+	bdev = lookup_bdev(dev_name, 0);
 	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 4a181fcb..5ca06095 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1662,7 +1662,7 @@ struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
 	struct block_device *bdev;
 	int err;
 
-	bdev = lookup_bdev(path);
+	bdev = lookup_bdev(path, 0);
 	if (IS_ERR(bdev))
 		return bdev;
 
@@ -2052,12 +2052,14 @@ 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;
@@ -2072,6 +2074,11 @@ 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 43612e2a..e5d47955 100644
--- a/fs/quota/quota.c
+++ b/fs/quota/quota.c
@@ -807,7 +807,7 @@ 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 2995a271..fce19c49 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2551,7 +2551,7 @@ static inline void unregister_chrdev(unsigned int major, const char *name)
 #define BLKDEV_MAJOR_MAX	512
 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
-- 
2.13.6

Re: [PATCH 01/11] block_dev: Support checking inode permissions in lookup_bdev()

[Coly Li]

On 22/12/2017 10:32 PM, Dongsu Park wrote:
> From: Seth Forshee <seth.forshee@canonical.com>
> 
> When looking up a block device by path no permission check is
> done to verify that the user has access to the block device inode
> at the specified path. In some cases it may be necessary to
> check permissions towards the inode, such as allowing
> unprivileged users to mount block devices in user namespaces.
> 
> Add an argument to lookup_bdev() to optionally perform this
> permission check. A value of 0 skips the permission check and
> behaves the same as before. A non-zero value specifies the mask
> of access rights required towards the inode at the specified
> path. The check is always skipped if the user has CAP_SYS_ADMIN.
> 
> All callers of lookup_bdev() currently pass a mask of 0, so this
> patch results in no functional change. Subsequent patches will
> add permission checks where appropriate.
> 
> Patch v4 is available: https://patchwork.kernel.org/patch/8943601/
> 
> Cc: dm-devel@redhat.com
> Cc: linux-bcache@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-mtd@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Jan Kara <jack@suse.com>
> Cc: Serge Hallyn <serge@hallyn.com>
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Dongsu Park <dongsu@kinvolk.io>

Hi Dongsu,

Could you please use a macro like NO_PERMISSION_CHECK to replace hard
coded 0 ? At least for me, I don't need to check what does 0 mean in the
new lookup_bdev().

Thanks.

Coly Li

> ---
>  drivers/md/bcache/super.c |  2 +-
>  drivers/md/dm-table.c     |  2 +-
>  drivers/mtd/mtdsuper.c    |  2 +-
>  fs/block_dev.c            | 13 ++++++++++---
>  fs/quota/quota.c          |  2 +-
>  include/linux/fs.h        |  2 +-
>  6 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index b4d28928..acc9d56c 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1967,7 +1967,7 @@ 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 88130b5d..bca5eaf4 100644
[snip]


-- 
Coly Li

Re: [PATCH 01/11] block_dev: Support checking inode permissions in lookup_bdev()

[Dongsu Park]

Hi,

On Fri, Dec 22, 2017 at 7:59 PM, Coly Li <i@coly.li> wrote:
> On 22/12/2017 10:32 PM, Dongsu Park wrote:
> Hi Dongsu,
>
> Could you please use a macro like NO_PERMISSION_CHECK to replace hard
> coded 0 ? At least for me, I don't need to check what does 0 mean in the
> new lookup_bdev().

I see. I'll do that.

Thanks,
Dongsu

> Thanks.
>
> Coly Li
>
>> ---
>>  drivers/md/bcache/super.c |  2 +-
>>  drivers/md/dm-table.c     |  2 +-
>>  drivers/mtd/mtdsuper.c    |  2 +-
>>  fs/block_dev.c            | 13 ++++++++++---
>>  fs/quota/quota.c          |  2 +-
>>  include/linux/fs.h        |  2 +-
>>  6 files changed, 15 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
>> index b4d28928..acc9d56c 100644
>> --- a/drivers/md/bcache/super.c
>> +++ b/drivers/md/bcache/super.c
>> @@ -1967,7 +1967,7 @@ 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 88130b5d..bca5eaf4 100644
> [snip]
>
>
> --
> Coly Li

Re: [PATCH 01/11] block_dev: Support checking inode permissions in lookup_bdev()

[Serge E. Hallyn]

On Fri, Dec 22, 2017 at 03:32:25PM +0100, Dongsu Park wrote:
> From: Seth Forshee <seth.forshee@canonical.com>
> 
> When looking up a block device by path no permission check is
> done to verify that the user has access to the block device inode
> at the specified path. In some cases it may be necessary to
> check permissions towards the inode, such as allowing
> unprivileged users to mount block devices in user namespaces.
> 
> Add an argument to lookup_bdev() to optionally perform this
> permission check. A value of 0 skips the permission check and
> behaves the same as before. A non-zero value specifies the mask
> of access rights required towards the inode at the specified
> path. The check is always skipped if the user has CAP_SYS_ADMIN.
> 
> All callers of lookup_bdev() currently pass a mask of 0, so this
> patch results in no functional change. Subsequent patches will
> add permission checks where appropriate.
> 
> Patch v4 is available: https://patchwork.kernel.org/patch/8943601/
> 
> Cc: dm-devel@redhat.com
> Cc: linux-bcache@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-mtd@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Jan Kara <jack@suse.com>
> Cc: Serge Hallyn <serge@hallyn.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
> ---
>  drivers/md/bcache/super.c |  2 +-
>  drivers/md/dm-table.c     |  2 +-
>  drivers/mtd/mtdsuper.c    |  2 +-
>  fs/block_dev.c            | 13 ++++++++++---
>  fs/quota/quota.c          |  2 +-
>  include/linux/fs.h        |  2 +-
>  6 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index b4d28928..acc9d56c 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1967,7 +1967,7 @@ 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 88130b5d..bca5eaf4 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -410,7 +410,7 @@ dev_t dm_get_dev_t(const char *path)
>  	dev_t dev;
>  	struct block_device *bdev;
>  
> -	bdev = lookup_bdev(path);
> +	bdev = lookup_bdev(path, 0);
>  	if (IS_ERR(bdev))
>  		dev = name_to_dev_t(path);
>  	else {
> diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
> index e43fea89..4a4d40c0 100644
> --- a/drivers/mtd/mtdsuper.c
> +++ b/drivers/mtd/mtdsuper.c
> @@ -180,7 +180,7 @@ 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);
> +	bdev = lookup_bdev(dev_name, 0);
>  	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 4a181fcb..5ca06095 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1662,7 +1662,7 @@ struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
>  	struct block_device *bdev;
>  	int err;
>  
> -	bdev = lookup_bdev(path);
> +	bdev = lookup_bdev(path, 0);
>  	if (IS_ERR(bdev))
>  		return bdev;
>  
> @@ -2052,12 +2052,14 @@ 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;
> @@ -2072,6 +2074,11 @@ 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 43612e2a..e5d47955 100644
> --- a/fs/quota/quota.c
> +++ b/fs/quota/quota.c
> @@ -807,7 +807,7 @@ 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 2995a271..fce19c49 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2551,7 +2551,7 @@ static inline void unregister_chrdev(unsigned int major, const char *name)
>  #define BLKDEV_MAJOR_MAX	512
>  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
> -- 
> 2.13.6

[PATCH 02/11] mtd: Check permissions towards mtd block device inode when mounting

[Dongsu Park]

From: Seth Forshee <seth.forshee@canonical.com>

Unprivileged users should not be able to mount mtd block devices
when they lack sufficient privileges towards the block device
inode.  Update mount_mtd() to validate that the user has the
required access to the inode at the specified path. The check
will be skipped for CAP_SYS_ADMIN, so privileged mounts will
continue working as before.

Patch v3 is available: https://patchwork.kernel.org/patch/7640011/

Cc: linux-mtd@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
---
 drivers/mtd/mtdsuper.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
index 4a4d40c0..3c8734f3 100644
--- a/drivers/mtd/mtdsuper.c
+++ b/drivers/mtd/mtdsuper.c
@@ -129,6 +129,7 @@ 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;
 
@@ -180,7 +181,10 @@ 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, 0);
+	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);
-- 
2.13.6

Re: [PATCH 02/11] mtd: Check permissions towards mtd block device inode when mounting

[Richard Weinberger]

Dongsu,

On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu@kinvolk.io> wrote:
> From: Seth Forshee <seth.forshee@canonical.com>
>
> Unprivileged users should not be able to mount mtd block devices
> when they lack sufficient privileges towards the block device
> inode.  Update mount_mtd() to validate that the user has the
> required access to the inode at the specified path. The check
> will be skipped for CAP_SYS_ADMIN, so privileged mounts will
> continue working as before.

What is the big picture of this?
Can in future an unprivileged user just mount UBIFS?
Please note that UBIFS sits on top of a character device and not a block device.

-- 
Thanks,
//richard

Re: [PATCH 02/11] mtd: Check permissions towards mtd block device inode when mounting

[Dongsu Park]

Hi,

On Fri, Dec 22, 2017 at 10:06 PM, Richard Weinberger
<richard.weinberger@gmail.com> wrote:
> Dongsu,
>
> On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu@kinvolk.io> wrote:
>> From: Seth Forshee <seth.forshee@canonical.com>
>>
>> Unprivileged users should not be able to mount mtd block devices
>> when they lack sufficient privileges towards the block device
>> inode.  Update mount_mtd() to validate that the user has the
>> required access to the inode at the specified path. The check
>> will be skipped for CAP_SYS_ADMIN, so privileged mounts will
>> continue working as before.
>
> What is the big picture of this?
> Can in future an unprivileged user just mount UBIFS?

I'm not sure I'm aware of all use cases w.r.t mtd & ubifs.
To my understanding, in these days many container runtimes allow
unprivileged users to run containers. (docker, lxc, runc, bubblewrap, etc)
That's why the kernel should deal with additional permission checks
that might have not been necessary in the past.
This MTD patch is one of those special cases.

> Please note that UBIFS sits on top of a character device and not a block device.

Aha, good to know.

Thanks,
Dongsu

> --
> Thanks,
> //richard

Re: [PATCH 02/11] mtd: Check permissions towards mtd block device inode when mounting

[Richard Weinberger]

Dongsu,

Am Samstag, 23. Dezember 2017, 13:18:30 CET schrieb Dongsu Park:
> Hi,
> 
> On Fri, Dec 22, 2017 at 10:06 PM, Richard Weinberger
> 
> <richard.weinberger@gmail.com> wrote:
> > Dongsu,
> > 
> > On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu@kinvolk.io> wrote:
> >> From: Seth Forshee <seth.forshee@canonical.com>
> >> 
> >> Unprivileged users should not be able to mount mtd block devices
> >> when they lack sufficient privileges towards the block device
> >> inode.  Update mount_mtd() to validate that the user has the
> >> required access to the inode at the specified path. The check
> >> will be skipped for CAP_SYS_ADMIN, so privileged mounts will
> >> continue working as before.
> > 
> > What is the big picture of this?
> > Can in future an unprivileged user just mount UBIFS?
> 
> I'm not sure I'm aware of all use cases w.r.t mtd & ubifs.
> To my understanding, in these days many container runtimes allow
> unprivileged users to run containers. (docker, lxc, runc, bubblewrap, etc)
> That's why the kernel should deal with additional permission checks
> that might have not been necessary in the past.
> This MTD patch is one of those special cases.

My fear is that a corner case is forgotten and all of a sudden someone can do 
funky things with MTD in a container...

Thanks,
//richard

Re: [PATCH 02/11] mtd: Check permissions towards mtd block device inode when mounting

[Serge E. Hallyn]

On Fri, Dec 22, 2017 at 03:32:26PM +0100, Dongsu Park wrote:
> From: Seth Forshee <seth.forshee@canonical.com>
> 
> Unprivileged users should not be able to mount mtd block devices
> when they lack sufficient privileges towards the block device
> inode.  Update mount_mtd() to validate that the user has the
> required access to the inode at the specified path. The check
> will be skipped for CAP_SYS_ADMIN, so privileged mounts will
> continue working as before.
> 
> Patch v3 is available: https://patchwork.kernel.org/patch/7640011/
> 
> Cc: linux-mtd@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Dongsu Park <dongsu@kinvolk.io>

Acked-by: Serge Hallyn <serge@hallyn.com>

> ---
>  drivers/mtd/mtdsuper.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
> index 4a4d40c0..3c8734f3 100644
> --- a/drivers/mtd/mtdsuper.c
> +++ b/drivers/mtd/mtdsuper.c
> @@ -129,6 +129,7 @@ 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;
>  
> @@ -180,7 +181,10 @@ 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, 0);
> +	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);
> -- 
> 2.13.6
> 
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