Re: [PATCH for v4.9 LTS 72/87] libfs: Modify mount_pseudo_xattr to be clear it is not a userspace mount

From: Levin, Alexander (Sasha Levin)
Date: Sat Jul 15 2017 - 11:47:10 EST

On Sat, Jul 15, 2017 at 02:50:30AM -0500, Eric W. Biederman wrote:
>*Scratches my head*
>Is there code in v4.9 where this matters? At the time I merged this to
>my knowledge there were no in kernel users that cared. Which is why I
>did not cc stable in the first place.

Hey Eric,

We're trying to go over older commits and figure out if they should have been in -stable even though they're not marked for stable.

Because of cases like these, they get to be reviewed a bunch more to verify that indeed should go into -stable, which is why you got this mail.

I'll drop this patch from the queue.

>"Levin, Alexander (Sasha Levin)" <alexander.levin@xxxxxxxxxxx> writes:
>> From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> [ Upstream commit 75422726b0f717d67db3283c2eb5bc14fa2619c5 ]
>> Add MS_KERNMOUNT to the flags that are passed.
>> Use sget_userns and force &init_user_ns instead of calling sget so that
>> even if called from a weird context the internal filesystem will be
>> considered to be in the intial user namespace.
>> Luis Ressel reported that the the failure to pass MS_KERNMOUNT into
>> mount_pseudo broke his in development graphics driver that uses the
>> generic drm infrastructure. I am not certain the deriver was bug
>> free in it's usage of that infrastructure but since
>> mount_pseudo_xattr can never be triggered by userspace it is clearer
>> and less error prone, and less problematic for the code to be explicit.
>> Reported-by: Luis Ressel <aranea@xxxxxxxx>
>> Tested-by: Luis Ressel <aranea@xxxxxxxx>
>> Acked-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxx>
>> ---
>> fs/libfs.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>> diff --git a/fs/libfs.c b/fs/libfs.c
>> index 48826d4da189..9588780ad43e 100644
>> --- a/fs/libfs.c
>> +++ b/fs/libfs.c
>> @@ -245,7 +245,8 @@ struct dentry *mount_pseudo_xattr(struct file_system_type *fs_type, char *name,
>> struct inode *root;
>> struct qstr d_name = QSTR_INIT(name, strlen(name));
>> - s = sget(fs_type, NULL, set_anon_super, MS_NOUSER, NULL);
>> + s = sget_userns(fs_type, NULL, set_anon_super, MS_KERNMOUNT|MS_NOUSER,
>> + &init_user_ns, NULL);
>> if (IS_ERR(s))
>> return ERR_CAST(s);