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Date: Sun, 23 Apr 2017 23:31:57 -0400
From: Matt Brown <matt@...tt.com>
To: "Serge E. Hallyn" <serge@...lyn.com>
Cc: jmorris@...ei.org, gregkh@...uxfoundation.org, jslaby@...e.com,
corbet@....net, keescook@...omium.org, akpm@...ux-foundation.org,
jannh@...gle.com, kernel-hardening@...ts.openwall.com,
linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tiocsti-restrict : Add owner user namespace to
tty_struct
On 04/23/2017 09:09 PM, Serge E. Hallyn wrote:
> Quoting Matt Brown (matt@...tt.com):
>> This patch adds struct user_namespace *owner_user_ns to the tty_struct.
>> Then it is set to current_user_ns() in the alloc_tty_struct function.
>>
>> This is done to facilitate capability checks against the original user
>> namespace that allocated the tty.
>>
>> E.g. ns_capable(tty->owner_user_ns,CAP_SYS_ADMIN)
>>
>> This combined with the use of user namespace's will allow hardening
>> protections to be built to mitigate container escapes that utilize TTY
>> ioctls such as TIOCSTI.
>>
>> See: https://bugzilla.redhat.com/show_bug.cgi?id=1411256
>>
>> Signed-off-by: Matt Brown <matt@...tt.com>
>> ---
>> drivers/tty/tty_io.c | 4 ++++
>> include/linux/tty.h | 2 ++
>> 2 files changed, 6 insertions(+)
>>
>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>> index e6d1a65..03d5ea2 100644
>> --- a/drivers/tty/tty_io.c
>> +++ b/drivers/tty/tty_io.c
>> @@ -161,6 +161,7 @@ static void release_tty(struct tty_struct *tty, int idx);
>> * @tty: tty struct to free
>> *
>> * Free the write buffers, tty queue and tty memory itself.
>> + * Decrement the owner_user_ns count.
>> *
>> * Locking: none. Must be called after tty is definitely unused
>> */
>> @@ -171,6 +172,7 @@ static void free_tty_struct(struct tty_struct *tty)
>> put_device(tty->dev);
>> kfree(tty->write_buf);
>> tty->magic = 0xDEADDEAD;
>> + atomic_dec(&tty->owner_user_ns->count);
>> kfree(tty);
>> }
>>
>> @@ -3191,6 +3193,8 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
>> tty->index = idx;
>> tty_line_name(driver, idx, tty->name);
>> tty->dev = tty_get_device(tty);
>> + tty->owner_user_ns = current_user_ns();
>> + atomic_inc(&tty->owner_user_ns->count);
>
> Hi,
>
> CONFIG_USER_NS is an option, so unfortunately you need to do
>
> tty->owner_user_ns = get_user_ns(current_user_ns());
>
Does this mean I need to also use put_user_ns(tty->owner_user_ns) in
free_tty_struct instead of atomic_dec(&tty->owner_user_ns->count) ?
>>
>> return tty;
>> }
>> diff --git a/include/linux/tty.h b/include/linux/tty.h
>> index 1017e904..d902d42 100644
>> --- a/include/linux/tty.h
>> +++ b/include/linux/tty.h
>> @@ -12,6 +12,7 @@
>> #include <uapi/linux/tty.h>
>> #include <linux/rwsem.h>
>> #include <linux/llist.h>
>> +#include <linux/user_namespace.h>
>>
>>
>> /*
>> @@ -333,6 +334,7 @@ struct tty_struct {
>> /* If the tty has a pending do_SAK, queue it here - akpm */
>> struct work_struct SAK_work;
>> struct tty_port *port;
>> + struct user_namespace *owner_user_ns;
>> };
>>
>> /* Each of a tty's open files has private_data pointing to tty_file_private */
>> --
>> 2.10.2
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