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Date: Wed, 27 Apr 2016 20:05:03 -0700
From: Peter Hurley <peter@...leysoftware.com>
To: Richard Guy Briggs <rgb@...hat.com>
Cc: linux-audit@...hat.com, pmoore@...hat.com,
linux-kernel@...r.kernel.org, sgrubb@...hat.com, eparis@...hat.com
Subject: Re: [PATCH V4] audit: add tty field to LOGIN event
On 04/27/2016 06:31 PM, Richard Guy Briggs wrote:
> On 16/04/22, Peter Hurley wrote:
>> On 04/21/2016 11:14 AM, Richard Guy Briggs wrote:
>>> The tty field was missing from AUDIT_LOGIN events.
>>>
>>> Refactor code to create a new function audit_get_tty(), using it to
>>> replace the call in audit_log_task_info() and to add it to
>>> audit_log_set_loginuid(). Lock and bump the kref to protect it, adding
>>> audit_put_tty() alias to decrement it.
>>>
>>> Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
>>> ---
>>>
>>> V4: Add missing prototype for audit_put_tty() when audit syscall is not
>>> enabled (MIPS).
>>>
>>> V3: Introduce audit_put_tty() alias to decrement kref.
>>>
>>> V2: Use kref to protect tty signal struct while in use.
>>>
>>> ---
>>>
>>> include/linux/audit.h | 24 ++++++++++++++++++++++++
>>> kernel/audit.c | 18 +++++-------------
>>> kernel/auditsc.c | 8 ++++++--
>>> 3 files changed, 35 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/include/linux/audit.h b/include/linux/audit.h
>>> index b40ed5d..32cdafb 100644
>>> --- a/include/linux/audit.h
>>> +++ b/include/linux/audit.h
>>> @@ -26,6 +26,7 @@
>>> #include <linux/sched.h>
>>> #include <linux/ptrace.h>
>>> #include <uapi/linux/audit.h>
>>> +#include <linux/tty.h>
>>>
>>> #define AUDIT_INO_UNSET ((unsigned long)-1)
>>> #define AUDIT_DEV_UNSET ((dev_t)-1)
>>> @@ -343,6 +344,23 @@ static inline unsigned int audit_get_sessionid(struct task_struct *tsk)
>>> return tsk->sessionid;
>>> }
>>>
>>> +static inline struct tty_struct *audit_get_tty(struct task_struct *tsk)
>>> +{
>>> + struct tty_struct *tty = NULL;
>>> + unsigned long flags;
>>> +
>>> + spin_lock_irqsave(&tsk->sighand->siglock, flags);
>>> + if (tsk->signal)
>>> + tty = tty_kref_get(tsk->signal->tty);
>>> + spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
>>
>>
>> Not that I'm objecting because I get that you're just refactoring
>> existing code, but I thought I'd point out some stuff.
>>
>> 1. There's no need to check if signal_struct is NULL (ie. tsk->signal)
>> because if it is, this will blow up trying to dereference the
>> sighand_struct (ie tsk->sighand).
>
> Ok. This logic goes back 10 years and one month less two days. (45d9bb0e)
>
>> 2. The existing usage is always tsk==current
>
> My understanding is that when it is called via:
>
> copy_process()
> audit_free()
> __audit_free()
> audit_log_exit()
> audit_log_task_info()
>
> then tsk != current.
While it's true that tsk != current here, everything relevant to tty
in task_struct is the same because the nascent task is not even half-done.
So tsk->sighand == current->sighand, tsk->signal == current->signal etc.
If you're uncomfortable with pass-through execution like that, then the
simple solution is:
struct tty_struct *tty = NULL;
/* tsk != current when copy_process() failed */
if (tsk == current)
tty = get_current_tty();
because tty_kref_put(tty) accepts NULL tty and (obviously) so does
tty_name(tty).
Regards,
Peter Hurley
> This appears to be the only case which appears to
> force lugging around tsk. This is noted in that commit referenced
> above.
>
>> 3. If the idea is to make this invulnerable to tsk being gone, then
>> the usage is unsafe anyway.
>>
>>
>> So ultimately (but not necessarily for this patch) I'd prefer that either
>> a. audit use existing tty api instead of open-coding, or
>> b. add any tty api functions required.
>
> This latter option did cross my mind...
>
>> Peter Hurley
>>
>>> + return tty;
>>> +}
>>> +
>>> +static inline void audit_put_tty(struct tty_struct *tty)
>>> +{
>>> + tty_kref_put(tty);
>>> +}
>>> +
>>> extern void __audit_ipc_obj(struct kern_ipc_perm *ipcp);
>>> extern void __audit_ipc_set_perm(unsigned long qbytes, uid_t uid, gid_t gid, umode_t mode);
>>> extern void __audit_bprm(struct linux_binprm *bprm);
>>> @@ -500,6 +518,12 @@ static inline unsigned int audit_get_sessionid(struct task_struct *tsk)
>>> {
>>> return -1;
>>> }
>>> +static inline struct tty_struct *audit_get_tty(struct task_struct *tsk)
>>> +{
>>> + return NULL;
>>> +}
>>> +static inline void audit_put_tty(struct tty_struct *tty)
>>> +{ }
>>> static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
>>> { }
>>> static inline void audit_ipc_set_perm(unsigned long qbytes, uid_t uid,
>>> diff --git a/kernel/audit.c b/kernel/audit.c
>>> index 3a3e5de..7edd776 100644
>>> --- a/kernel/audit.c
>>> +++ b/kernel/audit.c
>>> @@ -64,7 +64,6 @@
>>> #include <linux/security.h>
>>> #endif
>>> #include <linux/freezer.h>
>>> -#include <linux/tty.h>
>>> #include <linux/pid_namespace.h>
>>> #include <net/netns/generic.h>
>>>
>>> @@ -1873,21 +1872,14 @@ void audit_log_task_info(struct audit_buffer *ab, struct task_struct *tsk)
>>> {
>>> const struct cred *cred;
>>> char comm[sizeof(tsk->comm)];
>>> - char *tty;
>>> + struct tty_struct *tty;
>>>
>>> if (!ab)
>>> return;
>>>
>>> /* tsk == current */
>>> cred = current_cred();
>>> -
>>> - spin_lock_irq(&tsk->sighand->siglock);
>>> - if (tsk->signal && tsk->signal->tty && tsk->signal->tty->name)
>>> - tty = tsk->signal->tty->name;
>>> - else
>>> - tty = "(none)";
>>> - spin_unlock_irq(&tsk->sighand->siglock);
>>> -
>>> + tty = audit_get_tty(tsk);
>>> audit_log_format(ab,
>>> " ppid=%d pid=%d auid=%u uid=%u gid=%u"
>>> " euid=%u suid=%u fsuid=%u"
>>> @@ -1903,11 +1895,11 @@ void audit_log_task_info(struct audit_buffer *ab, struct task_struct *tsk)
>>> from_kgid(&init_user_ns, cred->egid),
>>> from_kgid(&init_user_ns, cred->sgid),
>>> from_kgid(&init_user_ns, cred->fsgid),
>>> - tty, audit_get_sessionid(tsk));
>>> -
>>> + tty ? tty_name(tty) : "(none)",
>>> + audit_get_sessionid(tsk));
>>> + audit_put_tty(tty);
>>> audit_log_format(ab, " comm=");
>>> audit_log_untrustedstring(ab, get_task_comm(comm, tsk));
>>> -
>>> audit_log_d_path_exe(ab, tsk->mm);
>>> audit_log_task_context(ab);
>>> }
>>> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
>>> index 195ffae..71e14d8 100644
>>> --- a/kernel/auditsc.c
>>> +++ b/kernel/auditsc.c
>>> @@ -1980,6 +1980,7 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
>>> {
>>> struct audit_buffer *ab;
>>> uid_t uid, oldloginuid, loginuid;
>>> + struct tty_struct *tty;
>>>
>>> if (!audit_enabled)
>>> return;
>>> @@ -1987,14 +1988,17 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
>>> uid = from_kuid(&init_user_ns, task_uid(current));
>>> oldloginuid = from_kuid(&init_user_ns, koldloginuid);
>>> loginuid = from_kuid(&init_user_ns, kloginuid),
>>> + tty = audit_get_tty(current);
>>>
>>> ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_LOGIN);
>>> if (!ab)
>>> return;
>>> audit_log_format(ab, "pid=%d uid=%u", task_pid_nr(current), uid);
>>> audit_log_task_context(ab);
>>> - audit_log_format(ab, " old-auid=%u auid=%u old-ses=%u ses=%u res=%d",
>>> - oldloginuid, loginuid, oldsessionid, sessionid, !rc);
>>> + audit_log_format(ab, " old-auid=%u auid=%u tty=%s old-ses=%u ses=%u res=%d",
>>> + oldloginuid, loginuid, tty ? tty_name(tty) : "(none)",
>>> + oldsessionid, sessionid, !rc);
>>> + audit_put_tty(tty);
>>> audit_log_end(ab);
>>> }
>>>
>>>
>>
>
> - RGB
>
> --
> Richard Guy Briggs <rgb@...hat.com>
> Kernel Security Engineering, Base Operating Systems, Red Hat
> Remote, Ottawa, Canada
> Voice: +1.647.777.2635, Internal: (81) 32635
>
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