Re: [PATCH V4] audit: add tty field to LOGIN event

On 16/04/27, Peter Hurley wrote: > 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(a)redhat.com&gt; >>>> --- >>>> >>>> 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. I agree this is true except in the case of !CLONE_SIGHAND, if it fails after copy_sighand() or copy_signal() then it would be null and would get freed before audit_free() is called. By the time tty gets copied from current in this case, it is past the point of failure in copy_process().

> 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). Given the circumstances above, this appears reasonable to me at first look.

>> 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(a)redhat.com&gt; >> Kernel Security Engineering, Base Operating Systems, Red Hat >> Remote, Ottawa, Canada >> Voice: +1.647.777.2635, Internal: (81) 32635 >> > - RGB -- Richard Guy Briggs <rgb(a)redhat.com&gt; Kernel Security Engineering, Base Operating Systems, Red Hat Remote, Ottawa, Canada Voice: +1.647.777.2635, Internal: (81) 32635