On sob, 2015-05-23 at 12:49 -0500, Eric W. Biederman wrote: > Lukasz Pawelczyk <l.pawelczyk@xxxxxxxxxxx> writes: > > > There is a rare case where current's nsproxy might be NULL but we are > > required to check for credentials and capabilities. It sometimes happens > > during an exit_group() syscall while destroying user's session (logging > > out). > > > > My understanding is that while we have to lock the task to get task's > > nsproxy and check whether it's NULL, for the 'current' we don't have to > > and it's expected not to be NULL. There is a code in the kernel > > currently that does current->nsproxy->user_ns without any checks. > > And include/linux/nsproxy.h confirms that: > > > > 2. when accessing (i.e. reading) current task's namespaces - no > > precautions should be taken - just dereference the pointers > > > > There seem to be no crash currently because of this, but with accessing > > nsproxy from LSM hooks there is. This is the backtrace: > > > > 0 smk_tskacc (task=0xffff88003b0b92e0, obj_known=0x2 <irq_stack_union+2>, mode=2, a=0xffff88003be53dd8) at security/smack/smack_access.c:261 > > 1 0xffffffff8130e2aa in smk_curacc (obj_known=<optimized out>, mode=<optimized out>, a=<optimized out>) at security/smack/smack_access.c:318 > > 2 0xffffffff8130a50d in smack_task_kill (p=0xffff88003b0b92e0, info=<optimized out>, sig=<optimized out>, secid=<optimized out>) at security/smack/smack_lsm.c:2071 > > 3 0xffffffff812ea4f6 in security_task_kill (p=<optimized out>, info=<optimized out>, sig=<optimized out>, secid=<optimized out>) at security/security.c:952 > > 4 0xffffffff8109ac80 in check_kill_permission (sig=15, info=0x0 <irq_stack_union>, t=0xffff88003b0b8000) at kernel/signal.c:796 > > 5 0xffffffff8109d3ab in group_send_sig_info (sig=15, info=0x0 <irq_stack_union>, p=0xffff88003b0b8000) at kernel/signal.c:1296 > > 6 0xffffffff8108e527 in forget_original_parent (father=<optimized out>) at kernel/exit.c:575 > > 7 exit_notify (group_dead=<optimized out>, tsk=<optimized out>) at kernel/exit.c:606 > > 8 do_exit (code=<optimized out>) at kernel/exit.c:775 > > 9 0xffffffff8108ec0f in do_group_exit (exit_code=0) at kernel/exit.c:891 > > 10 0xffffffff8108ec84 in SYSC_exit_group (error_code=<optimized out>) at kernel/exit.c:902 > > 11 SyS_exit_group (error_code=<optimized out>) at kernel/exit.c:900 > > > > This backtrace clearly shows that there is an LSM hook task_kill() that > > happens during an exit_group() syscall and that this happens after > > exit_task_namespaces(). LSM hooks with namespaces might need nsproxy to > > be able to check for capabilities. At this point this is impossible. The > > current's nsproxy is already NULL/destroyed. > > > > This is the case because exit_task_namespaces() is called before the > > exit_notify() where all of the above happens. This patch changes their > > order. > > Nacked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> > > current->nsproxy->user_ns does not exist, > and changing where exit_task_namespaces is fragile and I am really not > interested in messing with it right now, to solve a problem that does > not exist. I must have missed the moment where current->nsproxy->user_ns was removed. I obviously even don't use it in my patches anymore (replaced with cred->user_ns). Back when I started to write my patches and wanted to use current->nsproxy->user_ns in LSM hooks the problem was real. Fortunately current->cred->user_ns does not exhibit the same issue. I'll drop this patch. Sorry for the confusion. > > > > > Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@xxxxxxxxxxx> > > --- > > kernel/exit.c | 8 +++++++- > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/exit.c b/kernel/exit.c > > index 22fcc05..da1bb18 100644 > > --- a/kernel/exit.c > > +++ b/kernel/exit.c > > @@ -742,7 +742,6 @@ void do_exit(long code) > > exit_fs(tsk); > > if (group_dead) > > disassociate_ctty(1); > > - exit_task_namespaces(tsk); > > exit_task_work(tsk); > > exit_thread(); > > > > @@ -763,6 +762,13 @@ void do_exit(long code) > > > > TASKS_RCU(tasks_rcu_i = __srcu_read_lock(&tasks_rcu_exit_srcu)); > > exit_notify(tsk, group_dead); > > + > > + /* > > + * This should be after all things that potentially require > > + * process's namespaces (e.g. capability checks). > > + */ > > + exit_task_namespaces(tsk); > > + > > proc_exit_connector(tsk); > > #ifdef CONFIG_NUMA > > task_lock(tsk); > -- > To unsubscribe from this list: send the line "unsubscribe linux-security-module" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Lukasz Pawelczyk Samsung R&D Institute Poland Samsung Electronics -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
