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seccomp: release filter after task is fully dead

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The seccomp filter used to be released in free_task() which is called
asynchronously via call_rcu() and assorted mechanisms. Since we need
to inform tasks waiting on the seccomp notifier when a filter goes empty
we will notify them as soon as a task has been marked fully dead in
release_task(). To not split seccomp cleanup into two parts, move
filter release out of free_task() and into release_task() after we've
unhashed struct task from struct pid, exited signals, and unlinked it
from the threadgroups' thread list. We'll put the empty filter
notification infrastructure into it in a follow up patch.

This also renames put_seccomp_filter() to seccomp_filter_release() which
is a more descriptive name of what we're doing here especially once
we've added the empty filter notification mechanism in there.

We're also NULL-ing the task's filter tree entrypoint which seems
cleaner than leaving a dangling pointer in there. Note that this shouldn't
need any memory barriers since we're calling this when the task is in
release_task() which means it's EXIT_DEAD. So it can't modify its seccomp
filters anymore. You can also see this from the point where we're calling
seccomp_filter_release(). It's after __exit_signal() and at this point,
tsk->sighand will already have been NULLed which is required for
thread-sync and filter installation alike.

Cc: Tycho Andersen <tycho@tycho.ws>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matt Denton <mpdenton@google.com>
Cc: Sargun Dhillon <sargun@sargun.me>
Cc: Jann Horn <jannh@google.com>
Cc: Chris Palmer <palmer@google.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Robert Sesek <rsesek@google.com>
Cc: Jeffrey Vander Stoep <jeffv@google.com>
Cc: Linux Containers <containers@lists.linux-foundation.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200531115031.391515-2-christian.brauner@ubuntu.com
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
Christian Brauner 2020-05-31 13:50:29 +02:00 committed by Kees Cook
parent b707ddee11
commit 3a15fb6ed9
4 changed files with 40 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
include/linux/seccomp.h
View File

@ -84,10 +84,10 @@ static inline int seccomp_mode(struct seccomp *s)
#endif /* CONFIG_SECCOMP */
#ifdef CONFIG_SECCOMP_FILTER
extern void put_seccomp_filter(struct task_struct *tsk);
extern void seccomp_filter_release(struct task_struct *tsk);
extern void get_seccomp_filter(struct task_struct *tsk);
#else /* CONFIG_SECCOMP_FILTER */
static inline void put_seccomp_filter(struct task_struct *tsk)
static inline void seccomp_filter_release(struct task_struct *tsk)
{
return;
}

1
kernel/exit.c
View File

@ -217,6 +217,7 @@ repeat:
}
write_unlock_irq(&tasklist_lock);
seccomp_filter_release(p);
proc_flush_pid(thread_pid);
put_pid(thread_pid);
release_thread(p);

1
kernel/fork.c
View File

@ -473,7 +473,6 @@ void free_task(struct task_struct *tsk)
#endif
rt_mutex_debug_task_free(tsk);
ftrace_graph_exit_task(tsk);
put_seccomp_filter(tsk);
arch_release_task_struct(tsk);
if (tsk->flags & PF_KTHREAD)
free_kthread_struct(tsk);

62
kernel/seccomp.c
View File

@ -368,6 +368,42 @@ static inline pid_t seccomp_can_sync_threads(void)
return 0;
}
static inline void seccomp_filter_free(struct seccomp_filter *filter)
{
if (filter) {
bpf_prog_destroy(filter->prog);
kfree(filter);
}
}
static void __put_seccomp_filter(struct seccomp_filter *orig)
{
/* Clean up single-reference branches iteratively. */
while (orig && refcount_dec_and_test(&orig->refs)) {
struct seccomp_filter *freeme = orig;
orig = orig->prev;
seccomp_filter_free(freeme);
}
}
/**
* seccomp_filter_release - Detach the task from its filter tree
* and drop its reference count during
* exit.
*
* This function should only be called when the task is exiting as
* it detaches it from its filter tree. As such, READ_ONCE() and
* barriers are not needed here, as would normally be needed.
*/
void seccomp_filter_release(struct task_struct *tsk)
{
struct seccomp_filter *orig = tsk->seccomp.filter;
/* Detach task from its filter tree. */
tsk->seccomp.filter = NULL;
__put_seccomp_filter(orig);
}
/**
* seccomp_sync_threads: sets all threads to use current's filter
*
@ -397,7 +433,7 @@ static inline void seccomp_sync_threads(unsigned long flags)
* current's path will hold a reference. (This also
* allows a put before the assignment.)
*/
put_seccomp_filter(thread);
__put_seccomp_filter(thread->seccomp.filter);
smp_store_release(&thread->seccomp.filter,
caller->seccomp.filter);
atomic_set(&thread->seccomp.filter_count,
@ -571,30 +607,6 @@ void get_seccomp_filter(struct task_struct *tsk)
__get_seccomp_filter(orig);
}
static inline void seccomp_filter_free(struct seccomp_filter *filter)
{
if (filter) {
bpf_prog_destroy(filter->prog);
kfree(filter);
}
}
static void __put_seccomp_filter(struct seccomp_filter *orig)
{
/* Clean up single-reference branches iteratively. */
while (orig && refcount_dec_and_test(&orig->refs)) {
struct seccomp_filter *freeme = orig;
orig = orig->prev;
seccomp_filter_free(freeme);
}
}
/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
void put_seccomp_filter(struct task_struct *tsk)
{
__put_seccomp_filter(tsk->seccomp.filter);
}
static void seccomp_init_siginfo(kernel_siginfo_t *info, int syscall, int reason)
{
clear_siginfo(info);