Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited process killing

Christian Brauner
2019-04-11T10:30:26Z

On Wed, Apr 10, 2019 at 06:43:53PM -0700, Suren Baghdasaryan wrote:
> Add new SS_EXPEDITE flag to be used when sending SIGKILL via
> pidfd_send_signal() syscall to allow expedited memory reclaim of the
> victim process. The usage of this flag is currently limited to SIGKILL
> signal and only to privileged users.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> include/linux/sched/signal.h | 3 ++-
> include/linux/signal.h | 11 ++++++++++-
> ipc/mqueue.c | 2 +-
> kernel/signal.c | 37 ++++++++++++++++++++++++++++--------
> kernel/time/itimer.c | 2 +-
> 5 files changed, 43 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index e412c092c1e8..8a227633a058 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -327,7 +327,8 @@ extern int send_sig_info(int, struct kernel_siginfo *, struct task_struct *);
> extern void force_sigsegv(int sig, struct task_struct *p);
> extern int force_sig_info(int, struct kernel_siginfo *, struct task_struct *);
> extern int __kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp);
> -extern int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid);
> +extern int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid,
> + bool expedite);
> extern int kill_pid_info_as_cred(int, struct kernel_siginfo *, struct pid *,
> const struct cred *);
> extern int kill_pgrp(struct pid *pid, int sig, int priv);
> diff --git a/include/linux/signal.h b/include/linux/signal.h
> index 9702016734b1..34b7852aa4a0 100644
> --- a/include/linux/signal.h
> +++ b/include/linux/signal.h
> @@ -446,8 +446,17 @@ int __save_altstack(stack_t __user *, unsigned long);
> } while (0);
>
> #ifdef CONFIG_PROC_FS
> +
> +/*
> + * SS_FLAGS values used in pidfd_send_signal:
> + *
> + * SS_EXPEDITE indicates desire to expedite the operation.
> + */
> +#define SS_EXPEDITE 0x00000001

Does this make sense as an SS_* flag?
How does this relate to the signal stack?
Is there any intention to ever use this flag with stack_t?

New flags should be PIDFD_SIGNAL_*. (E.g. the thread flag will be
PIDFD_SIGNAL_THREAD.)
And since this is exposed to userspace in contrast to the mm internal
naming it should be something more easily understandable like
PIDFD_SIGNAL_MM_RECLAIM{_FASTER} or something.

> +
> struct seq_file;
> extern void render_sigset_t(struct seq_file *, const char *, sigset_t *);
> -#endif
> +
> +#endif /* CONFIG_PROC_FS */
>
> #endif /* _LINUX_SIGNAL_H */
> diff --git a/ipc/mqueue.c b/ipc/mqueue.c
> index aea30530c472..27c66296e08e 100644
> --- a/ipc/mqueue.c
> +++ b/ipc/mqueue.c
> @@ -720,7 +720,7 @@ static void __do_notify(struct mqueue_inode_info *info)
> rcu_read_unlock();
>
> kill_pid_info(info->notify.sigev_signo,
> - &sig_i, info->notify_owner);
> + &sig_i, info->notify_owner, false);
> break;
> case SIGEV_THREAD:
> set_cookie(info->notify_cookie, NOTIFY_WOKENUP);
> diff --git a/kernel/signal.c b/kernel/signal.c
> index f98448cf2def..02ed4332d17c 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -43,6 +43,7 @@
> #include <linux/compiler.h>
> #include <linux/posix-timers.h>
> #include <linux/livepatch.h>
> +#include <linux/oom.h>
>
> #define CREATE_TRACE_POINTS
> #include <trace/events/signal.h>
> @@ -1394,7 +1395,8 @@ int __kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp)
> return success ? 0 : retval;
> }
>
> -int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid)
> +int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid,
> + bool expedite)
> {
> int error = -ESRCH;
> struct task_struct *p;
> @@ -1402,8 +1404,17 @@ int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid)
> for (;;) {
> rcu_read_lock();
> p = pid_task(pid, PIDTYPE_PID);
> - if (p)
> + if (p) {
> error = group_send_sig_info(sig, info, p, PIDTYPE_TGID);
> +
> + /*
> + * Ignore expedite_reclaim return value, it is best
> + * effort only.
> + */
> + if (!error && expedite)
> + expedite_reclaim(p);

SIGKILL will take the whole thread group down so the reclaim should make
sense here.

> + }
> +
> rcu_read_unlock();
> if (likely(!p || error != -ESRCH))
> return error;
> @@ -1420,7 +1431,7 @@ static int kill_proc_info(int sig, struct kernel_siginfo *info, pid_t pid)
> {
> int error;
> rcu_read_lock();
> - error = kill_pid_info(sig, info, find_vpid(pid));
> + error = kill_pid_info(sig, info, find_vpid(pid), false);
> rcu_read_unlock();
> return error;
> }
> @@ -1487,7 +1498,7 @@ static int kill_something_info(int sig, struct kernel_siginfo *info, pid_t pid)
>
> if (pid > 0) {
> rcu_read_lock();
> - ret = kill_pid_info(sig, info, find_vpid(pid));
> + ret = kill_pid_info(sig, info, find_vpid(pid), false);
> rcu_read_unlock();
> return ret;
> }
> @@ -1704,7 +1715,7 @@ EXPORT_SYMBOL(kill_pgrp);
>
> int kill_pid(struct pid *pid, int sig, int priv)
> {
> - return kill_pid_info(sig, __si_special(priv), pid);
> + return kill_pid_info(sig, __si_special(priv), pid, false);
> }
> EXPORT_SYMBOL(kill_pid);
>
> @@ -3577,10 +3588,20 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> struct pid *pid;
> kernel_siginfo_t kinfo;
>
> - /* Enforce flags be set to 0 until we add an extension. */
> - if (flags)
> + /* Enforce no unknown flags. */
> + if (flags & ~SS_EXPEDITE)
> return -EINVAL;
>
> + if (flags & SS_EXPEDITE) {
> + /* Enforce SS_EXPEDITE to be used with SIGKILL only. */
> + if (sig != SIGKILL)
> + return -EINVAL;

Not super fond of this being a SIGKILL specific flag but I get why.

> +
> + /* Limit expedited killing to privileged users only. */
> + if (!capable(CAP_SYS_NICE))
> + return -EPERM;

Do you have a specific (DOS or other) attack vector in mind that renders
ns_capable unsuitable?

> + }
> +
> f = fdget_raw(pidfd);
> if (!f.file)
> return -EBADF;
> @@ -3614,7 +3635,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> prepare_kill_siginfo(sig, &kinfo);
> }
>
> - ret = kill_pid_info(sig, &kinfo, pid);
> + ret = kill_pid_info(sig, &kinfo, pid, (flags & SS_EXPEDITE) != 0);
>
> err:
> fdput(f);
> diff --git a/kernel/time/itimer.c b/kernel/time/itimer.c
> index 02068b2d5862..c926483cdb53 100644
> --- a/kernel/time/itimer.c
> +++ b/kernel/time/itimer.c
> @@ -140,7 +140,7 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
> struct pid *leader_pid = sig->pids[PIDTYPE_TGID];
>
> trace_itimer_expire(ITIMER_REAL, leader_pid, 0);
> - kill_pid_info(SIGALRM, SEND_SIG_PRIV, leader_pid);
> + kill_pid_info(SIGALRM, SEND_SIG_PRIV, leader_pid, false);
>
> return HRTIMER_NORESTART;
> }
> --
> 2.21.0.392.gf8f6787159e-goog
>