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Peter Anvin" , x86@kernel.org, Peter Zijlstra , Arnd Bergmann , "Eric W. Biederman" , Khalid Aziz , Kate Stewart , Helge Deller , Greg Kroah-Hartman , Al Viro , Andrew Morton , Christian Brauner , Catalin Marinas , Will Deacon , Dave Martin , Mauro Carvalho Chehab , Michal Hocko , Rik van Riel , "Kirill A. Shutemov" , Roman Gushchin , Marcos Paulo de Souza , Oleg Nesterov , Dominik Brodowski , Cyrill Gorcunov , Yang Shi , Jann Horn , Kees Cook , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, "Victor Kamensky (kamensky)" , xe-linux-external@cisco.com, Stefan Strogin Subject: Re: [PATCH] kernel/signal: Signal-based pre-coredump notification Message-ID: <20181013104446.b3z26rwmiripqfks@gmail.com> Reply-To: christian@brauner.io References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 12, 2018 at 05:33:35PM -0700, Enke Chen wrote: > For simplicity and consistency, this patch provides an implementation > for signal-based fault notification prior to the coredump of a child > process. A new prctl command, PR_SET_PREDUMP_SIG, is defined that can > be used by an application to express its interest and to specify the > signal (SIGCHLD or SIGUSR1 or SIGUSR2) for such a notification. A new > signal code (si_code), CLD_PREDUMP, is also defined for SIGCHLD. > > Background: > > As the coredump of a process may take time, in certain time-sensitive > applications it is necessary for a parent process (e.g., a process > manager) to be notified of a child's imminent death before the coredump > so that the parent process can act sooner, such as re-spawning an > application process, or initiating a control-plane fail-over. > > Currently there are two ways for a parent process to be notified of a > child process's state change. One is to use the POSIX signal, and > another is to use the kernel connector module. The specific events and > actions are summarized as follows: > > Process Event POSIX Signal Connector-based > ---------------------------------------------------------------------- > ptrace_attach() do_notify_parent_cldstop() proc_ptrace_connector() > SIGCHLD / CLD_STOPPED > > ptrace_detach() do_notify_parent_cldstop() proc_ptrace_connector() > SIGCHLD / CLD_CONTINUED > > pre_coredump/ N/A proc_coredump_connector() > get_signal() > > post_coredump/ do_notify_parent() proc_exit_connector() > do_exit() SIGCHLD / exit_signal > ---------------------------------------------------------------------- > > As shown in the table, the signal-based pre-coredump notification is not > currently available. In some cases using a connector-based notification > can be quite complicated (e.g., when a process manager is written in shell > scripts and thus is subject to certain inherent limitations), and a > signal-based notification would be simpler and better suited. > > Signed-off-by: Enke Chen > --- > arch/x86/kernel/signal_compat.c | 2 +- > include/linux/sched.h | 4 ++ > include/linux/signal.h | 5 +++ > include/uapi/asm-generic/siginfo.h | 3 +- > include/uapi/linux/prctl.h | 4 ++ > kernel/fork.c | 1 + > kernel/signal.c | 51 +++++++++++++++++++++++++ > kernel/sys.c | 77 ++++++++++++++++++++++++++++++++++++++ > 8 files changed, 145 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c > index 9ccbf05..a3deba8 100644 > --- a/arch/x86/kernel/signal_compat.c > +++ b/arch/x86/kernel/signal_compat.c > @@ -30,7 +30,7 @@ static inline void signal_compat_build_tests(void) > BUILD_BUG_ON(NSIGSEGV != 7); > BUILD_BUG_ON(NSIGBUS != 5); > BUILD_BUG_ON(NSIGTRAP != 5); > - BUILD_BUG_ON(NSIGCHLD != 6); > + BUILD_BUG_ON(NSIGCHLD != 7); > BUILD_BUG_ON(NSIGSYS != 1); > > /* This is part of the ABI and can never change in size: */ > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 09026ea..cfb9645 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -696,6 +696,10 @@ struct task_struct { > int exit_signal; > /* The signal sent when the parent dies: */ > int pdeath_signal; > + > + /* The signal sent prior to a child's coredump: */ > + int predump_signal; > + > /* JOBCTL_*, siglock protected: */ > unsigned long jobctl; > > diff --git a/include/linux/signal.h b/include/linux/signal.h > index 706a499..7cb976d 100644 > --- a/include/linux/signal.h > +++ b/include/linux/signal.h > @@ -256,6 +256,11 @@ static inline int valid_signal(unsigned long sig) > return sig <= _NSIG ? 1 : 0; > } > > +static inline int valid_predump_signal(int sig) > +{ > + return (sig == SIGCHLD) || (sig == SIGUSR1) || (sig == SIGUSR2); > +} > + > struct timespec; > struct pt_regs; > enum pid_type; > diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h > index cb3d6c2..1a47cef 100644 > --- a/include/uapi/asm-generic/siginfo.h > +++ b/include/uapi/asm-generic/siginfo.h > @@ -267,7 +267,8 @@ struct { \ > #define CLD_TRAPPED 4 /* traced child has trapped */ > #define CLD_STOPPED 5 /* child has stopped */ > #define CLD_CONTINUED 6 /* stopped child has continued */ > -#define NSIGCHLD 6 > +#define CLD_PREDUMP 7 /* child is about to dump core */ > +#define NSIGCHLD 7 > > /* > * SIGPOLL (or any other signal without signal specific si_codes) si_codes > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h > index c0d7ea0..79f0a8a 100644 > --- a/include/uapi/linux/prctl.h > +++ b/include/uapi/linux/prctl.h > @@ -219,4 +219,8 @@ struct prctl_mm_map { > # define PR_SPEC_DISABLE (1UL << 2) > # define PR_SPEC_FORCE_DISABLE (1UL << 3) > > +/* Whether to receive signal prior to child's coredump */ > +#define PR_SET_PREDUMP_SIG 54 > +#define PR_GET_PREDUMP_SIG 55 > + > #endif /* _LINUX_PRCTL_H */ > diff --git a/kernel/fork.c b/kernel/fork.c > index 07cddff..c296c11 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1985,6 +1985,7 @@ static __latent_entropy struct task_struct *copy_process( > p->dirty_paused_when = 0; > > p->pdeath_signal = 0; > + p->predump_signal = 0; > INIT_LIST_HEAD(&p->thread_group); > p->task_works = NULL; > > diff --git a/kernel/signal.c b/kernel/signal.c > index 312b43e..eb4a483 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -2337,6 +2337,44 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info) > return signr; > } > > +/* > + * Let the parent, if so desired, know about the imminent death of a child > + * prior to its coredump. > + * > + * Locking logic is similar to do_notify_parent_cldstop(). > + */ > +static void do_notify_parent_predump(struct task_struct *tsk) > +{ > + struct sighand_struct *sighand; > + struct task_struct *parent; > + struct kernel_siginfo info; > + unsigned long flags; > + int sig; > + > + parent = tsk->real_parent; > + sig = parent->predump_signal; > + > + /* Check again with "tasklist_lock" locked by the caller */ > + if (!valid_predump_signal(sig)) > + return; > + > + clear_siginfo(&info); > + info.si_signo = sig; > + if (sig == SIGCHLD) > + info.si_code = CLD_PREDUMP; > + > + rcu_read_lock(); > + info.si_pid = task_pid_nr_ns(tsk, task_active_pid_ns(parent)); > + info.si_uid = from_kuid_munged(task_cred_xxx(parent, user_ns), > + task_uid(tsk)); > + rcu_read_unlock(); > + > + sighand = parent->sighand; > + spin_lock_irqsave(&sighand->siglock, flags); > + __group_send_sig_info(sig, &info, parent); > + spin_unlock_irqrestore(&sighand->siglock, flags); > +} > + > bool get_signal(struct ksignal *ksig) > { > struct sighand_struct *sighand = current->sighand; > @@ -2497,6 +2535,19 @@ bool get_signal(struct ksignal *ksig) > current->flags |= PF_SIGNALED; > > if (sig_kernel_coredump(signr)) { > + /* > + * Notify the parent prior to the coredump if the > + * parent is interested in such a notificaiton. > + */ > + int p_sig = current->real_parent->predump_signal; > + > + if (valid_predump_signal(p_sig)) { > + read_lock(&tasklist_lock); > + do_notify_parent_predump(current); > + read_unlock(&tasklist_lock); > + cond_resched(); > + } > + > if (print_fatal_signals) > print_fatal_signal(ksig->info.si_signo); > proc_coredump_connector(current); > diff --git a/kernel/sys.c b/kernel/sys.c > index 123bd73..43eb250d 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2258,6 +2258,76 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which, > return -EINVAL; > } > > +static int prctl_get_predump_signal(struct task_struct *tsk, pid_t pid, > + int __user *addr) > +{ > + struct task_struct *p; > + int error; > + > + /* For the current task, the common case */ > + if (pid == 0) { > + put_user(tsk->predump_signal, addr); > + return 0; > + } > + > + error = -ESRCH; > + rcu_read_lock(); > + p = find_task_by_vpid(pid); > + if (p) { > + error = 0; > + put_user(p->predump_signal, addr); > + } > + rcu_read_unlock(); > + return error; > +} > + > +/* > + * Returns true if current's euid is same as p's uid or euid, > + * or has CAP_SYS_ADMIN. > + * > + * Called with rcu_read_lock, creds are safe. > + * > + * Adapted from set_one_prio_perm(). > + */ > +static bool set_predump_signal_perm(struct task_struct *p) > +{ > + const struct cred *cred = current_cred(), *pcred = __task_cred(p); > + > + return uid_eq(pcred->uid, cred->euid) || > + uid_eq(pcred->euid, cred->euid) || > + capable(CAP_SYS_ADMIN); So before proceeding I'd like to discuss at least two points: - how does this interact with the dumpability of a process? - do we need the capable(CAP_SYS_ADMIN) restriction to init_user_ns? Seems we could make this work per-user-ns just like PRCTL_SET_PDEATHSIG does? > +} > + > +static int prctl_set_predump_signal(struct task_struct *tsk, pid_t pid, int sig) > +{ > + struct task_struct *p; > + int error; > + > + /* 0 is valid for disabling the feature */ > + if (sig && !valid_predump_signal(sig)) > + return -EINVAL; > + > + /* For the current task, the common case */ > + if (pid == 0) { > + tsk->predump_signal = sig; > + return 0; > + } > + > + error = -ESRCH; > + rcu_read_lock(); > + p = find_task_by_vpid(pid); > + if (p) { > + if (!set_predump_signal_perm(p)) > + error = -EPERM; > + else { > + error = 0; > + p->predump_signal = sig; > + } > + } > + rcu_read_unlock(); > + return error; > +} > + > SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > unsigned long, arg4, unsigned long, arg5) > { > @@ -2476,6 +2546,13 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which, > return -EINVAL; > error = arch_prctl_spec_ctrl_set(me, arg2, arg3); > break; > + case PR_SET_PREDUMP_SIG: > + error = prctl_set_predump_signal(me, (pid_t)arg2, (int)arg3); > + break; > + case PR_GET_PREDUMP_SIG: > + error = prctl_get_predump_signal(me, (pid_t)arg2, > + (int __user *)arg3); > + break; > default: > error = -EINVAL; > break; > -- > 1.8.3.1