pidfd: allow to override signal scope in pidfd_send_signal()

Right now we determine the scope of the signal based on the type of
pidfd. There are use-cases where it's useful to override the scope of
the signal. For example in [1]. Add flags to determine the scope of the
signal:

(1) PIDFD_SIGNAL_THREAD: send signal to specific thread reference by @pidfd
(2) PIDFD_SIGNAL_THREAD_GROUP: send signal to thread-group of @pidfd
(2) PIDFD_SIGNAL_PROCESS_GROUP: send signal to process-group of @pidfd

Since we now allow specifying PIDFD_SEND_PROCESS_GROUP for
pidfd_send_signal() to send signals to process groups we need to adjust
the check restricting si_code emulation by userspace to account for
PIDTYPE_PGID.

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://github.com/systemd/systemd/issues/31093
Link: https://lore.kernel.org/r/20240210-chihuahua-hinzog-3945b6abd44a@brauner
Link: https://lore.kernel.org/r/20240214123655.GB16265@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
index 2e64614..72ec000 100644 (file)
--- a/include/uapi/linux/pidfd.h
+++ b/include/uapi/linux/pidfd.h
@@ -10,4 +10,9 @@
 #define PIDFD_NONBLOCK O_NONBLOCK
 #define PIDFD_THREAD   O_EXCL
 
+/* Flags for pidfd_send_signal(). */
+#define PIDFD_SIGNAL_THREAD            (1UL << 0)
+#define PIDFD_SIGNAL_THREAD_GROUP      (1UL << 1)
+#define PIDFD_SIGNAL_PROCESS_GROUP     (1UL << 2)
+
 #endif /* _UAPI_LINUX_PIDFD_H */

diff --git a/kernel/signal.c b/kernel/signal.c
index 8b81696..bdca529 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1905,16 +1905,19 @@ int send_sig_fault_trapno(int sig, int code, void __user *addr, int trapno,
        return send_sig_info(info.si_signo, &info, t);
 }
 
-int kill_pgrp(struct pid *pid, int sig, int priv)
+static int kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp)
 {
        int ret;
-
        read_lock(&tasklist_lock);
-       ret = __kill_pgrp_info(sig, __si_special(priv), pid);
+       ret = __kill_pgrp_info(sig, info, pgrp);
        read_unlock(&tasklist_lock);
-
        return ret;
 }
+
+int kill_pgrp(struct pid *pid, int sig, int priv)
+{
+       return kill_pgrp_info(sig, __si_special(priv), pid);
+}
 EXPORT_SYMBOL(kill_pgrp);
 
 int kill_pid(struct pid *pid, int sig, int priv)
@@ -3873,6 +3876,10 @@ static struct pid *pidfd_to_pid(const struct file *file)
        return tgid_pidfd_to_pid(file);
 }
 
+#define PIDFD_SEND_SIGNAL_FLAGS                            \
+       (PIDFD_SIGNAL_THREAD | PIDFD_SIGNAL_THREAD_GROUP | \
+        PIDFD_SIGNAL_PROCESS_GROUP)
+
 /**
  * sys_pidfd_send_signal - Signal a process through a pidfd
  * @pidfd:  file descriptor of the process
@@ -3897,7 +3904,11 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
        enum pid_type type;
 
        /* Enforce flags be set to 0 until we add an extension. */
-       if (flags)
+       if (flags & ~PIDFD_SEND_SIGNAL_FLAGS)
+               return -EINVAL;
+
+       /* Ensure that only a single signal scope determining flag is set. */
+       if (hweight32(flags & PIDFD_SEND_SIGNAL_FLAGS) > 1)
                return -EINVAL;
 
        f = fdget(pidfd);
@@ -3915,10 +3926,24 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
        if (!access_pidfd_pidns(pid))
                goto err;
 
-       if (f.file->f_flags & PIDFD_THREAD)
+       switch (flags) {
+       case 0:
+               /* Infer scope from the type of pidfd. */
+               if (f.file->f_flags & PIDFD_THREAD)
+                       type = PIDTYPE_PID;
+               else
+                       type = PIDTYPE_TGID;
+               break;
+       case PIDFD_SIGNAL_THREAD:
                type = PIDTYPE_PID;
-       else
+               break;
+       case PIDFD_SIGNAL_THREAD_GROUP:
                type = PIDTYPE_TGID;
+               break;
+       case PIDFD_SIGNAL_PROCESS_GROUP:
+               type = PIDTYPE_PGID;
+               break;
+       }
 
        if (info) {
                ret = copy_siginfo_from_user_any(&kinfo, info);
@@ -3931,14 +3956,17 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 
                /* Only allow sending arbitrary signals to yourself. */
                ret = -EPERM;
-               if ((task_pid(current) != pid) &&
+               if ((task_pid(current) != pid || type > PIDTYPE_TGID) &&
                    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
                        goto err;
        } else {
                prepare_kill_siginfo(sig, &kinfo, type);
        }
 
-       ret = kill_pid_info_type(sig, &kinfo, pid, type);
+       if (type == PIDTYPE_PGID)
+               ret = kill_pgrp_info(sig, &kinfo, pid);
+       else
+               ret = kill_pid_info_type(sig, &kinfo, pid, type);
 err:
        fdput(f);
        return ret;