Date:   Fri, 7 Dec 2018 10:47:31 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        luto@...nel.org, arnd@...db.de, ebiederm@...ssion.com,
        serge@...lyn.com, jannh@...gle.com, akpm@...ux-foundation.org,
        oleg@...hat.com, cyphar@...har.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, dancol@...gle.com,
        timmurray@...gle.com, linux-man@...r.kernel.org,
        keescook@...omium.org, fweimer@...hat.com, tglx@...utronix.de,
        x86@...nel.org
Subject: Re: [PATCH v4] signal: add taskfd_send_signal() syscall

On Thu, Dec 06, 2018 at 01:18:58PM +0100, Christian Brauner wrote:
> The kill() syscall operates on process identifiers (pid). After a process
> has exited its pid can be reused by another process. If a caller sends a
> signal to a reused pid it will end up signaling the wrong process. This
> issue has often surfaced and there has been a push [1] to address this
> problem.
> 
> This patch uses file descriptors (fd) from proc/<pid> as stable handles on
> struct pid. Even if a pid is recycled the handle will not change. The fd
> can be used to send signals to the process it refers to.
> Thus, the new syscall taskfd_send_signal() is introduced to solve this
> problem. Instead of pids it operates on process fds (taskfd).
> 
> /* prototype and argument /*
> long taskfd_send_signal(int taskfd, int sig, siginfo_t *info, unsigned int flags);
> 
> In addition to the taskfd and signal argument it takes an additional
> siginfo_t and flags argument. If the siginfo_t argument is NULL then
> taskfd_send_signal() behaves like kill(). If it is not NULL
> taskfd_send_signal() behaves like rt_sigqueueinfo().
> The flags argument is added to allow for future extensions of this syscall.
> It currently needs to be passed as 0. Failing to do so will cause EINVAL.
> 
> /* taskfd_send_signal() replaces multiple pid-based syscalls */
> The taskfd_send_signal() syscall currently takes on the job of the
> following syscalls that operate on pids:
> - kill(2)
> - rt_sigqueueinfo(2)
> The syscall is defined in such a way that it can also operate on thread fds
> instead of process fds. In a future patchset I will extend it to operate on
> taskfds from /proc/<pid>/task/<tid> at which point it will additionally
> take on the job of:
> - tgkill(2)
> - rt_tgsigqueueinfo(2)
> Right now this is intentionally left out to keep this patchset as simple as
> possible (cf. [4]). If a taskfd of /proc/<pid>/task/<tid> is passed
> EOPNOTSUPP will be returned to give userspace a way to detect when I add
> support for such taskfds (cf. [10]).
> 
> /* naming */
> The original prefix of the syscall was "procfd_". However, it has been
> pointed out that since this syscall will eventually operate on both
> processes and threads the name should reflect this (cf. [12]). The best
> possible candidate even from a userspace perspective seems to be "task".
> Although "task" is used internally we are alreday deviating from POSIX by
> using file descriptors to processes in the first place so it seems fine to
> use the "taskfd_" prefix.
> 
> The name taskfd_send_signal() was also chosen to reflect the fact that it
> takes on the job of multiple syscalls. It is intentional that the name is
> not reminiscent of neither kill(2) nor rt_sigqueueinfo(2). Not the fomer
> because it might imply that taskfd_send_signal() is only a replacement for
> kill(2) and not the latter because it is a hazzle to remember the correct
> spelling (especially for non-native speakers) and because it is not
> descriptive enough of what the syscall actually does. The name
> "taskfd_send_signal" makes it very clear that its job is to send signals.
> 
> /* O_PATH file descriptors */
> taskfds opened as O_PATH fds cannot be used to send signals to a process
> (cf. [2]). Signaling processes through taskfds is the equivalent of writing
> to a file. Thus, this is not an operation that operates "purely at the
> file descriptor level" as required by the open(2) manpage.
> 
> /* zombies */
> Zombies can be signaled just as any other process. No special error will be
> reported since a zombie state is an unreliable state (cf. [3]).
> 
> /* cross-namespace signals */
> The patch currently enforces that the signaler and signalee either are in
> the same pid namespace or that the signaler's pid namespace is an ancestor
> of the signalee's pid namespace. This is done for the sake of simplicity
> and because it is unclear to what values certain members of struct
> siginfo_t would need to be set to (cf. [5], [6]).
> 
> /* compat syscalls */
> It became clear that we would like to avoid adding compat syscalls (cf.
> [7]). The compat syscall handling is now done in kernel/signal.c itself by
> adding __copy_siginfo_from_user_generic() which lets us avoid compat
> syscalls (cf. [8]). It should be noted that the addition of
> __copy_siginfo_from_user_any() is caused by a bug in the original
> implementation of rt_sigqueueinfo(2) (cf. 12).
> With upcoming rework for syscall handling things might improve
> significantly (cf. [11]) and __copy_siginfo_from_user_any() will not gain
> any additional callers.
> 
> /* testing */
> This patch was tested on x64 and x86.
> 
> /* userspace usage */
> An asciinema recording for the basic functionality can be found under [9].
> With this patch a process can be killed via:
> 
>  #define _GNU_SOURCE
>  #include <errno.h>
>  #include <fcntl.h>
>  #include <signal.h>
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <string.h>
>  #include <sys/stat.h>
>  #include <sys/syscall.h>
>  #include <sys/types.h>
>  #include <unistd.h>
> 
>  static inline int do_taskfd_send_signal(int taskfd, int sig, siginfo_t *info,
>                                          unsigned int flags)
>  {
>  #ifdef __NR_taskfd_send_signal
>          return syscall(__NR_taskfd_send_signal, taskfd, sig, info, flags);
>  #else
>          return -ENOSYS;
>  #endif
>  }
> 
>  int main(int argc, char *argv[])
>  {
>          int fd, ret, saved_errno, sig;
> 
>          if (argc < 3)
>                  exit(EXIT_FAILURE);
> 
>          fd = open(argv[1], O_DIRECTORY | O_CLOEXEC);
>          if (fd < 0) {
>                  printf("%s - Failed to open \"%s\"\n", strerror(errno), argv[1]);
>                  exit(EXIT_FAILURE);
>          }
> 
>          sig = atoi(argv[2]);
> 
>          printf("Sending signal %d to process %s\n", sig, argv[1]);
>          ret = do_taskfd_send_signal(fd, sig, NULL, 0);
> 
>          saved_errno = errno;
>          close(fd);
>          errno = saved_errno;
> 
>          if (ret < 0) {
>                  printf("%s - Failed to send signal %d to process %s\n",
>                         strerror(errno), sig, argv[1]);
>                  exit(EXIT_FAILURE);
>          }
> 
>          exit(EXIT_SUCCESS);
>  }
> 
> [1]:  https://lore.kernel.org/lkml/20181029221037.87724-1-dancol@google.com/
> [2]:  https://lore.kernel.org/lkml/874lbtjvtd.fsf@oldenburg2.str.redhat.com/
> [3]:  https://lore.kernel.org/lkml/20181204132604.aspfupwjgjx6fhva@brauner.io/
> [4]:  https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
> [5]:  https://lore.kernel.org/lkml/20181121213946.GA10795@mail.hallyn.com/
> [6]:  https://lore.kernel.org/lkml/20181120103111.etlqp7zop34v6nv4@brauner.io/
> [7]:  https://lore.kernel.org/lkml/36323361-90BD-41AF-AB5B-EE0D7BA02C21@amacapital.net/
> [8]:  https://lore.kernel.org/lkml/87tvjxp8pc.fsf@xmission.com/
> [9]:  https://asciinema.org/a/IQjuCHew6bnq1cr78yuMv16cy
> [10]: https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
> [11]: https://lore.kernel.org/lkml/F53D6D38-3521-4C20-9034-5AF447DF62FF@amacapital.net/
> [12]: https://lore.kernel.org/lkml/87zhtjn8ck.fsf@xmission.com/
> 
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> Cc: Serge Hallyn <serge@...lyn.com>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Andy Lutomirsky <luto@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Aleksa Sarai <cyphar@...har.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Florian Weimer <fweimer@...hat.com>
> Signed-off-by: Christian Brauner <christian@...uner.io>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
> Changelog:
> v4:
> - updated asciinema to use "taskfd_" prefix
> - s/procfd_send_signal/taskfd_send_signal/g
> - s/proc_is_tgid_procfd/tgid_taskfd_to_pid/b
> - s/proc_is_tid_procfd/tid_taskfd_to_pid/b
> - s/__copy_siginfo_from_user_generic/__copy_siginfo_from_user_any/g
> - make it clear that __copy_siginfo_from_user_any() is a workaround caused
>   by a bug in the original implementation of rt_sigqueueinfo()
> - when spoofing signals turn them into regular kill signals if si_code is
>   set to SI_USER
> - make proc_is_t{g}id_procfd() return struct pid to allow proc_pid() to
>   stay private to fs/proc/
> v3:
> - add __copy_siginfo_from_user_generic() to avoid adding compat syscalls
> - s/procfd_signal/procfd_send_signal/g
> - change type of flags argument from int to unsigned int
> - add comment about what happens to zombies
> - add proc_is_tid_procfd()
> - return EOPNOTSUPP when /proc/<pid>/task/<tid> fd is passed so userspace
>   has a way of knowing that tidfds are not supported currently.
> v2:
> - define __NR_procfd_signal in unistd.h
> - wire up compat syscall
> - s/proc_is_procfd/proc_is_tgid_procfd/g
> - provide stubs when CONFIG_PROC_FS=n
> - move proc_pid() to linux/proc_fs.h header
> - use proc_pid() to grab struct pid from /proc/<pid> fd
> v1:
> - patch introduced
> ---
>  arch/x86/entry/syscalls/syscall_32.tbl |   1 +
>  arch/x86/entry/syscalls/syscall_64.tbl |   1 +
>  fs/proc/base.c                         |  20 +++-
>  include/linux/proc_fs.h                |  12 +++
>  include/linux/syscalls.h               |   3 +
>  include/uapi/asm-generic/unistd.h      |   4 +-
>  kernel/signal.c                        | 132 +++++++++++++++++++++++--
>  7 files changed, 164 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 3cf7b533b3d1..7efb63fd0617 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -398,3 +398,4 @@
>  384	i386	arch_prctl		sys_arch_prctl			__ia32_compat_sys_arch_prctl
>  385	i386	io_pgetevents		sys_io_pgetevents		__ia32_compat_sys_io_pgetevents
>  386	i386	rseq			sys_rseq			__ia32_sys_rseq
> +387	i386	taskfd_send_signal	sys_taskfd_send_signal		__ia32_sys_taskfd_send_signal
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index f0b1709a5ffb..be894f4a84e9 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -343,6 +343,7 @@
>  332	common	statx			__x64_sys_statx
>  333	common	io_pgetevents		__x64_sys_io_pgetevents
>  334	common	rseq			__x64_sys_rseq
> +335	common	taskfd_send_signal	__x64_sys_taskfd_send_signal
>  
>  #
>  # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ce3465479447..b8b88bfee455 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -716,8 +716,6 @@ static int proc_pid_permission(struct inode *inode, int mask)
>  	return generic_permission(inode, mask);
>  }
>  
> -
> -
>  static const struct inode_operations proc_def_inode_operations = {
>  	.setattr	= proc_setattr,
>  };
> @@ -3038,6 +3036,15 @@ static const struct file_operations proc_tgid_base_operations = {
>  	.llseek		= generic_file_llseek,
>  };
>  
> +struct pid *tgid_taskfd_to_pid(const struct file *file)
> +{
> +	if (!d_is_dir(file->f_path.dentry) ||
> +	    (file->f_op != &proc_tgid_base_operations))
> +		return ERR_PTR(-EBADF);
> +
> +	return proc_pid(file_inode(file));
> +}
> +
>  static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
>  {
>  	return proc_pident_lookup(dir, dentry,
> @@ -3422,6 +3429,15 @@ static const struct file_operations proc_tid_base_operations = {
>  	.llseek		= generic_file_llseek,
>  };
>  
> +struct pid *tid_taskfd_to_pid(const struct file *file)
> +{
> +	if (!d_is_dir(file->f_path.dentry) ||
> +	    (file->f_op != &proc_tid_base_operations))
> +		return ERR_PTR(-EBADF);
> +
> +	return proc_pid(file_inode(file));
> +}
> +
>  static const struct inode_operations proc_tid_base_inode_operations = {
>  	.lookup		= proc_tid_base_lookup,
>  	.getattr	= pid_getattr,
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index d0e1f1522a78..96817415c420 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -73,6 +73,8 @@ struct proc_dir_entry *proc_create_net_single_write(const char *name, umode_t mo
>  						    int (*show)(struct seq_file *, void *),
>  						    proc_write_t write,
>  						    void *data);
> +extern struct pid *tgid_taskfd_to_pid(const struct file *file);
> +extern struct pid *tid_taskfd_to_pid(const struct file *file);
>  
>  #else /* CONFIG_PROC_FS */
>  
> @@ -114,6 +116,16 @@ static inline int remove_proc_subtree(const char *name, struct proc_dir_entry *p
>  #define proc_create_net(name, mode, parent, state_size, ops) ({NULL;})
>  #define proc_create_net_single(name, mode, parent, show, data) ({NULL;})
>  
> +static inline struct pid *tgid_taskfd_to_pid(const struct file *file)
> +{
> +	return ERR_PTR(-EBADF);
> +}
> +
> +static inline struct pid *tid_taskfd_to_pid(const struct file *file)
> +{
> +	return ERR_PTR(-EBADF);
> +}
> +
>  #endif /* CONFIG_PROC_FS */
>  
>  struct net;
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 2ac3d13a915b..5ffe194ef29b 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -907,6 +907,9 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
>  			  unsigned mask, struct statx __user *buffer);
>  asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
>  			 int flags, uint32_t sig);
> +asmlinkage long sys_taskfd_send_signal(int taskfd, int sig,
> +				       siginfo_t __user *info,
> +				       unsigned int flags);
>  
>  /*
>   * Architecture-specific system calls
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 538546edbfbd..9343dca63fd9 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -738,9 +738,11 @@ __SYSCALL(__NR_statx,     sys_statx)
>  __SC_COMP(__NR_io_pgetevents, sys_io_pgetevents, compat_sys_io_pgetevents)
>  #define __NR_rseq 293
>  __SYSCALL(__NR_rseq, sys_rseq)
> +#define __NR_taskfd_send_signal 294
> +__SYSCALL(__NR_taskfd_send_signal, sys_taskfd_send_signal)
>  
>  #undef __NR_syscalls
> -#define __NR_syscalls 294
> +#define __NR_syscalls 295
>  
>  /*
>   * 32 bit systems traditionally used different
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9a32bc2088c9..a00a4bcb7605 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -19,7 +19,9 @@
>  #include <linux/sched/task.h>
>  #include <linux/sched/task_stack.h>
>  #include <linux/sched/cputime.h>
> +#include <linux/file.h>
>  #include <linux/fs.h>
> +#include <linux/proc_fs.h>
>  #include <linux/tty.h>
>  #include <linux/binfmts.h>
>  #include <linux/coredump.h>
> @@ -3286,6 +3288,16 @@ COMPAT_SYSCALL_DEFINE4(rt_sigtimedwait, compat_sigset_t __user *, uthese,
>  }
>  #endif
>  
> +static inline void prepare_kill_siginfo(int sig, struct kernel_siginfo *info)
> +{
> +	clear_siginfo(info);
> +	info->si_signo = sig;
> +	info->si_errno = 0;
> +	info->si_code = SI_USER;
> +	info->si_pid = task_tgid_vnr(current);
> +	info->si_uid = from_kuid_munged(current_user_ns(), current_uid());
> +}
> +
>  /**
>   *  sys_kill - send a signal to a process
>   *  @pid: the PID of the process
> @@ -3295,16 +3307,124 @@ SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
>  {
>  	struct kernel_siginfo info;
>  
> -	clear_siginfo(&info);
> -	info.si_signo = sig;
> -	info.si_errno = 0;
> -	info.si_code = SI_USER;
> -	info.si_pid = task_tgid_vnr(current);
> -	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
> +	prepare_kill_siginfo(sig, &info);
>  
>  	return kill_something_info(sig, &info, pid);
>  }
>  
> +/*
> + * Verify that the signaler and signalee either are in the same pid namespace
> + * or that the signaler's pid namespace is an ancestor of the signalee's pid
> + * namespace.
> + */
> +static bool may_signal_taskfd(struct pid *pid)

may_signal_taskfd seems like a misnomer here.  It leads the reader to think it
is about permission, when it's really about accessibility.

> +{
> +	struct pid_namespace *active = task_active_pid_ns(current);
> +	struct pid_namespace *p = ns_of_pid(pid);
> +
> +	for (;;) {
> +		if (!p)
> +			return false;
> +		if (p == active)
> +			break;
> +		p = p->parent;
> +	}
> +
> +	return true;
> +}
> +
> +static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
> +{
> +#ifdef CONFIG_COMPAT
> +	/*
> +	 * Avoid hooking up compat syscalls and instead handle necessary
> +	 * conversions here. Note, this is a stop-gap measure and should not be
> +	 * considered a generic solution.
> +	 */
> +	if (in_compat_syscall())
> +		return copy_siginfo_from_user32(
> +			kinfo, (struct compat_siginfo __user *)info);
> +#endif
> +	return copy_siginfo_from_user(kinfo, info);
> +}
> +
> +/**
> + *  sys_taskfd_send_signal - send a signal to a process through a task file
> + *                           descriptor
> + *  @taskfd: the file descriptor of the process
> + *  @sig:    signal to be sent
> + *  @info:   the signal info
> + *  @flags:  future flags to be passed
> + *
> + *  Return: 0 on success, negative errno on failure
> + */
> +SYSCALL_DEFINE4(taskfd_send_signal, int, taskfd, int, sig,
> +		siginfo_t __user *, info, unsigned int, flags)
> +{
> +	int ret;
> +	struct fd f;
> +	struct pid *pid;
> +	kernel_siginfo_t kinfo;
> +
> +	/* Enforce flags be set to 0 until we add an extension. */
> +	if (flags)
> +		return -EINVAL;
> +
> +	f = fdget_raw(taskfd);
> +	if (!f.file)
> +		return -EBADF;
> +
> +	pid = tid_taskfd_to_pid(f.file);
> +	if (!IS_ERR(pid)) {
> +		/*
> +		 * Give userspace a way to detect /proc/<pid>/task/<tid>
> +		 * support when we add it.
> +		 */
> +		ret = -EOPNOTSUPP;
> +		goto err;
> +	}
> +
> +	/* Is this a procfd? */
> +	pid = tgid_taskfd_to_pid(f.file);
> +	if (IS_ERR(pid)) {
> +		ret = PTR_ERR(pid);
> +		goto err;
> +	}
> +
> +	ret = -EINVAL;
> +	if (!may_signal_taskfd(pid))
> +		goto err;
> +
> +	if (info) {
> +		ret = copy_siginfo_from_user_any(&kinfo, info);
> +		if (unlikely(ret))
> +			goto err;
> +
> +		ret = -EINVAL;
> +		if (unlikely(sig != kinfo.si_signo))
> +			goto err;
> +
> +		if ((task_pid(current) != pid) &&
> +		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)) {
> +			/* Only allow sending arbitrary signals to yourself. */
> +			ret = -EPERM;
> +			if (kinfo.si_code != SI_USER)
> +				goto err;
> +
> +			/* Turn this into a regular kill signal. */
> +			prepare_kill_siginfo(sig, &kinfo);
> +		}
> +	} else {
> +		prepare_kill_siginfo(sig, &kinfo);
> +	}
> +
> +	ret = kill_pid_info(sig, &kinfo, pid);
> +
> +err:
> +	fdput(f);
> +	return ret;
> +}
> +
>  static int
>  do_send_specific(pid_t tgid, pid_t pid, int sig, struct kernel_siginfo *info)
>  {
> -- 
> 2.19.1

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