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    SubjectRe: [PATCH v4] signal: add taskfd_send_signal() syscall
    Christian Brauner <christian@brauner.io> writes:

    > 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).

    I am not yet thrilled with the taskfd naming.
    Is there any plan to support sesssions and process groups?

    I am concerned about using kill_pid_info. It does this:


    rcu_read_lock();
    p = pid_task(pid, PIDTYPE_PID);
    if (p)
    error = group_send_sig_info(sig, info, p, PIDTYPE_TGID);
    rcu_read_unlock();

    That pid_task(PIDTYPE_PID) is fine for existing callers that need bug
    compatibility. For new interfaces I would strongly prefer pid_task(PIDTYPE_TGID).


    Eric

    >
    > /* 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@arndb.de>
    > Cc: "Eric W. Biederman" <ebiederm@xmission.com>
    > Cc: Serge Hallyn <serge@hallyn.com>
    > Cc: Jann Horn <jannh@google.com>
    > Cc: Andy Lutomirsky <luto@kernel.org>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Oleg Nesterov <oleg@redhat.com>
    > Cc: Aleksa Sarai <cyphar@cyphar.com>
    > Cc: Al Viro <viro@zeniv.linux.org.uk>
    > Cc: Florian Weimer <fweimer@redhat.com>
    > Signed-off-by: Christian Brauner <christian@brauner.io>
    > Reviewed-by: Kees Cook <keescook@chromium.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)
    > +{
    > + 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)
    > {

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