[PATCH v1 2/2] signal: add procfd_signal() syscall
From: Christian Brauner <christian@brauner.io>
Date: 2018-11-19 20:56:59
Also in:
linux-fsdevel, linux-man, lkml
Subsystem:
abi/api, filesystems (vfs and infrastructure), proc filesystem, the rest, x86 architecture (32-bit and 64-bit), x86 entry code · Maintainers:
Alexander Viro, Christian Brauner, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen, Andy Lutomirski
The kill() syscall operates on process identifiers. 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.
A prior patch has introduced the ability to get a file descriptor
referencing struct pid by opening /proc/<pid>. This guarantees a stable
handle on a process which can be used to send signals to the referenced
process. Discussion has shown that a dedicated syscall is preferable over
ioctl()s. Thus, the new syscall procfd_signal() is introduced to solve
this problem. It operates on a process file descriptor.
The syscall takes an additional siginfo_t and flags argument. If siginfo_t
is NULL then procfd_signal() behaves like kill() if it is not NULL it
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.
With this patch a process can be killed via:
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int ret;
char buf[1000];
if (argc < 2)
exit(EXIT_FAILURE);
ret = snprintf(buf, sizeof(buf), "/proc/%s", argv[1]);
if (ret < 0)
exit(EXIT_FAILURE);
int fd = open(buf, O_DIRECTORY | O_CLOEXEC);
if (fd < 0) {
printf("%s - Failed to open \"%s\"\n", strerror(errno), buf);
exit(EXIT_FAILURE);
}
ret = syscall(__NR_procfd_signal, fd, SIGKILL, NULL, 0);
if (ret < 0) {
printf("Failed to send SIGKILL \"%s\"\n", strerror(errno));
close(fd);
exit(EXIT_FAILURE);
}
close(fd);
exit(EXIT_SUCCESS);
}
[1]: https://lkml.org/lkml/2018/11/18/130
Cc: "Eric W. Biederman" <redacted>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <redacted>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Aleksa Sarai <redacted>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christian Brauner <christian@brauner.io>
---
Changelog:
v1:
- patch introduced
---
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
fs/proc/base.c | 6 ++
include/linux/proc_fs.h | 1 +
include/linux/syscalls.h | 2 +
kernel/signal.c | 76 ++++++++++++++++++++++++--
6 files changed, 81 insertions(+), 6 deletions(-)
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 3cf7b533b3d1..e637eab883e9 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 procfd_signal sys_procfd_signal __ia32_sys_procfd_signal
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f0b1709a5ffb..e95f6741ab42 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 procfd_signal __x64_sys_procfd_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 6365a4fea314..a12c9de92bd0 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c@@ -3055,6 +3055,12 @@ static const struct file_operations proc_tgid_base_operations = { .release = proc_tgid_release, }; +bool proc_is_procfd(const struct file *file) +{ + return d_is_dir(file->f_path.dentry) && + (file->f_op == &proc_tgid_base_operations); +} + static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags) { return proc_pident_lookup(dir, dentry,
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index d0e1f1522a78..2d53a47fba34 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h@@ -73,6 +73,7 @@ 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 bool proc_is_procfd(const struct file *file); #else /* CONFIG_PROC_FS */
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 2ac3d13a915b..a5ca8cb84566 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h@@ -907,6 +907,8 @@ 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_procfd_signal(int fd, int sig, siginfo_t __user *info, + int flags); /* * Architecture-specific system calls
diff --git a/kernel/signal.c b/kernel/signal.c
index 9a32bc2088c9..e8a8929de710 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 +3307,68 @@ 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); } +/** + * sys_procfd_signal - send a signal to a process through a process file + * descriptor + * @fd: the file descriptor of the process + * @sig: signal to be sent + * @info: the signal info + * @flags: future flags to be passed + */ +SYSCALL_DEFINE4(procfd_signal, int, fd, int, sig, siginfo_t __user *, info, + int, flags) +{ + int ret; + struct pid *pid; + kernel_siginfo_t kinfo; + struct fd f; + + /* Enforce flags be set to 0 until we add an extension. */ + if (flags) + return -EINVAL; + + f = fdget_raw(fd); + if (!f.file) + return -EBADF; + + ret = -EINVAL; + /* Is this a process file descriptor? */ + if (!proc_is_procfd(f.file)) + goto err; + + pid = f.file->private_data; + if (!pid) + goto err; + + if (info) { + ret = __copy_siginfo_from_user(sig, &kinfo, info); + if (unlikely(ret)) + goto err; + /* + * Not even root can pretend to send signals from the kernel. + * Nor can they impersonate a kill()/tgkill(), which adds + * source info. + */ + ret = -EPERM; + if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) && + (task_pid(current) != pid)) + goto err; + } 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