Re: [PATCH v7 4/9] coredump: add coredump socket

Kuniyuki Iwashima
2025-05-15T17:01:19Z

From: Christian Brauner <brauner@kernel.org>
Date: Thu, 15 May 2025 00:03:37 +0200
> Coredumping currently supports two modes:
>
> (1) Dumping directly into a file somewhere on the filesystem.
> (2) Dumping into a pipe connected to a usermode helper process
> spawned as a child of the system_unbound_wq or kthreadd.
>
> For simplicity I'm mostly ignoring (1). There's probably still some
> users of (1) out there but processing coredumps in this way can be
> considered adventurous especially in the face of set*id binaries.
>
> The most common option should be (2) by now. It works by allowing
> userspace to put a string into /proc/sys/kernel/core_pattern like:
>
> |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h
>
> The "|" at the beginning indicates to the kernel that a pipe must be
> used. The path following the pipe indicator is a path to a binary that
> will be spawned as a usermode helper process. Any additional parameters
> pass information about the task that is generating the coredump to the
> binary that processes the coredump.
>
> In the example core_pattern shown above systemd-coredump is spawned as a
> usermode helper. There's various conceptual consequences of this
> (non-exhaustive list):
>
> - systemd-coredump is spawned with file descriptor number 0 (stdin)
> connected to the read-end of the pipe. All other file descriptors are
> closed. That specifically includes 1 (stdout) and 2 (stderr). This has
> already caused bugs because userspace assumed that this cannot happen
> (Whether or not this is a sane assumption is irrelevant.).
>
> - systemd-coredump will be spawned as a child of system_unbound_wq. So
> it is not a child of any userspace process and specifically not a
> child of PID 1. It cannot be waited upon and is in a weird hybrid
> upcall which are difficult for userspace to control correctly.
>
> - systemd-coredump is spawned with full kernel privileges. This
> necessitates all kinds of weird privilege dropping excercises in
> userspace to make this safe.
>
> - A new usermode helper has to be spawned for each crashing process.
>
> This series adds a new mode:
>
> (3) Dumping into an AF_UNIX socket.
>
> Userspace can set /proc/sys/kernel/core_pattern to:
>
> @/path/to/coredump.socket
>
> The "@" at the beginning indicates to the kernel that an AF_UNIX
> coredump socket will be used to process coredumps.
>
> The coredump socket must be located in the initial mount namespace.
> When a task coredumps it opens a client socket in the initial network
> namespace and connects to the coredump socket.
>
> - The coredump server uses SO_PEERPIDFD to get a stable handle on the
> connected crashing task. The retrieved pidfd will provide a stable
> reference even if the crashing task gets SIGKILLed while generating
> the coredump.
>
> - By setting core_pipe_limit non-zero userspace can guarantee that the
> crashing task cannot be reaped behind it's back and thus process all
> necessary information in /proc/<pid>. The SO_PEERPIDFD can be used to
> detect whether /proc/<pid> still refers to the same process.
>
> The core_pipe_limit isn't used to rate-limit connections to the
> socket. This can simply be done via AF_UNIX sockets directly.
>
> - The pidfd for the crashing task will grow new information how the task
> coredumps.
>
> - The coredump server should mark itself as non-dumpable.
>
> - A container coredump server in a separate network namespace can simply
> bind to another well-know address and systemd-coredump fowards
> coredumps to the container.
>
> - Coredumps could in the future also be handled via per-user/session
> coredump servers that run only with that users privileges.
>
> The coredump server listens on the coredump socket and accepts a
> new coredump connection. It then retrieves SO_PEERPIDFD for the
> client, inspects uid/gid and hands the accepted client to the users
> own coredump handler which runs with the users privileges only
> (It must of coure pay close attention to not forward crashing suid
> binaries.).
>
> The new coredump socket will allow userspace to not have to rely on
> usermode helpers for processing coredumps and provides a safer way to
> handle them instead of relying on super privileged coredumping helpers
> that have and continue to cause significant CVEs.
>
> This will also be significantly more lightweight since no fork()+exec()
> for the usermodehelper is required for each crashing process. The
> coredump server in userspace can e.g., just keep a worker pool.
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> fs/coredump.c | 133 ++++++++++++++++++++++++++++++++++++++++++++++++----
> include/linux/net.h | 1 +
> net/unix/af_unix.c | 53 ++++++++++++++++-----
> 3 files changed, 166 insertions(+), 21 deletions(-)
>
> diff --git a/fs/coredump.c b/fs/coredump.c
> index a70929c3585b..e1256ebb89c1 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -44,7 +44,11 @@
> #include <linux/sysctl.h>
> #include <linux/elf.h>
> #include <linux/pidfs.h>
> +#include <linux/net.h>
> +#include <linux/socket.h>
> +#include <net/net_namespace.h>
> #include <uapi/linux/pidfd.h>
> +#include <uapi/linux/un.h>
>
> #include <linux/uaccess.h>
> #include <asm/mmu_context.h>
> @@ -79,6 +83,7 @@ unsigned int core_file_note_size_limit = CORE_FILE_NOTE_SIZE_DEFAULT;
> enum coredump_type_t {
> COREDUMP_FILE = 1,
> COREDUMP_PIPE = 2,
> + COREDUMP_SOCK = 3,
> };
>
> struct core_name {
> @@ -232,13 +237,16 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> cn->corename = NULL;
> if (*pat_ptr == '|')
> cn->core_type = COREDUMP_PIPE;
> + else if (*pat_ptr == '@')
> + cn->core_type = COREDUMP_SOCK;
> else
> cn->core_type = COREDUMP_FILE;
> if (expand_corename(cn, core_name_size))
> return -ENOMEM;
> cn->corename[0] = '\0';
>
> - if (cn->core_type == COREDUMP_PIPE) {
> + switch (cn->core_type) {
> + case COREDUMP_PIPE: {
> int argvs = sizeof(core_pattern) / 2;
> (*argv) = kmalloc_array(argvs, sizeof(**argv), GFP_KERNEL);
> if (!(*argv))
> @@ -247,6 +255,33 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> ++pat_ptr;
> if (!(*pat_ptr))
> return -ENOMEM;
> + break;
> + }
> + case COREDUMP_SOCK: {
> + /* skip the @ */
> + pat_ptr++;
> + err = cn_printf(cn, "%s", pat_ptr);
> + if (err)
> + return err;
> +
> + /* Require absolute paths. */
> + if (cn->corename[0] != '/')
> + return -EINVAL;
> +
> + /*
> + * Currently no need to parse any other options.
> + * Relevant information can be retrieved from the peer
> + * pidfd retrievable via SO_PEERPIDFD by the receiver or
> + * via /proc/<pid>, using the SO_PEERPIDFD to guard
> + * against pid recycling when opening /proc/<pid>.
> + */
> + return 0;
> + }
> + case COREDUMP_FILE:
> + break;
> + default:
> + WARN_ON_ONCE(true);
> + return -EINVAL;
> }
>
> /* Repeat as long as we have more pattern to process and more output
> @@ -393,11 +428,20 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> * If core_pattern does not include a %p (as is the default)
> * and core_uses_pid is set, then .%pid will be appended to
> * the filename. Do not do this for piped commands. */
> - if (!(cn->core_type == COREDUMP_PIPE) && !pid_in_pattern && core_uses_pid) {
> - err = cn_printf(cn, ".%d", task_tgid_vnr(current));
> - if (err)
> - return err;
> + if (!pid_in_pattern && core_uses_pid) {
> + switch (cn->core_type) {
> + case COREDUMP_FILE:
> + return cn_printf(cn, ".%d", task_tgid_vnr(current));
> + case COREDUMP_PIPE:
> + break;
> + case COREDUMP_SOCK:
> + break;
> + default:
> + WARN_ON_ONCE(true);
> + return -EINVAL;
> + }
> }
> +
> return 0;
> }
>
> @@ -801,6 +845,55 @@ void do_coredump(const kernel_siginfo_t *siginfo)
> }
> break;
> }
> + case COREDUMP_SOCK: {
> +#ifdef CONFIG_UNIX
> + struct file *file __free(fput) = NULL;
> + struct sockaddr_un addr = {
> + .sun_family = AF_UNIX,
> + };
> + ssize_t addr_len;
> + struct socket *socket;
> +
> + retval = strscpy(addr.sun_path, cn.corename, sizeof(addr.sun_path));
> + if (retval < 0)
> + goto close_fail;
> + addr_len = offsetof(struct sockaddr_un, sun_path) + retval + 1;
> +
> + /*
> + * It is possible that the userspace process which is
> + * supposed to handle the coredump and is listening on
> + * the AF_UNIX socket coredumps. Userspace should just
> + * mark itself non dumpable.
> + */
> +
> + retval = sock_create_kern(&init_net, AF_UNIX, SOCK_STREAM, 0, &socket);
> + if (retval < 0)
> + goto close_fail;
> +
> + file = sock_alloc_file(socket, 0, NULL);
> + if (IS_ERR(file)) {
> + sock_release(socket);
> + goto close_fail;
> + }
> +
> + retval = kernel_connect(socket, (struct sockaddr *)(&addr),
> + addr_len, O_NONBLOCK | SOCK_COREDUMP);
> + if (retval) {
> + if (retval == -EAGAIN)
> + coredump_report_failure("Coredump socket %s receive queue full", addr.sun_path);
> + else
> + coredump_report_failure("Coredump socket connection %s failed %d", addr.sun_path, retval);
> + goto close_fail;
> + }
> +
> + cprm.limit = RLIM_INFINITY;
> + cprm.file = no_free_ptr(file);
> +#else
> + coredump_report_failure("Core dump socket support %s disabled", cn.corename);
> + goto close_fail;
> +#endif
> + break;
> + }
> default:
> WARN_ON_ONCE(true);
> goto close_fail;
> @@ -838,8 +931,32 @@ void do_coredump(const kernel_siginfo_t *siginfo)
> file_end_write(cprm.file);
> free_vma_snapshot(&cprm);
> }
> - if ((cn.core_type == COREDUMP_PIPE) && core_pipe_limit)
> - wait_for_dump_helpers(cprm.file);
> +
> + /*
> + * When core_pipe_limit is set we wait for the coredump server
> + * or usermodehelper to finish before exiting so it can e.g.,
> + * inspect /proc/<pid>.
> + */
> + if (core_pipe_limit) {
> + switch (cn.core_type) {
> + case COREDUMP_PIPE:
> + wait_for_dump_helpers(cprm.file);
> + break;
> + case COREDUMP_SOCK: {
> + /*
> + * We use a simple read to wait for the coredump
> + * processing to finish. Either the socket is
> + * closed or we get sent unexpected data. In
> + * both cases, we're done.
> + */
> + __kernel_read(cprm.file, &(char){ 0 }, 1, NULL);
> + break;
> + }
> + default:
> + break;
> + }
> + }
> +
> close_fail:
> if (cprm.file)
> filp_close(cprm.file, NULL);
> @@ -1069,7 +1186,7 @@ EXPORT_SYMBOL(dump_align);
> void validate_coredump_safety(void)
> {
> if (suid_dumpable == SUID_DUMP_ROOT &&
> - core_pattern[0] != '/' && core_pattern[0] != '|') {
> + core_pattern[0] != '/' && core_pattern[0] != '|' && core_pattern[0] != '@') {
>
> coredump_report_failure("Unsafe core_pattern used with fs.suid_dumpable=2: "
> "pipe handler or fully qualified core dump path required. "
> diff --git a/include/linux/net.h b/include/linux/net.h
> index 0ff950eecc6b..139c85d0f2ea 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -81,6 +81,7 @@ enum sock_type {
> #ifndef SOCK_NONBLOCK
> #define SOCK_NONBLOCK O_NONBLOCK
> #endif
> +#define SOCK_COREDUMP O_NOCTTY
>
> #endif /* ARCH_HAS_SOCKET_TYPES */
>
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 472f8aa9ea15..a9d1c9ba2961 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -85,10 +85,13 @@
> #include <linux/file.h>
> #include <linux/filter.h>
> #include <linux/fs.h>
> +#include <linux/fs_struct.h>
> #include <linux/init.h>
> #include <linux/kernel.h>
> #include <linux/mount.h>
> #include <linux/namei.h>
> +#include <linux/net.h>
> +#include <linux/pidfs.h>
> #include <linux/poll.h>
> #include <linux/proc_fs.h>
> #include <linux/sched/signal.h>
> @@ -100,7 +103,6 @@
> #include <linux/splice.h>
> #include <linux/string.h>
> #include <linux/uaccess.h>
> -#include <linux/pidfs.h>
> #include <net/af_unix.h>
> #include <net/net_namespace.h>
> #include <net/scm.h>
> @@ -1146,7 +1148,7 @@ static int unix_release(struct socket *sock)
> }
>
> static struct sock *unix_find_bsd(struct sockaddr_un *sunaddr, int addr_len,
> - int type)
> + int type, unsigned int flags)
^^^
nit: int flags

> {
> struct inode *inode;
> struct path path;
> @@ -1154,13 +1156,38 @@ static struct sock *unix_find_bsd(struct sockaddr_un *sunaddr, int addr_len,
> int err;
>
> unix_mkname_bsd(sunaddr, addr_len);
> - err = kern_path(sunaddr->sun_path, LOOKUP_FOLLOW, &path);
> - if (err)
> - goto fail;
>
> - err = path_permission(&path, MAY_WRITE);
> - if (err)
> - goto path_put;
> + if (flags & SOCK_COREDUMP) {
> + struct path root;
> + struct cred *kcred;
> + const struct cred *cred;

nit: please keep these in the reverse xmas tree order.
https://docs.kernel.org/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs

> +
> + err = -ENOMEM;

While at it, please move this in the "if (!kcred)" as it's only
used for this.

Otherwise looks good to me. I think you can just fix up nits
before pushing to the vfs tree unless there is any other feedback.

Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Thanks!

> + kcred = prepare_kernel_cred(&init_task);
> + if (!kcred)
> + goto fail;
> +
> + task_lock(&init_task);
> + get_fs_root(init_task.fs, &root);
> + task_unlock(&init_task);
> +
> + cred = override_creds(kcred);
> + err = vfs_path_lookup(root.dentry, root.mnt, sunaddr->sun_path,
> + LOOKUP_BENEATH | LOOKUP_NO_SYMLINKS |
> + LOOKUP_NO_MAGICLINKS, &path);
> + put_cred(revert_creds(cred));
> + path_put(&root);
> + if (err)
> + goto fail;
> + } else {
> + err = kern_path(sunaddr->sun_path, LOOKUP_FOLLOW, &path);
> + if (err)
> + goto fail;
> +
> + err = path_permission(&path, MAY_WRITE);
> + if (err)
> + goto path_put;
> + }
>
> err = -ECONNREFUSED;
> inode = d_backing_inode(path.dentry);
> @@ -1210,12 +1237,12 @@ static struct sock *unix_find_abstract(struct net *net,
>
> static struct sock *unix_find_other(struct net *net,
> struct sockaddr_un *sunaddr,
> - int addr_len, int type)
> + int addr_len, int type, int flags)
> {
> struct sock *sk;
>
> if (sunaddr->sun_path[0])
> - sk = unix_find_bsd(sunaddr, addr_len, type);
> + sk = unix_find_bsd(sunaddr, addr_len, type, flags);
> else
> sk = unix_find_abstract(net, sunaddr, addr_len, type);
>
> @@ -1473,7 +1500,7 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
> }
>
> restart:
> - other = unix_find_other(sock_net(sk), sunaddr, alen, sock->type);
> + other = unix_find_other(sock_net(sk), sunaddr, alen, sock->type, 0);
> if (IS_ERR(other)) {
> err = PTR_ERR(other);
> goto out;
> @@ -1620,7 +1647,7 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
>
> restart:
> /* Find listening sock. */
> - other = unix_find_other(net, sunaddr, addr_len, sk->sk_type);
> + other = unix_find_other(net, sunaddr, addr_len, sk->sk_type, flags);
> if (IS_ERR(other)) {
> err = PTR_ERR(other);
> goto out_free_skb;
> @@ -2089,7 +2116,7 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
> if (msg->msg_namelen) {
> lookup:
> other = unix_find_other(sock_net(sk), msg->msg_name,
> - msg->msg_namelen, sk->sk_type);
> + msg->msg_namelen, sk->sk_type, 0);
> if (IS_ERR(other)) {
> err = PTR_ERR(other);
> goto out_free;
>
> --
> 2.47.2
>