[PATCH v4 05/11] coredump: add coredump socket

Christian Brauner
2025-05-07T16:14:09Z

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 abstract AF_UNIX socket.

Userspace can set /proc/sys/kernel/core_pattern to:

@linuxafsk/coredump_socket

The "@" at the beginning indicates to the kernel that the abstract
AF_UNIX coredump socket will be used to process coredumps.

The coredump socket uses the fixed address "linuxafsk/coredump.socket"
for now.

The coredump socket is located in the initial network namespace. To bind
the coredump socket userspace must hold CAP_SYS_ADMIN in the initial
user namespace. Listening and reading can happen from whatever
unprivileged context is necessary to safely process coredumps.

When a task coredumps it opens a client socket in the initial network
namespace and connects to the coredump socket.

- The coredump server should use 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 socket directly.

- The pidfd for the crashing task will contain information how the task
coredumps. The PIDFD_GET_INFO ioctl gained a new flag
PIDFD_INFO_COREDUMP which can be used to retreive the coredump
information.

If the coredump gets a new coredump client connection the kernel
guarantees that PIDFD_INFO_COREDUMP information is available.
Currently the following information is provided in the new
@coredump_mask extension to struct pidfd_info:

* PIDFD_COREDUMPED is raised if the task did actually coredump.
* PIDFD_COREDUMP_SKIP is raised if the task skipped coredumping (e.g.,
undumpable).
* PIDFD_COREDUMP_USER is raised if this is a regular coredump and
doesn't need special care by the coredump server.
* IDFD_COREDUMP_ROOT is raised if the generated coredump should be
treated as sensitive and the coredump server should restrict to the
generated coredump to sufficiently privileged users.

- Since unix_stream_connect() runs bpf programs during connect it's
possible to even redirect or multiplex coredumps to other sockets.

- The coredump server should mark itself as non-dumpable.
To capture coredumps for the coredump server itself a bpf program
should be run at connect to redirect it to another socket in
userspace. This can be useful for debugging crashing coredump servers.

- A container coredump server in a separate network namespace can simply
bind to linuxafsk/coredump.socket and systemd-coredump fowards
coredumps to the container.

- Fwiw, one idea is to handle coredumps via per-user/session coredump
servers that run 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.

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.

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 just keep a worker pool.

This is easy to test:

(a) coredump processing (we're using socat):

> cat coredump_socket.sh
#!/bin/bash

set -x

sudo bash -c "echo '@linuxafsk/coredump.socket' > /proc/sys/kernel/core_pattern"
sudo socat --statistics abstract-listen:linuxafsk/coredump.socket,fork FILE:core_file,create,append,trunc

(b) trigger a coredump:

user1@localhost:~/data/scripts$ cat crash.c
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
fprintf(stderr, "%u\n", (1 / 0));
_exit(0);
}

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/coredump.c | 136 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 128 insertions(+), 8 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index b2eda7b176e4..d61e15d855d2 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 +255,34 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
++pat_ptr;
if (!(*pat_ptr))
return -ENOMEM;
+ break;
+ }
+ case COREDUMP_SOCK: {
+ err = cn_printf(cn, "%s", pat_ptr);
+ if (err)
+ return err;
+
+ /*
+ * We can potentially allow this to be changed later but
+ * I currently see no reason to.
+ */
+ if (strcmp(cn->corename, "@linuxafsk/coredump.socket"))
+ 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 +429,20 @@ 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;
}

@@ -583,6 +628,17 @@ static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
return 0;
}

+#ifdef CONFIG_UNIX
+static const struct sockaddr_un coredump_unix_socket = {
+ .sun_family = AF_UNIX,
+ .sun_path = "\0linuxafsk/coredump.socket",
+};
+/* Without trailing NUL byte. */
+#define COREDUMP_UNIX_SOCKET_ADDR_SIZE \
+ (offsetof(struct sockaddr_un, sun_path) + \
+ sizeof("\0linuxafsk/coredump.socket") - 1)
+#endif
+
void do_coredump(const kernel_siginfo_t *siginfo)
{
struct core_state core_state;
@@ -801,6 +857,45 @@ void do_coredump(const kernel_siginfo_t *siginfo)
}
break;
}
+ case COREDUMP_SOCK: {
+#ifdef CONFIG_UNIX
+ struct file *file __free(fput) = NULL;
+ struct socket *socket;
+
+ /*
+ * 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 *)(&coredump_unix_socket),
+ COREDUMP_UNIX_SOCKET_ADDR_SIZE, O_NONBLOCK);
+ if (retval) {
+ if (retval == -EAGAIN)
+ coredump_report_failure("Skipping as coredump socket connection %s couldn't complete immediately", cn.corename);
+ 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 +933,33 @@ 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: {
+ char buf[1];
+ /*
+ * 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, buf, 1, NULL);
+ break;
+ }
+ default:
+ break;
+ }
+ }
+
close_fail:
if (cprm.file)
filp_close(cprm.file, NULL);
@@ -1069,7 +1189,7 @@ 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. "

--
2.47.2