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Merge tag 'vfs-6.16-rc1.coredump' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs
Pull coredump updates from Christian Brauner:
"This adds support for sending coredumps over an AF_UNIX socket. It
also makes (implicit) use of the new SO_PEERPIDFD ability to hand out
pidfds for reaped peer tasks
The new coredump socket will allow userspace to not have to rely on
usermode helpers for processing coredumps and provides a saf way to
handle them instead of relying on super privileged coredumping helpers
This will also be significantly more lightweight since the kernel
doens't have to do a fork()+exec() for each crashing process to spawn
a usermodehelper. Instead the kernel just connects to the AF_UNIX
socket and userspace can process it concurrently however it sees fit.
Support for userspace is incoming starting with systemd-coredump
There's more work coming in that direction next cycle. The rest below
goes into some details and background
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 the core_pattern shown causes the kernel to spawn
systemd-coredump 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 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. That is a huge attack vector right now
- 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
* PIDFD_COREDUMP_ROOT is raised if the generated coredump should
be treated as sensitive and the coredump server should restrict
access to the generated coredump to sufficiently privileged
users"
* tag 'vfs-6.16-rc1.coredump' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
mips, net: ensure that SOCK_COREDUMP is defined
selftests/coredump: add tests for AF_UNIX coredumps
selftests/pidfd: add PIDFD_INFO_COREDUMP infrastructure
coredump: validate socket name as it is written
coredump: show supported coredump modes
pidfs, coredump: add PIDFD_INFO_COREDUMP
coredump: add coredump socket
coredump: reflow dump helpers a little
coredump: massage do_coredump()
coredump: massage format_corename()
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.