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FreeBSD-EN-18:16.ptrace                                         Errata Notice
                                                          The FreeBSD Project

Topic:          kernel panic upon ptrace attach to stopped process

Category:       core
Module:         kernel
Announced:      2018-12-19
Credits:        John Baldwin, Konstantin Belousov
Affects:        FreeBSD 11.2
Corrected:      2018-11-09 17:43:23 UTC (stable/11, 11.2-STABLE)
                2018-12-19 17:52:56 UTC (releng/11.2, 11.2-RELEASE-p7)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

ptrace(2) is a system call used by debuggers and related utilities
to take control of a process and inspect its state.  To use the
interface, a debugger must first attach to a target process.  Once
attached, the ptrace interface allows the debugger to intercept events,
such as signal delivery, involving the target process.

II.  Problem Description

The ptrace(2) implementation in FreeBSD 11.2 contains a bug such that
a ptrace attach operation will trigger a kernel panic if the target
process is in a stopped state.

III. Impact

Users debugging a problem with, for example, gdb, may cause the system to
crash.

IV.  Workaround

No workaround is available.

V.   Solution

Perform one of the following:

1) Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date, and reboot.

2) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +30 "Rebooting for FreeBSD errata update"

3) To update your system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

[FreeBSD 11.2]
# fetch https://security.FreeBSD.org/patches/EN-18:16/ptrace.patch
# fetch https://security.FreeBSD.org/patches/EN-18:16/ptrace.patch.asc
# gpg --verify ptrace.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/11/                                                        r340290
releng/11.2/                                                      r342224
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-18:16.ptrace.asc>
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