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Modernize ASLR advice in etc/PROBLEMS

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* etc/PROBLEMS (Segfault during 'make'): Modernize advice for
seccomp, Docker, and NetBSD (Bug#23529).
This commit is contained in:
Paul Eggert 2016-05-18 01:05:00 -07:00
parent 1fe1e0a8e7
commit 3b5e38cde1
1 changed files with 56 additions and 37 deletions
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93
etc/PROBLEMS
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@ -2600,51 +2600,70 @@ See <URL:http://debbugs.gnu.org/327>, <URL:http://debbugs.gnu.org/821>.
** Dumping
*** Segfault during 'make bootstrap' under the Linux kernel.
*** Segfault during 'make'
In Red Hat Linux kernels, "Exec-shield" functionality is enabled by
default, which creates a different memory layout that can break the
emacs dumper. Emacs tries to handle this at build time, but if this
fails, the following instructions may be useful.
If Emacs segfaults when 'make' executes one of these commands:
Exec-shield is enabled on your system if
LC_ALL=C ./temacs -batch -l loadup bootstrap
LC_ALL=C ./temacs -batch -l loadup dump
the problem may be due to inadequate workarounds for address space
layout randomization (ASLR), an operating system feature that
randomizes the virtual address space of a process. ASLR is commonly
enabled in Linux and NetBSD kernels, and is intended to deter exploits
of pointer-related bugs in applications. If ASLR is enabled, the
command:
cat /proc/sys/kernel/randomize_va_space # GNU/Linux
sysctl security.pax.aslr.global # NetBSD
outputs a nonzero value.
These segfaults should not occur on most modern systems, because the
Emacs build procedure uses the command 'setfattr' or 'paxctl' to mark
the Emacs executable as requiring non-randomized address space, and
Emacs uses the 'personality' system call to disable address space
randomization when dumping. However, older kernels may not support
'setfattr', 'paxctl', or 'personality', and newer Linux kernels have a
secure computing mode (seccomp) that can be configured to disable the
'personality' call.
It may be possible to work around the 'personality' problem in a newer
Linux kernel by configuring seccomp to allow the 'personality' call.
For example, if you are building Emacs under Docker, you can run the
Docker container with a security profile that allows 'personality' by
using Docker's --security-opt option with an appropriate profile; see
<https://docs.docker.com/engine/security/seccomp/>.
To work around the ASLR problem in either an older or a newer kernel,
you can temporarily disable the feature while building Emacs. On
GNU/Linux you can do so using the following command (as root).
echo 0 > /proc/sys/kernel/randomize_va_space
You can re-enable the feature when you are done, by echoing the
original value back to the file. NetBSD uses a different command,
e.g., 'sysctl -w security.pax.aslr.global=0'.
Alternatively, you can try using the 'setarch' command when building
temacs like this, where -R disables address space randomization:
setarch $(uname -m) -R make
ASLR is not the only problem that can break Emacs dumping. Another
issue is that in Red Hat Linux kernels, Exec-shield is enabled by
default, and this creates a different memory layout. Emacs should
handle this at build time, but if this fails the following
instructions may be useful. Exec-shield is enabled on your system if
cat /proc/sys/kernel/exec-shield
prints a value other than 0. (Please read your system documentation
for more details on Exec-shield and associated commands.)
Additionally, Linux kernel versions since 2.6.12 randomize the virtual
address space of a process by default. If this feature is enabled on
your system, then
cat /proc/sys/kernel/randomize_va_space
prints a value other than 0.
When these features are enabled, building Emacs may segfault during
the execution of this command:
./temacs --batch --load loadup [dump|bootstrap]
To work around this problem, you can temporarily disable these
features while building Emacs. You can do so using the following
commands (as root). Remember to re-enable them when you are done,
by echoing the original values back to the files.
prints a nonzero value. You can temporarily disable it as follows:
echo 0 > /proc/sys/kernel/exec-shield
echo 0 > /proc/sys/kernel/randomize_va_space
Or, on x86, you can try using the 'setarch' command when running
temacs, like this:
setarch i386 -R ./temacs --batch --load loadup [dump|bootstrap]
or
setarch i386 -R make
(The -R option disables address space randomization.)
As with randomize_va_space, you can re-enable Exec-shield when you are
done, by echoing the original value back to the file.
*** temacs prints "Pure Lisp storage exhausted".