Library for creating Python executables compatible with LaTeX restricted shell escape
Project description
latexrestricted
— Python library for creating executables compatible with LaTeX restricted shell escape
This Python package is designed to simplify the process of creating Python executables compatible with LaTeX restricted shell escape. Restricted shell escape allows LaTeX to run trusted executables as part of compiling documents. These executables have restricted access to the file system and restricted ability to launch subprocesses.
latexrestricted
provides access to LaTeX configuration and implements
wrappers around Python's
pathlib.Path
and
subprocess.run()
that follow LaTeX security settings.
Usage considerations
Importing
latexrestricted
should be imported as soon as possible during the
initialization of an executable. When it is imported, it sets the current
working directory as the TeX working directory. If it is imported after the
current working directory is changed, then the TeX working directory will be
set incorrectly and security restrictions will fail.
Failure modes on systems with multiple TeX installations
latexrestricted
works correctly when used on systems with no more than one
TeX Live installation and no more than one
MiKTeX installation. If multiple TeX Live
installations or multiple MiKTeX installations are present, it is not always
possible for latexrestricted
to determine the correct TeX Live or MiKTeX
installation, and in these cases the first installation on PATH
is used.
In these cases, latexrestricted
may fail to return the correct TeX
configuration values, and there is no way to detect this. See
LatexConfig._init_tex_paths()
for implementation details.
-
Multiple TeX Live installations: The correct installation will be used under non-Windows operating systems. Under Windows, the first installation on
PATH
is used. -
Multiple MiKTeX installations: Under all operating systems, the first installation on
PATH
is used.
The user can avoid any issues with multiple installations by modifying PATH
to put the correct TeX Live or MiKTeX installation first.
LaTeX configuration
from latexrestricted import latex_config
The latex_config
instance of the LatexConfig
class provides access to
LaTeX configuration and related environment variables. If there are errors in
determining LaTeX configuration (for example, a TeX installation cannot be
located), then latexrestricted.LatexConfigError
is raised.
latex_config
attributes and properties
Paths:
-
tex_cwd: str
: Current working directory whenlatexrestricted
was first imported. -
texlive_bin: str | None
andtexlive_kpsewhich: str | None
: If an executable usinglatexrestricted
is launched via\ShellEscape
with TeX Live, these will be the paths to the TeX Livebin/
directory and the TeX Livekpsewhich
executable. Otherwise, both areNone
.TeX Live is detected by the absence of a
TEXSYSTEM
environment variable, or by this variable having a value other thanmiktex
. Under non-Windows operating systems, theSELFAUTOLOC
environment variable set bykpathsea
is used to locate the TeX Live binary directory, so it will be correct even on systems with multiple TeX Live installations. Under Windows, shell escape executables are often launched with TeX Live's executable wrapperrunscript.exe
, which overwritesSELFAUTOLOC
with the location of the wrapper. In this case, the TeX Live binary directory is located by using Python'sshutil.which()
to searchPATH
for atlmgr
executable with accompanyingkpsewhich
. On systems with multiple TeX Live installations, this will give the first TeX Live installation onPATH
, which is not guaranteed to be correct unless the user ensures that the correct installation has precedence onPATH
. -
miktex_bin: str | None
,miktex_initexmf: str | None
, andmiktex_kpsewhich: str | None
: If an executable usinglatexrestricted
is launched via\ShellEscape
with MiKTeX, these will be the paths to the MiKTeXbin/
directory and the MiKTeXinitexmf
andkpsewhich
executables. Otherwise, all areNone
.MiKTeX is detected by checking the
TEXSYSTEM
environment variable for the valuemiktex
.TEXSYSTEM
only declares that MiKTeX is in use; unlike the TeX Live case withSELFAUTOLOC
,TEXSYSTEM
does not give the location of TeX binaries. The location of TeX binaries is determined using Python'sshutil.which()
to searchPATH
for aninitexmf
executable with accompanyingkpsewhich
. On systems with multiple MiKTeX installations, this will give the first MiKTeX installation onPATH
, which is not guaranteed to be correct unless the user ensures that the correct installation has precedence onPATH
.
File system access:
-
can_read_dotfiles: bool
,can_read_anywhere: bool
,can_write_dotfiles: bool
,can_write_anywhere: bool
: These summarize the file system security settings for TeX Live (openin_any
andopenout_any
intexmf.cnf
) and MiKTeX ([Core]AllowUnsafeInputFiles
and[Core]AllowUnsafeOutputFiles
inmiktex.ini
). The*dotfile
properties describe whether dotfiles (files with names beginning with.
) can be read/written. The*anywhere
properties describe whether files anywhere in the file system can be read/written, or only those under the current working directory,TEXMFOUTPUT
, andTEXMF_OUTPUT_DIRECTORY
. -
can_restricted_shell_escape: bool
: This describes whether restricted shell escape is possible. It is true when restricted shell escape is enabled and also when full shell escape is enabled. It is based on TeX Live'sshell_escape
intexmf.cnf
and MiKTeX's[Core]ShellCommandMode
inmiktex.ini
. -
prohibited_write_file_extensions: frozenset[str] | None
: Under Windows (including Cygwin), this is a frozen set of file extensions that cannot be used in writing files. Under other operating systems, this isNone
.All file extensions are lower case with a leading period (for example,
.exe
). These are determined from thePATHEXT
environment variable, or use a default fallback ifPATHEXT
is not defined or when under Cygwin.
Other LaTeX configuration variables and environment variables:
-
TEXMFHOME: str | None
: Value ofTEXMFHOME
obtained fromkpsewhich
orinitexmf
. -
TEXMFOUTPUT: str | None
: Value ofTEXMFOUTPUT
obtained from environment variable if defined and otherwise fromkpsewhich
orinitexmf
. -
TEXMF_OUTPUT_DIRECTORY: str | None
: Value ofTEXMF_OUTPUT_DIRECTORY
obtained from environment variable. -
restricted_shell_escape_commands: frozenset[str]
: Permitted restricted shell escape executables. Obtained from TeX Live'sshell_escape_commands
intexmf.cnf
or MiKTeX's[Core]AllowedShellCommands[]
inmiktex.ini
.Note that this will contain permitted restricted shell escape executables even when shell escape is completely disabled; it cannot be used to determine whether restricted shell escape is permitted. See
can_restricted_shell_escape
to determine whether restricted shell escape is permitted.
latex_config
methods
-
kpsewhich_find_config_file(file: str)
: Usekpsewhich
in a subprocess to find a configuration file (kpsewhich -f othertext <file>
). Returnskpsewhich
output as a string orNone
if there was no output. -
kpsewhich_find_file(file: str, *, cache: bool = False)
: Usekpsewhich
in a subprocess to find a file (kpsewhich <file>
). Returnskpsewhich
output as a string orNone
if there was no output. The optional argumentcache
cacheskpsewhich
output to minimize subprocesses. This can be useful when the file system is not being modified andkpsewhich
is simply returning values from its own cache.
Restricted file system access
TeX limits file system access. The file system security settings for TeX Live
(openin_any
and openout_any
in texmf.cnf
) and MiKTeX
([Core]AllowUnsafeInputFiles
and [Core]AllowUnsafeOutputFiles
in
miktex.ini
) determine whether dotfiles can be read/written and whether files
anywhere in the file system can be read/written, or only those under the
current working directory, TEXMFOUTPUT
, and TEXMF_OUTPUT_DIRECTORY
.
The latexrestricted
package provides RestrictedPath
subclasses of Python's
pathlib.Path
that respect
these security settings or enforce more stringent security. Under Python 3.8,
these subclasses backport the methods .is_relative_to()
and .with_stem()
from Python 3.9. When these subclasses are used to modify the file system,
latexrestricted.PathSecurityError
is raised if reading/writing a given path
is not permitted.
With RestrictedPath
classes, relative paths are always relative to the TeX
working directory. If the current working directory has been changed to
another location (for example, via os.chdir()
), then it will temporarily be
switched back to the TeX working directory during any RestrictedPath
operations that access the file system.
The SafeWrite*
classes should be preferred unless access to additional
write locations is absolutely necessary. When multiple TeX Live
installations are present under Windows or multiple MiKTeX installations are
present under all operating systems, there is no guarantee that
latexrestricted
will find the correct installation and thus use the correct
TeX configuration, unless PATH
has been modified to put the correct
installation first.
from latexrestricted import <RestrictedPathClass>
RestrictedPath
classes
BaseRestrictedPath
This is the base class for RestrictedPath
classes. It cannot be used
directly. Subclasses define methods .readable_dir()
, .readable_file()
,
.writable_dir()
, and .writable_file()
that determine whether a given path
is readable/writable. Most methods for opening, reading, writing, replacing,
and deleting files as well as methods for creating and deleting directories
are supported. Methods related to modifying file permissions and creating
links are not supported. Unsupported methods raise NotImplementedError
.
StringRestrictedPath
classes
-
StringRestrictedPath
: This follows the approach taken in TeX's file system security. TeX configuration determines whether dotfiles are readable/writable and which locations are readable/writable. Paths are analyzed as strings; the file system is never consulted. When read/write locations are restricted, paths are restricted using the following criteria:-
All relative paths are relative to the TeX working directory.
-
All absolute paths must be under
TEXMF_OUTPUT_DIRECTORY
andTEXMFOUTPUT
. -
Paths cannot contain
..
to access a parent directory, even if the parent directory is a valid location.
When read/write locations are restricted, it is still possible to access locations outside the TeX working directory,
TEXMF_OUTPUT_DIRECTORY
, andTEXMFOUTPUT
if there are symlinks in those locations.Under Windows (including Cygwin), writing files with file extensions in
PATHEXT
(for example,.exe
) is also disabled. -
-
SafeStringRestrictedPath
: Same asStringRestrictedPath
, except that TeX configuration is ignored and all security settings are at maximum: dotfiles cannot be read/written, and all reading/writing is limited to the TeX working directory,TEXMF_OUTPUT_DIRECTORY
, andTEXMFOUTPUT
. -
SafeWriteStringRestrictedPath
: Same asStringRestrictedPath
, except that TeX configuration for writing is ignored and all security settings related to writing are at maximum.
ResolvedRestrictedPath
classes
-
ResolvedRestrictedPath
: This resolves any symlinks in paths using the file system before determining whether paths are readable/writable. TeX configuration determines whether dotfiles are readable/writable and which locations are readable/writable. When read/write locations are restricted, paths are restricted using the following criteria:-
Resolved paths must be under the TeX working directory, resolved
TEXMF_OUTPUT_DIRECTORY
, or resolvedTEXMFOUTPUT
. -
All relative paths are resolved relative to the TeX working directory.
-
Unlike
StringRestrictedPath
, paths are allowed to contain..
, andTEXMF_OUTPUT_DIRECTORY
andTEXMFOUTPUT
can be accessed via relative paths. This is possible since paths are fully resolved with the file system before being compared with permitted read/write locations.
Because paths are resolved before being compared with permitted read/write locations, it is not possible to access locations outside the TeX working directory,
TEXMF_OUTPUT_DIRECTORY
, andTEXMFOUTPUT
via symlinks in those locations.Under Windows (including Cygwin), writing files with file extensions in
PATHEXT
(for example,.exe
) is also disabled. -
-
SafeResolvedRestrictedPath
: Same asResolvedRestrictedPath
, except that TeX configuration is ignored and all security settings are at maximum: dotfiles cannot be read/written, and all other reading/writing is limited to the TeX working directory,TEXMF_OUTPUT_DIRECTORY
, andTEXMFOUTPUT
. -
SafeWriteResolvedRestrictedPath
: Same asResolvedRestrictedPath
, except that TeX configuration for writing is ignored and all security settings related to writing are at maximum.
RestrictedPath
class methods
-
tex_cwd() -> Self
: TeX working directory. -
TEXMFOUTPUT() -> Self | None
: Path ofTEXMFOUTPUT
from LaTeX configuration or environment variable, orNone
if not defined. -
TEXMF_OUTPUT_DIRECTORY() -> Self | None
: Path ofTEXMF_OUTPUT_DIRECTORY
from environment variable, orNone
if not defined. -
tex_roots() -> frozenset[Self]
: All root locations where TeX might write output, under default configuration with restricted access to the file system. This includestex_cwd()
plusTEXMFOUTPUT()
andTEXMF_OUTPUT_DIRECTORY()
if they are defined. -
tex_roots_resolved() -> frozenset[Self]
: Same astex_roots()
except all paths are resolved with the file system. -
tex_roots_with_resolved() -> frozenset[Self]
: Union oftex_roots()
andtex_roots_resolved()
. -
tex_openout_roots() -> tuple[Self]
: Locations where TeX will attempt to write with\openout
, in order. The first element of the tuple isTEXMF_OUTPUT_DIRECTORY()
if notNone
and otherwisetex_cwd()
. IfTEXMFOUTPUT()
is notNone
and is not already in the tuple, then it is the second element.TeX attempts to write to
TEXMFOUTPUT
(if defined) when the default write location (TEXMF_OUTPUT_DIRECTORY
if defined, else TeX working directory) is read-only. -
tex_texmfoutput_roots() -> frozenset[Self]
:TEXMFOUTPUT()
and/orTEXMF_OUTPUT_DIRECTORY()
if they are defined.
Restricted subprocesses
When LaTeX runs with restricted shell escape, only executables specified in
TeX configuration can be executed via \ShellEscape
. The latexrestricted
package allows these same executables to run in subprocesses via
restricted_run()
. This is a wrapper around Python's
subprocess.run()
.
from latexrestricted import restricted_run
restricted_run(args: list[str], allow_restricted_executables: bool = False)
-
It is always possible to run
kpsewhich
andinitexmf
. These are necessary to access TeX configuration values.Running other executables allowed by TeX configuration for restricted shell escape requires the optional argument
allow_restricted_executables=True
. In this case, TeX configuration is checked to determine whether restricted shell escape is enabled, although this should be redundant iflatexrestricted
itself is being used in a restricted shell escape executable. -
When
allow_restricted_executables=True
, the executable must be in the same directory askpsewhich
orinitexmf
, as previously located during TeX configuration detection, or the executable must exist onPATH
, as found by Python'sshutil.which()
.The executable must not be in a location writable by LaTeX. For added security, locations writable by LaTeX cannot be under the executable parent directory.
-
The executable cannot be a batch file (no
*.bat
or*.cmd
): https://docs.python.org/3/library/subprocess.html#security-considerations. This is enforced by requiring*.exe
under Windows and completely prohibiting*.bat
and*.cmd
everywhere. -
The subprocess runs with
shell=False
.
restricted_run()
will raise latexrestricted.ExecutableNotFoundError
if an
executable (args[0]
) cannot be found in the same directory as kpsewhich
or
initexmf
, or on PATH
. It will raise
latexrestricted.UnapprovedExecutableError
when the executable is not in the
approved list of executables from LaTeX configuration
(latex_config.restricted_shell_escape_commands
). It will raise
latexrestricted.ExecutablePathSecurityError
if the executable is found and
is in the approved list, but is in an insecure location relative to locations
writable by LaTeX.
Security limitations with TeX Live and TEXMFOUTPUT
TeX Live allows TEXMFOUTPUT
to be set in a texmf.cnf
config file. In this
case, latexrestricted
will retrieve the value of TEXMFOUTPUT
by running
kpsewhich --var-value TEXMFOUTPUT
in a subprocess. If the user has modified
TEXMFOUTPUT
to an unsafe value in a texmf.cnf
config file, then the
kpsewhich
executable (and all other TeX-related executables) are potentially
compromised, and latexrestricted
cannot detect this until after running
kpsewhich
.
It is possible to set TEXMFOUTPUT
to unsafe values in a texmf.cnf
config
file. For example, if TEXMFOUTPUT
is set to a location in the file system
that contains executables, this could allow LaTeX documents to modify those
executables or their resources. With TeX Live, latexrestricted
retrieves
the value of TEXMFOUTPUT
by running kpsewhich
. However, the kpsewhich
executable itself could be compromised if TEXMFOUTPUT
is set to a directory
that contains the kpsewhich
executable (or other parts of a TeX
installation). If latexrestricted
detects an unsafe value of TEXMFOUTPUT
,
it raises latexrestricted.LatexConfigError
, but this is only possible
after running the potentially compromised kpsewhich
executable to obtain
the value of TEXMFOUTPUT
. (And if kpsewhich
is compromised, there is
always the possibility that it will not return the true value of
TEXMFOUTPUT
.)
While raising a security-related error after running the potentially
compromised executable is not ideal, this will typically have a negligible
impact on overall security. If TEXMFOUTPUT
is set to a directory that
contains the kpsewhich
executable (or other parts of a TeX installation),
then all other TeX-related executables are also potentially compromised. If
latexrestricted
is being used in a Python executable designed for LaTeX
shell escape, then presumably a LaTeX executable is already running, and LaTeX
may invoke additional trusted executables such as kpsewhich
in shells.
Thus, before latexrestricted
ever runs kpsewhich
to retrieve the value of
TEXMFOUTPUT
, potentially compromised executables would already be running.
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