Adding documentation for S.A.R.A. LSM. Signed-off-by: Salvatore Mesoraca <s.mesoraca16@xxxxxxxxx> --- Documentation/admin-guide/LSM/SARA.rst | 177 ++++++++++++++++++++++++ Documentation/admin-guide/LSM/index.rst | 1 + Documentation/admin-guide/kernel-parameters.txt | 24 ++++ 3 files changed, 202 insertions(+) create mode 100644 Documentation/admin-guide/LSM/SARA.rst diff --git a/Documentation/admin-guide/LSM/SARA.rst b/Documentation/admin-guide/LSM/SARA.rst new file mode 100644 index 0000000..fdde04c --- /dev/null +++ b/Documentation/admin-guide/LSM/SARA.rst @@ -0,0 +1,177 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======== +S.A.R.A. +======== + +S.A.R.A. (S.A.R.A. is Another Recursive Acronym) is a stacked Linux Security +Module that aims to collect heterogeneous security measures, providing a common +interface to manage them. +As of today it consists of one submodule: + +- WX Protection + + +The kernel-space part is complemented by its user-space counterpart: `saractl` +[2]_. +A test suite for WX Protection, called `sara-test` [4]_, is also available. +More information about where to find these tools and the full S.A.R.A. +documentation are in the `External Links and Documentation`_ section. + +------------------------------------------------------------------------------- + +S.A.R.A.'s Submodules +===================== + +WX Protection +------------- +WX Protection aims to improve user-space programs security by applying: + +- `W^X enforcement`_ +- `W!->X (once writable never executable) mprotect restriction`_ +- `Executable MMAP prevention`_ + +All of the above features can be enabled or disabled both system wide +or on a per executable basis through the use of configuration files managed by +`saractl` [2]_. + +It is important to note that some programs may have issues working with +WX Protection. In particular: + +- **W^X enforcement** will cause problems to any programs that needs + memory pages mapped both as writable and executable at the same time e.g. + programs with executable stack markings in the *PT_GNU_STACK* segment. +- **W!->X mprotect restriction** will cause problems to any program that + needs to generate executable code at run time or to modify executable + pages e.g. programs with a *JIT* compiler built-in or linked against a + *non-PIC* library. +- **Executable MMAP prevention** can work only with programs that have at least + partial *RELRO* support. It's disabled automatically for programs that + lack this feature. It will cause problems to any program that uses *dlopen* + or tries to do an executable mmap. Unfortunately this feature is the one + that could create most problems and should be enabled only after careful + evaluation. + +To extend the scope of the above features, despite the issues that they may +cause, they are complemented by **/proc/PID/attr/sara/wxprot** interface +and **trampoline emulation**. + +At the moment, WX Protection (unless specified otherwise) should work on +any architecture supporting the NX bit, including, but not limited to: +`x86_64`, `x86_32` (with PAE), `ARM` and `ARM64`. + +Parts of WX Protection are inspired by some of the features available in PaX. + +For further information about configuration file format and user-space +utilities please take a look at the full documentation [1]_. + +W^X enforcement +^^^^^^^^^^^^^^^ +W^X means that a program can't have a page of memory that is marked, at the +same time, writable and executable. This also allow to detect many bad +behaviours that make life much more easy for attackers. Programs running with +this feature enabled will be more difficult to exploit in the case they are +affected by some vulnerabilities, because the attacker will be forced +to make more steps in order to exploit them. +This feature also blocks accesses to /proc/*/mem files that would allow to +write the current process read-only memory, bypassing any protection. + +W!->X (once writable never executable) mprotect restriction +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +"Once writable never executable" means that any page that could have been +marked as writable in the past won't ever be allowed to be marked (e.g. via +an mprotect syscall) as executable. +This goes on the same track as W^X, but is much stricter and prevents +the runtime creation of new executable code in memory. +Obviously, this feature does not prevent a program from creating a new file and +*mmapping* it as executable, however, it will be way more difficult for +attackers to exploit vulnerabilities if this feature is enabled. + +Executable MMAP prevention +^^^^^^^^^^^^^^^^^^^^^^^^^^ +This feature prevents the creation of new executable mmaps after the dynamic +libraries have been loaded. When used in combination with **W!->X mprotect +restriction** this feature will completely prevent the creation of new +executable code from the current thread. +Obviously, this feature does not prevent cases in which an attacker uses an +*execve* to start a completely new program. This kind of restriction, if +needed, can be applied using one of the other LSM that focuses on MAC. +Please be aware that this feature can break many programs and so it should be +enabled after careful evaluation. + +/proc/PID/attr/sara/wxprot interface +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +The `procattr` interface can be used by a thread to discover which +WX Protection features are enabled and/or to tighten them: protection +can't be softened via procattr. +The interface is simple: it's a text file with an hexadecimal +number in it representing enabled features (more information can be +found in the `Flags values`_ section). Via this interface it is also +possible to perform a complete memory scan to remove the write permission +from pages that are both writable and executable, please note that this +change will also affect other threads of the same process. + +Protections that prevent the runtime creation of executable code +can be troublesome for all those programs that actually need to do it +e.g. programs shipping with a JIT compiler built-in. +This feature can be use to run the JIT compiler with few restrictions +while enforcing full WX Protection in the rest of the program. + +The preferred way to access this interface is via `libsara` [3]_. +If you don't want it as a dependency, you can just statically link it +in your project or copy/paste parts of it. +To make things simpler `libsara` is the only part of S.A.R.A. released under +*CC0 - No Rights Reserved* license. + +Trampoline emulation +^^^^^^^^^^^^^^^^^^^^ +Some programs need to generate part of their code at runtime. Luckily enough, +in some cases they only generate well-known code sequences (the +*trampolines*) that can be easily recognized and emulated by the kernel. +This way WX Protection can still be active, so a potential attacker won't be +able to generate arbitrary sequences of code, but just those that are +explicitly allowed. This is not ideal, but it's still better than having WX +Protection completely disabled. + +In particular S.A.R.A. is able to recognize trampolines used by GCC for nested +C functions and libffi's trampolines. +This feature is available only on `x86_32` and `x86_64`. + +Flags values +^^^^^^^^^^^^ +Flags are represented as a 16 bit unsigned integer in which every bit indicates +the status of a given feature: + ++------------------------------+----------+ +| Feature | Value | ++==============================+==========+ +| W!->X Heap | 0x0001 | ++------------------------------+----------+ +| W!->X Stack | 0x0002 | ++------------------------------+----------+ +| W!->X Other memory | 0x0004 | ++------------------------------+----------+ +| W^X | 0x0008 | ++------------------------------+----------+ +| Don't enforce, just complain | 0x0010 | ++------------------------------+----------+ +| Be Verbose | 0x0020 | ++------------------------------+----------+ +| Executable MMAP prevention | 0x0040 | ++------------------------------+----------+ +| Force W^X on setprocattr | 0x0080 | ++------------------------------+----------+ +| Trampoline emulation | 0x0100 | ++------------------------------+----------+ +| Children will inherit flags | 0x0200 | ++------------------------------+----------+ + +------------------------------------------------------------------------------- + +External Links and Documentation +================================ + +.. [1] `Documentation <https://sara.smeso.it>`_ +.. [2] `saractl <https://github.com/smeso/saractl>`_ +.. [3] `libsara <https://github.com/smeso/libsara>`_ +.. [4] `sara-test <https://github.com/smeso/sara-test>`_ diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst index a6ba95f..81b50e4 100644 --- a/Documentation/admin-guide/LSM/index.rst +++ b/Documentation/admin-guide/LSM/index.rst @@ -47,3 +47,4 @@ subdirectories. tomoyo Yama SafeSetID + SARA diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 138f666..3d6e86d 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4230,6 +4230,30 @@ 1 -- enable. Default value is set via kernel config option. + sara.enabled= [SARA] Disable or enable S.A.R.A. at boot time. + If disabled this way S.A.R.A. can't be enabled + again. + Format: { "0" | "1" } + See security/sara/Kconfig help text + 0 -- disable. + 1 -- enable. + Default value is set via kernel config option. + + sara.wxprot_enabled= [SARA] + Disable or enable S.A.R.A. WX Protection + at boot time. + Format: { "0" | "1" } + See security/sara/Kconfig help text + 0 -- disable. + 1 -- enable. + Default value is set via kernel config option. + + sara.wxprot_default_flags= [SARA] + Set S.A.R.A. WX Protection default flags. + Format: <integer> + See S.A.R.A. documentation. + Default value is set via kernel config option. + serialnumber [BUGS=X86-32] shapers= [NET] -- 1.9.1
