On Thu, Dec 05, 2024 at 05:09:19PM +0100, Mickaël Salaün wrote: > The new SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and > their *_LOCKED counterparts are designed to be set by processes setting > up an execution environment, such as a user session, a container, or a > security sandbox. Unlike other securebits, these ones can be set by > unprivileged processes. Like seccomp filters or Landlock domains, the > securebits are inherited across processes. > > When SECBIT_EXEC_RESTRICT_FILE is set, programs interpreting code should > control executable resources according to execveat(2) + AT_EXECVE_CHECK > (see previous commit). > > When SECBIT_EXEC_DENY_INTERACTIVE is set, a process should deny > execution of user interactive commands (which excludes executable > regular files). > > Being able to configure each of these securebits enables system > administrators or owner of image containers to gradually validate the > related changes and to identify potential issues (e.g. with interpreter > or audit logs). > > It should be noted that unlike other security bits, the > SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE bits are > dedicated to user space willing to restrict itself. Because of that, > they only make sense in the context of a trusted environment (e.g. > sandbox, container, user session, full system) where the process > changing its behavior (according to these bits) and all its parent > processes are trusted. Otherwise, any parent process could just execute > its own malicious code (interpreting a script or not), or even enforce a > seccomp filter to mask these bits. > > Such a secure environment can be achieved with an appropriate access > control (e.g. mount's noexec option, file access rights, LSM policy) and > an enlighten ld.so checking that libraries are allowed for execution > e.g., to protect against illegitimate use of LD_PRELOAD. > > Ptrace restrictions according to these securebits would not make sense > because of the processes' trust assumption. > > Scripts may need some changes to deal with untrusted data (e.g. stdin, > environment variables), but that is outside the scope of the kernel. > > See chromeOS's documentation about script execution control and the > related threat model: > https://www.chromium.org/chromium-os/developer-library/guides/security/noexec-shell-scripts/ > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > Cc: Christian Brauner <brauner@xxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Paul Moore <paul@xxxxxxxxxxxxxx> > Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx> > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> > Link: https://lore.kernel.org/r/20241205160925.230119-3-mic@xxxxxxxxxxx > --- > > Changes since v21: > * Extend user documentation with exception regarding tailored execution > environments (e.g. chromeOS's libc) as discussed with Jeff. > > Changes since v20: > * Move UAPI documentation to a dedicated RST file and format it. > > Changes since v19: > * Replace SECBIT_SHOULD_EXEC_CHECK and SECBIT_SHOULD_EXEC_RESTRICT with > SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE: > https://lore.kernel.org/all/20240710.eiKohpa4Phai@xxxxxxxxxxx/ > * Remove the ptrace restrictions, suggested by Andy. > * Improve documentation according to the discussion with Jeff. > > New design since v18: > https://lore.kernel.org/r/20220104155024.48023-3-mic@xxxxxxxxxxx > --- > Documentation/userspace-api/check_exec.rst | 107 +++++++++++++++++++++ > include/uapi/linux/securebits.h | 24 ++++- > security/commoncap.c | 29 ++++-- > 3 files changed, 153 insertions(+), 7 deletions(-) > > diff --git a/Documentation/userspace-api/check_exec.rst b/Documentation/userspace-api/check_exec.rst > index 393dd7ca19c4..05dfe3b56f71 100644 > --- a/Documentation/userspace-api/check_exec.rst > +++ b/Documentation/userspace-api/check_exec.rst > @@ -5,6 +5,31 @@ > Executability check > =================== > > +The ``AT_EXECVE_CHECK`` :manpage:`execveat(2)` flag, and the > +``SECBIT_EXEC_RESTRICT_FILE`` and ``SECBIT_EXEC_DENY_INTERACTIVE`` securebits > +are intended for script interpreters and dynamic linkers to enforce a > +consistent execution security policy handled by the kernel. See the > +`samples/check-exec/inc.c`_ example. > + > +Whether an interpreter should check these securebits or not depends on the > +security risk of running malicious scripts with respect to the execution > +environment, and whether the kernel can check if a script is trustworthy or > +not. For instance, Python scripts running on a server can use arbitrary > +syscalls and access arbitrary files. Such interpreters should then be > +enlighten to use these securebits and let users define their security policy. > +However, a JavaScript engine running in a web browser should already be > +sandboxed and then should not be able to harm the user's environment. > + > +Script interpreters or dynamic linkers built for tailored execution environments > +(e.g. hardened Linux distributions or hermetic container images) could use > +``AT_EXECVE_CHECK`` without checking the related securebits if backward > +compatibility is handled by something else (e.g. atomic update ensuring that > +all legitimate libraries are allowed to be executed). It is then recommended > +for script interpreters and dynamic linkers to check the securebits at run time > +by default, but also to provide the ability for custom builds to behave like if > +``SECBIT_EXEC_RESTRICT_FILE`` or ``SECBIT_EXEC_DENY_INTERACTIVE`` were always > +set to 1 (i.e. always enforce restrictions). Jeff, does this work for you? I'll update the IMA patch with a last version but otherwise it should be good: https://lore.kernel.org/all/20241210.Wie6ion7Aich@xxxxxxxxxxx/ > + > AT_EXECVE_CHECK > =============== > > @@ -35,3 +60,85 @@ be executable, which also requires integrity guarantees. > To avoid race conditions leading to time-of-check to time-of-use issues, > ``AT_EXECVE_CHECK`` should be used with ``AT_EMPTY_PATH`` to check against a > file descriptor instead of a path. > + > +SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE > +========================================================== > + > +When ``SECBIT_EXEC_RESTRICT_FILE`` is set, a process should only interpret or > +execute a file if a call to :manpage:`execveat(2)` with the related file > +descriptor and the ``AT_EXECVE_CHECK`` flag succeed. > + > +This secure bit may be set by user session managers, service managers, > +container runtimes, sandboxer tools... Except for test environments, the > +related ``SECBIT_EXEC_RESTRICT_FILE_LOCKED`` bit should also be set. > + > +Programs should only enforce consistent restrictions according to the > +securebits but without relying on any other user-controlled configuration. > +Indeed, the use case for these securebits is to only trust executable code > +vetted by the system configuration (through the kernel), so we should be > +careful to not let untrusted users control this configuration. > + > +However, script interpreters may still use user configuration such as > +environment variables as long as it is not a way to disable the securebits > +checks. For instance, the ``PATH`` and ``LD_PRELOAD`` variables can be set by > +a script's caller. Changing these variables may lead to unintended code > +executions, but only from vetted executable programs, which is OK. For this to > +make sense, the system should provide a consistent security policy to avoid > +arbitrary code execution e.g., by enforcing a write xor execute policy. > + > +When ``SECBIT_EXEC_DENY_INTERACTIVE`` is set, a process should never interpret > +interactive user commands (e.g. scripts). However, if such commands are passed > +through a file descriptor (e.g. stdin), its content should be interpreted if a > +call to :manpage:`execveat(2)` with the related file descriptor and the > +``AT_EXECVE_CHECK`` flag succeed. > + > +For instance, script interpreters called with a script snippet as argument > +should always deny such execution if ``SECBIT_EXEC_DENY_INTERACTIVE`` is set. > + > +This secure bit may be set by user session managers, service managers, > +container runtimes, sandboxer tools... Except for test environments, the > +related ``SECBIT_EXEC_DENY_INTERACTIVE_LOCKED`` bit should also be set. > + > +Here is the expected behavior for a script interpreter according to combination > +of any exec securebits: > + > +1. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` > + > + Always interpret scripts, and allow arbitrary user commands (default). > + > + No threat, everyone and everything is trusted, but we can get ahead of > + potential issues thanks to the call to :manpage:`execveat(2)` with > + ``AT_EXECVE_CHECK`` which should always be performed but ignored by the > + script interpreter. Indeed, this check is still important to enable systems > + administrators to verify requests (e.g. with audit) and prepare for > + migration to a secure mode. > + > +2. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` > + > + Deny script interpretation if they are not executable, but allow > + arbitrary user commands. > + > + The threat is (potential) malicious scripts run by trusted (and not fooled) > + users. That can protect against unintended script executions (e.g. ``sh > + /tmp/*.sh``). This makes sense for (semi-restricted) user sessions. > + > +3. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` > + > + Always interpret scripts, but deny arbitrary user commands. > + > + This use case may be useful for secure services (i.e. without interactive > + user session) where scripts' integrity is verified (e.g. with IMA/EVM or > + dm-verity/IPE) but where access rights might not be ready yet. Indeed, > + arbitrary interactive commands would be much more difficult to check. > + > +4. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` > + > + Deny script interpretation if they are not executable, and also deny > + any arbitrary user commands. > + > + The threat is malicious scripts run by untrusted users (but trusted code). > + This makes sense for system services that may only execute trusted scripts. > + > +.. Links > +.. _samples/check-exec/inc.c: > + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/check-exec/inc.c
