On Mon, Nov 25, 2024 at 09:38:51AM -0800, Jeff Xu wrote: > On Fri, Nov 22, 2024 at 6:50 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > On Thu, Nov 21, 2024 at 10:27:40AM -0800, Jeff Xu wrote: > > > On Thu, Nov 21, 2024 at 5:40 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > > > > > On Wed, Nov 20, 2024 at 08:06:07AM -0800, Jeff Xu wrote: > > > > > On Wed, Nov 20, 2024 at 1:42 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > > > > > > > > > On Tue, Nov 19, 2024 at 05:17:00PM -0800, Jeff Xu wrote: > > > > > > > On Tue, Nov 12, 2024 at 11:22 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > Add a new AT_EXECVE_CHECK flag to execveat(2) to check if a file would > > > > > > > > be allowed for execution. The main use case is for script interpreters > > > > > > > > and dynamic linkers to check execution permission according to the > > > > > > > > kernel's security policy. Another use case is to add context to access > > > > > > > > logs e.g., which script (instead of interpreter) accessed a file. As > > > > > > > > any executable code, scripts could also use this check [1]. > > > > > > > > > > > > > > > > This is different from faccessat(2) + X_OK which only checks a subset of > > > > > > > > access rights (i.e. inode permission and mount options for regular > > > > > > > > files), but not the full context (e.g. all LSM access checks). The main > > > > > > > > use case for access(2) is for SUID processes to (partially) check access > > > > > > > > on behalf of their caller. The main use case for execveat(2) + > > > > > > > > AT_EXECVE_CHECK is to check if a script execution would be allowed, > > > > > > > > according to all the different restrictions in place. Because the use > > > > > > > > of AT_EXECVE_CHECK follows the exact kernel semantic as for a real > > > > > > > > execution, user space gets the same error codes. > > > > > > > > > > > > > > > > An interesting point of using execveat(2) instead of openat2(2) is that > > > > > > > > it decouples the check from the enforcement. Indeed, the security check > > > > > > > > can be logged (e.g. with audit) without blocking an execution > > > > > > > > environment not yet ready to enforce a strict security policy. > > > > > > > > > > > > > > > > LSMs can control or log execution requests with > > > > > > > > security_bprm_creds_for_exec(). However, to enforce a consistent and > > > > > > > > complete access control (e.g. on binary's dependencies) LSMs should > > > > > > > > restrict file executability, or mesure executed files, with > > > > > > > > security_file_open() by checking file->f_flags & __FMODE_EXEC. > > > > > > > > > > > > > > > > Because AT_EXECVE_CHECK is dedicated to user space interpreters, it > > > > > > > > doesn't make sense for the kernel to parse the checked files, look for > > > > > > > > interpreters known to the kernel (e.g. ELF, shebang), and return ENOEXEC > > > > > > > > if the format is unknown. Because of that, security_bprm_check() is > > > > > > > > never called when AT_EXECVE_CHECK is used. > > > > > > > > > > > > > > > > It should be noted that script interpreters cannot directly use > > > > > > > > execveat(2) (without this new AT_EXECVE_CHECK flag) because this could > > > > > > > > lead to unexpected behaviors e.g., `python script.sh` could lead to Bash > > > > > > > > being executed to interpret the script. Unlike the kernel, script > > > > > > > > interpreters may just interpret the shebang as a simple comment, which > > > > > > > > should not change for backward compatibility reasons. > > > > > > > > > > > > > > > > Because scripts or libraries files might not currently have the > > > > > > > > executable permission set, or because we might want specific users to be > > > > > > > > allowed to run arbitrary scripts, the following patch provides a dynamic > > > > > > > > configuration mechanism with the SECBIT_EXEC_RESTRICT_FILE and > > > > > > > > SECBIT_EXEC_DENY_INTERACTIVE securebits. > > > > > > > > > > > > > > > > This is a redesign of the CLIP OS 4's O_MAYEXEC: > > > > > > > > https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch > > > > > > > > This patch has been used for more than a decade with customized script > > > > > > > > interpreters. Some examples can be found here: > > > > > > > > https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC > > > > > > > > > > > > > > > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > > > > > > > > Cc: Christian Brauner <brauner@xxxxxxxxxx> > > > > > > > > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > > > > > > > > Cc: Paul Moore <paul@xxxxxxxxxxxxxx> > > > > > > > > Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx> > > > > > > > > Link: https://docs.python.org/3/library/io.html#io.open_code [1] > > > > > > > > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> > > > > > > > > Link: https://lore.kernel.org/r/20241112191858.162021-2-mic@xxxxxxxxxxx > > > > > > > > --- > > > > > > > > > > > > > > > > Changes since v20: > > > > > > > > * Rename AT_CHECK to AT_EXECVE_CHECK, requested by Amir Goldstein and > > > > > > > > Serge Hallyn. > > > > > > > > * Move the UAPI documentation to a dedicated RST file. > > > > > > > > * Add Reviewed-by: Serge Hallyn > > > > > > > > > > > > > > > > Changes since v19: > > > > > > > > * Remove mention of "role transition" as suggested by Andy. > > > > > > > > * Highlight the difference between security_bprm_creds_for_exec() and > > > > > > > > the __FMODE_EXEC check for LSMs (in commit message and LSM's hooks) as > > > > > > > > discussed with Jeff. > > > > > > > > * Improve documentation both in UAPI comments and kernel comments > > > > > > > > (requested by Kees). > > > > > > > > > > > > > > > > New design since v18: > > > > > > > > https://lore.kernel.org/r/20220104155024.48023-3-mic@xxxxxxxxxxx > > > > > > > > --- > > > > > > > > Documentation/userspace-api/check_exec.rst | 34 ++++++++++++++++++++++ > > > > > > > > Documentation/userspace-api/index.rst | 1 + > > > > > > > > fs/exec.c | 20 +++++++++++-- > > > > > > > > include/linux/binfmts.h | 7 ++++- > > > > > > > > include/uapi/linux/fcntl.h | 4 +++ > > > > > > > > kernel/audit.h | 1 + > > > > > > > > kernel/auditsc.c | 1 + > > > > > > > > security/security.c | 10 +++++++ > > > > > > > > 8 files changed, 75 insertions(+), 3 deletions(-) > > > > > > > > create mode 100644 Documentation/userspace-api/check_exec.rst > > > > > > > > > > > > > > > > diff --git a/Documentation/userspace-api/check_exec.rst b/Documentation/userspace-api/check_exec.rst > > > > > > > > new file mode 100644 > > > > > > > > index 000000000000..ad1aeaa5f6c0 > > > > > > > > --- /dev/null > > > > > > > > +++ b/Documentation/userspace-api/check_exec.rst > > > > > > > > @@ -0,0 +1,34 @@ > > > > > > > > +=================== > > > > > > > > +Executability check > > > > > > > > +=================== > > > > > > > > + > > > > > > > > +AT_EXECVE_CHECK > > > > > > > > +=============== > > > > > > > > + > > > > > > > > +Passing the ``AT_EXECVE_CHECK`` flag to :manpage:`execveat(2)` only performs a > > > > > > > > +check on a regular file and returns 0 if execution of this file would be > > > > > > > > +allowed, ignoring the file format and then the related interpreter dependencies > > > > > > > > +(e.g. ELF libraries, script's shebang). > > > > > > > > + > > > > > > > > +Programs should always perform this check to apply kernel-level checks against > > > > > > > > +files that are not directly executed by the kernel but passed to a user space > > > > > > > > +interpreter instead. All files that contain executable code, from the point of > > > > > > > > +view of the interpreter, should be checked. However the result of this check > > > > > > > > +should only be enforced according to ``SECBIT_EXEC_RESTRICT_FILE`` or > > > > > > > > +``SECBIT_EXEC_DENY_INTERACTIVE.``. > > > > > > > Regarding "should only" > > > > > > > Userspace (e.g. libc) could decide to enforce even when > > > > > > > SECBIT_EXEC_RESTRICT_FILE=0), i.e. if it determines not-enforcing > > > > > > > doesn't make sense. > > > > > > > > > > > > User space is always in control, but I don't think it would be wise to > > > > > > not follow the configuration securebits (in a generic system) because > > > > > > this could result to unattended behaviors (I don't have a specific one > > > > > > in mind but...). That being said, configuration and checks are > > > > > > standalones and specific/tailored systems are free to do the checks they > > > > > > want. > > > > > > > > > > > In the case of dynamic linker, we can always enforce honoring the > > > > > execveat(AT_EXECVE_CHECK) result, right ? I can't think of a case not > > > > > to, the dynamic linker doesn't need to check the > > > > > SECBIT_EXEC_RESTRICT_FILE bit. > > > > > > > > If the library file is not allowed to be executed by *all* access > > > > control systems (not just mount and file permission, but all LSMs), then > > > > the AT_EXECVE_CHECK will fail, which is OK as long as it is not a hard > > > > requirement. > > > Yes. specifically for the library loading case, I can't think of a > > > case where we need to by-pass LSMs. (letting user space to by-pass > > > LSM check seems questionable in concept, and should only be used when > > > there aren't other solutions). In the context of SELINUX enforcing > > > mode, we will want to enforce it. In the context of process level LSM > > > such as landlock, the process can already decide for itself by > > > selecting the policy for its own domain, it is unnecessary to use > > > another opt-out solution. > > > > My answer wasn't clear. The execveat(AT_EXECVE_CHECK) can and should > > always be done, but user space should only enforce restrictions > > according to the securebits. > > > I knew this part (AT_EXESCVE_CHECK is called always) > Since the securebits are enforced by userspace, setting it to 0 is > equivalent to opt-out enforcement, that is what I meant by opt-out. OK, that was confusing because these bits are set to 0 by default (for compatibility reasons). > > > It doesn't make sense to talk about user space "bypassing" kernel > > checks. This patch series provides a feature to enable user space to > > enforce (at its level) the same checks as the kernel. > > > > There is no opt-out solution, but compatibility configuration bits > > through securebits (which can also be set by LSMs). > > > > To answer your question about the dynamic linker, there should be no > > difference of behavior with a script interpreter. Both should check > > executability but only enforce restriction according to the securebits > > (as explained in the documentation). Doing otherwise on a generic > > distro could lead to unexpected behaviors (e.g. if a user enforced a > > specific SELinux policy that doesn't allow execution of library files). > > > > > > > > There is one case where I see a difference: > > > ld.so a.out (when a.out is on non-exec mount) > > > > > > If the dynamic linker doesn't read SECBIT_EXEC_RESTRICT_FILE setting, > > > above will always fail. But that is more of a bugfix. > > > > No, the dynamic linker should only enforce restrictions according to the > > securebits, otherwise a user space update (e.g. with a new dynamic > > linker ignoring the securebits) could break an existing system. > > > OK. upgrade is a valid concern. Previously, I was just thinking about > a new LSM based on this check, not existing LSM policies. > Do you happen to know which SELinux policy/LSM could break ? i.e. it > will be applied to libraries once we add AT_EXESCVE_CHECK in the > dynamic linker. We cannot assume anything about LSM policies because of custom and private ones. > We could give heads up and prepare for that. > > > > > > > >Relying on the securebits to know if this is a hard > > > > requirement or not enables system administrator and distros to control > > > > this potential behavior change. > > > > > > > I think, for the dynamic linker, it can be a hard requirement. > > > > Not on a generic distro. > > > Ok. Maybe this can be done through a configuration option for the > dynamic linker. Yes, we could have a built-time option (disabled by default) for the dynamic linker to enforce that. > > The consideration I have is: securebits is currently designed to > control both dynamic linker and shell scripts. > The case for dynamic linker is simpler than scripts cases, (non-exec > mount, and perhaps some LSM policies for libraries) and distributions > such as ChromeOS can enforce the dynamic linker case ahead of scripts > interrupter cases, i.e. without waiting for python/shell being > upgraded, that can take sometimes. For secure systems, the end goal is to always enforce such restrictions, so once interpretation/execution of a set of file types (e.g. ELF libraries) are tested enough in such a system, we can remove the securebits checks for the related library/executable (e.g. ld.so) and consider that they are always set, independently of the current user/credentials.
