[PATCH v21 1/6] exec: Add a new AT_EXECVE_CHECK flag to execveat(2)
Mickaël Salaün
mic at digikod.net
Fri Nov 22 14:50:35 UTC 2024
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 at digikod.net> 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 at digikod.net> 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 at digikod.net> 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 at zeniv.linux.org.uk>
> > > > > > Cc: Christian Brauner <brauner at kernel.org>
> > > > > > Cc: Kees Cook <keescook at chromium.org>
> > > > > > Cc: Paul Moore <paul at paul-moore.com>
> > > > > > Reviewed-by: Serge Hallyn <serge at hallyn.com>
> > > > > > Link: https://docs.python.org/3/library/io.html#io.open_code [1]
> > > > > > Signed-off-by: Mickaël Salaün <mic at digikod.net>
> > > > > > Link: https://lore.kernel.org/r/20241112191858.162021-2-mic@digikod.net
> > > > > > ---
> > > > > >
> > > > > > 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@digikod.net
> > > > > > ---
> > > > > > 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.
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.
>
> >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.
>
> For scripts, the cases are more complicated and we can't just enforce
> it, therefore have to rely on security bits to give a pre-process
> level control.
>
> > >
> > > script interpreters need to check this though, because the apps might
> > > need to adjust/test the scripts they are calling, so
> > > SECBIT_EXEC_RESTRICT_FILE can be used to opt-out the enforcement.
> > >
> > > > > When SECBIT_EXEC_RESTRICT_FILE=1, userspace is bound to enforce.
> > > > >
> > > > > > +
> > > > > > +The main purpose of this flag is to improve the security and consistency of an
> > > > > > +execution environment to ensure that direct file execution (e.g.
> > > > > > +``./script.sh``) and indirect file execution (e.g. ``sh script.sh``) lead to
> > > > > > +the same result. For instance, this can be used to check if a file is
> > > > > > +trustworthy according to the caller's environment.
> > > > > > +
> > > > > > +In a secure environment, libraries and any executable dependencies should also
> > > > > > +be checked. For instance, dynamic linking should make sure that all libraries
> > > > > > +are allowed for execution to avoid trivial bypass (e.g. using ``LD_PRELOAD``).
> > > > > > +For such secure execution environment to make sense, only trusted code should
> > > > > > +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.
> > > > > > diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
> > > > > > index 274cc7546efc..6272bcf11296 100644
> > > > > > --- a/Documentation/userspace-api/index.rst
> > > > > > +++ b/Documentation/userspace-api/index.rst
> > > > > > @@ -35,6 +35,7 @@ Security-related interfaces
> > > > > > mfd_noexec
> > > > > > spec_ctrl
> > > > > > tee
> > > > > > + check_exec
> > > > > >
> > > > > > Devices and I/O
> > > > > > ===============
> > > > > > diff --git a/fs/exec.c b/fs/exec.c
> > > > > > index 6c53920795c2..bb83b6a39530 100644
> > > > > > --- a/fs/exec.c
> > > > > > +++ b/fs/exec.c
> > > > > > @@ -891,7 +891,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
> > > > > > .lookup_flags = LOOKUP_FOLLOW,
> > > > > > };
> > > > > >
> > > > > > - if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
> > > > > > + if ((flags &
> > > > > > + ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH | AT_EXECVE_CHECK)) != 0)
> > > > > > return ERR_PTR(-EINVAL);
> > > > > > if (flags & AT_SYMLINK_NOFOLLOW)
> > > > > > open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
> > > > > > @@ -1545,6 +1546,21 @@ static struct linux_binprm *alloc_bprm(int fd, struct filename *filename, int fl
> > > > > > }
> > > > > > bprm->interp = bprm->filename;
> > > > > >
> > > > > > + /*
> > > > > > + * At this point, security_file_open() has already been called (with
> > > > > > + * __FMODE_EXEC) and access control checks for AT_EXECVE_CHECK will
> > > > > > + * stop just after the security_bprm_creds_for_exec() call in
> > > > > > + * bprm_execve(). Indeed, the kernel should not try to parse the
> > > > > > + * content of the file with exec_binprm() nor change the calling
> > > > > > + * thread, which means that the following security functions will be
> > > > > > + * not called:
> > > > > > + * - security_bprm_check()
> > > > > > + * - security_bprm_creds_from_file()
> > > > > > + * - security_bprm_committing_creds()
> > > > > > + * - security_bprm_committed_creds()
> > > > > > + */
> > > > > > + bprm->is_check = !!(flags & AT_EXECVE_CHECK);
> > > > > > +
> > > > > > retval = bprm_mm_init(bprm);
> > > > > > if (!retval)
> > > > > > return bprm;
> > > > > > @@ -1839,7 +1855,7 @@ static int bprm_execve(struct linux_binprm *bprm)
> > > > > >
> > > > > > /* Set the unchanging part of bprm->cred */
> > > > > > retval = security_bprm_creds_for_exec(bprm);
> > > > > > - if (retval)
> > > > > > + if (retval || bprm->is_check)
> > > > > > goto out;
> > > > > >
> > > > > > retval = exec_binprm(bprm);
> > > > > > diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> > > > > > index e6c00e860951..8ff0eb3644a1 100644
> > > > > > --- a/include/linux/binfmts.h
> > > > > > +++ b/include/linux/binfmts.h
> > > > > > @@ -42,7 +42,12 @@ struct linux_binprm {
> > > > > > * Set when errors can no longer be returned to the
> > > > > > * original userspace.
> > > > > > */
> > > > > > - point_of_no_return:1;
> > > > > > + point_of_no_return:1,
> > > > > > + /*
> > > > > > + * Set by user space to check executability according to the
> > > > > > + * caller's environment.
> > > > > > + */
> > > > > > + is_check:1;
> > > > > > struct file *executable; /* Executable to pass to the interpreter */
> > > > > > struct file *interpreter;
> > > > > > struct file *file;
> > > > > > diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> > > > > > index 87e2dec79fea..2e87f2e3a79f 100644
> > > > > > --- a/include/uapi/linux/fcntl.h
> > > > > > +++ b/include/uapi/linux/fcntl.h
> > > > > > @@ -154,6 +154,10 @@
> > > > > > usable with open_by_handle_at(2). */
> > > > > > #define AT_HANDLE_MNT_ID_UNIQUE 0x001 /* Return the u64 unique mount ID. */
> > > > > >
> > > > > > +/* Flags for execveat2(2). */
> > > > > > +#define AT_EXECVE_CHECK 0x10000 /* Only perform a check if execution
> > > > > > + would be allowed. */
> > > > > > +
> > > > > > #if defined(__KERNEL__)
> > > > > > #define AT_GETATTR_NOSEC 0x80000000
> > > > > > #endif
> > > > > > diff --git a/kernel/audit.h b/kernel/audit.h
> > > > > > index a60d2840559e..8ebdabd2ab81 100644
> > > > > > --- a/kernel/audit.h
> > > > > > +++ b/kernel/audit.h
> > > > > > @@ -197,6 +197,7 @@ struct audit_context {
> > > > > > struct open_how openat2;
> > > > > > struct {
> > > > > > int argc;
> > > > > > + bool is_check;
> > > > > > } execve;
> > > > > > struct {
> > > > > > char *name;
> > > > > > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > > > > > index cd57053b4a69..8d9ba5600cf2 100644
> > > > > > --- a/kernel/auditsc.c
> > > > > > +++ b/kernel/auditsc.c
> > > > > > @@ -2662,6 +2662,7 @@ void __audit_bprm(struct linux_binprm *bprm)
> > > > > >
> > > > > > context->type = AUDIT_EXECVE;
> > > > > > context->execve.argc = bprm->argc;
> > > > > > + context->execve.is_check = bprm->is_check;
> > > > > Where is execve.is_check used ?
> > > >
> > > > It is used in bprm_execve(), exposed to the audit framework, and
> > > > potentially used by LSMs.
> > > >
> > > bprm_execve() uses bprm->is_check, not the context->execve.is_check.
> >
> > Correct, this is only for audit but not used yet.
> >
> > Paul, Eric, do you want me to remove this field, leave it, or extend
> > this patch like this?
> >
> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index 8d9ba5600cf2..12cf89fa224a 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -1290,6 +1290,8 @@ static void audit_log_execve_info(struct audit_context *context,
> > }
> > } while (arg < context->execve.argc);
> >
> > + audit_log_format(*ab, " check=%d", context->execve.is_check);
> > +
> > /* NOTE: the caller handles the final audit_log_end() call */
> >
> > out:
> >
> > >
> > >
> > > > >
> > > > >
> > > > > > }
> > > > > >
> > > > > >
> > > > > > diff --git a/security/security.c b/security/security.c
> > > > > > index c5981e558bc2..456361ec249d 100644
> > > > > > --- a/security/security.c
> > > > > > +++ b/security/security.c
> > > > > > @@ -1249,6 +1249,12 @@ int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
> > > > > > * to 1 if AT_SECURE should be set to request libc enable secure mode. @bprm
> > > > > > * contains the linux_binprm structure.
> > > > > > *
> > > > > > + * If execveat(2) is called with the AT_EXECVE_CHECK flag, bprm->is_check is
> > > > > > + * set. The result must be the same as without this flag even if the execution
> > > > > > + * will never really happen and @bprm will always be dropped.
> > > > > > + *
> > > > > > + * This hook must not change current->cred, only @bprm->cred.
> > > > > > + *
> > > > > > * Return: Returns 0 if the hook is successful and permission is granted.
> > > > > > */
> > > > > > int security_bprm_creds_for_exec(struct linux_binprm *bprm)
> > > > > > @@ -3100,6 +3106,10 @@ int security_file_receive(struct file *file)
> > > > > > * Save open-time permission checking state for later use upon file_permission,
> > > > > > * and recheck access if anything has changed since inode_permission.
> > > > > > *
> > > > > > + * We can check if a file is opened for execution (e.g. execve(2) call), either
> > > > > > + * directly or indirectly (e.g. ELF's ld.so) by checking file->f_flags &
> > > > > > + * __FMODE_EXEC .
> > > > > > + *
> > > > > > * Return: Returns 0 if permission is granted.
> > > > > > */
> > > > > > int security_file_open(struct file *file)
> > > > > > --
> > > > > > 2.47.0
> > > > > >
> > > > > >
> > >
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