[PATCH v21 1/6] exec: Add a new AT_EXECVE_CHECK flag to execveat(2)
Jeff Xu
jeffxu at chromium.org
Wed Nov 20 01:17:00 UTC 2024
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.
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 ?
> }
>
>
> 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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