[PATCH v20 1/6] exec: Add a new AT_CHECK flag to execveat(2)

[Amir Goldstein]

On Fri, Oct 11, 2024 at 8:45 PM Mickaël Salaün <mic at digikod.net> wrote:
>
> Add a new AT_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_CHECK
> is to check if a script execution would be allowed, according to all the
> different restrictions in place.  Because the use of AT_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_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_CHECK is used.
>
> It should be noted that script interpreters cannot directly use
> execveat(2) (without this new AT_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>
> Cc: 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/20241011184422.977903-2-mic@digikod.net
> ---
>
> 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
> ---
>  fs/exec.c                  | 18 ++++++++++++++++--
>  include/linux/binfmts.h    |  7 ++++++-
>  include/uapi/linux/fcntl.h | 31 +++++++++++++++++++++++++++++++
>  kernel/audit.h             |  1 +
>  kernel/auditsc.c           |  1 +
>  security/security.c        | 10 ++++++++++
>  6 files changed, 65 insertions(+), 3 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 6c53920795c2..163c659d9ae6 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -891,7 +891,7 @@ 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_CHECK)) != 0)
>                 return ERR_PTR(-EINVAL);
>         if (flags & AT_SYMLINK_NOFOLLOW)
>                 open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
> @@ -1545,6 +1545,20 @@ 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_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_CHECK);
> +
>         retval = bprm_mm_init(bprm);
>         if (!retval)
>                 return bprm;
> @@ -1839,7 +1853,7 @@ 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..e606815b1c5a 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -154,6 +154,37 @@
>                                            usable with open_by_handle_at(2). */
>  #define AT_HANDLE_MNT_ID_UNIQUE        0x001   /* Return the u64 unique mount ID. */
>
> +/*
> + * AT_CHECK 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.  See securebits.h
> + * documentation and the samples/check-exec/inc.c example.
> + *
> + * 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_CHECK should be used with AT_EMPTY_PATH to check against a file
> + * descriptor instead of a path.
> + */

If you ask me, the very elaborate comment above belongs to execveat(2)
man page and is way too verbose for a uapi header.

> +#define AT_CHECK               0x10000

Please see the comment "Per-syscall flags for the *at(2) family of syscalls."
above. If this is a per-syscall flag please use one of the per-syscall
flags, e.g.:

/* Flags for execveat2(2) */
#define AT_EXECVE_CHECK     0x0001   /* Only perform a check if
execution would be allowed */


Thanks,
Amir.