exec: Add a new AT_EXECVE_CHECK flag to execveat(2)

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 measure 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 <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Kees Cook <[email protected]>
Acked-by: Paul Moore <[email protected]>
Reviewed-by: Serge Hallyn <[email protected]>
Reviewed-by: Jeff Xu <[email protected]>
Tested-by: Jeff Xu <[email protected]>
Link: https://docs.python.org/3/library/io.html#io.open_code [1]
Signed-off-by: Mickaël Salaün <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: l0kod

Documentation/userspace-api/check_exec.rst

@@ -0,0 +1,37 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright © 2024 Microsoft Corporation
+
+===================
+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.``.
+
+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.

Documentation/userspace-api/index.rst

@@ -35,6 +35,7 @@ Security-related interfaces
    mfd_noexec
    spec_ctrl
    tee
+   check_exec
 
 Devices and I/O
 ===============

fs/exec.c

@@ -892,7 +892,8 @@ 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;
@@ -1541,6 +1542,21 @@ 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 not
+	 * be 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;
@@ -1836,7 +1852,7 @@ 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);

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;

include/uapi/linux/fcntl.h

@@ -155,4 +155,8 @@
 #define AT_HANDLE_MNT_ID_UNIQUE	0x001	/* Return the u64 unique mount ID. */
 #define AT_HANDLE_CONNECTABLE	0x002	/* Request a connectable file handle */
 
+/* Flags for execveat2(2). */
+#define AT_EXECVE_CHECK		0x10000	/* Only perform a check if execution
+					   would be allowed. */
+
 #endif /* _UAPI_LINUX_FCNTL_H */

security/security.c

@@ -1248,6 +1248,12 @@ 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)
@@ -3098,6 +3104,10 @@ 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)