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@xxxxxxxxxxxxxxxxxx> Cc: Christian Brauner <brauner@xxxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Paul Moore <paul@xxxxxxxxxxxxxx> Cc: 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/20241011184422.977903-2-mic@xxxxxxxxxxx --- 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 --- 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. + */ +#define AT_CHECK 0x10000 + #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; } diff --git a/security/security.c b/security/security.c index 6875eb4a59fc..2f7d2c6949d7 100644 --- a/security/security.c +++ b/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_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) -- 2.46.1
