On Tue, Nov 12, 2024 at 11:22 AM Mickaël Salaün <mic@xxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> > Cc: Christian Brauner <brauner@xxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Paul Moore <paul@xxxxxxxxxxxxxx> > Reviewed-by: 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/20241112191858.162021-2-mic@xxxxxxxxxxx > --- > > 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@xxxxxxxxxxx > --- > 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 > >
