On Thu, Jul 04, 2024 at 09:01:33PM +0200, Mickaël Salaün 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 than faccessat(2) which only checks file access > rights, but not the full context e.g. mount point's noexec, stack limit, > and all potential LSM extra checks (e.g. argv, envp, credentials). > Since the use of AT_CHECK follows the exact kernel semantic as for a > real execution, user space gets the same error codes. Nice! I much prefer this method of going through the exec machinery so we always have a single code path for these kinds of checks. > 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. I'd like some additional comments in the code that reminds us that access control checks have finished past a certain point. [...] > diff --git a/fs/exec.c b/fs/exec.c > index 40073142288f..ea2a1867afdc 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -931,7 +931,7 @@ 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; [...] > + * 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. I want this enforced by the kernel. Let's not leave trivial ToCToU foot-guns around. i.e.: if ((flags & AT_CHECK) == AT_CHECK && (flags & AT_EMPTY_PATH) == 0) return ERR_PTR(-EBADF); -- Kees Cook
