On Thu, Jul 04, 2024 at 05:04:03PM -0700, Kees Cook wrote: > 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. Where in the code? Just before the bprm->is_check assignment? > > [...] > > 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); There are valid use cases relying on pathnames. See Linus's comment: https://lore.kernel.org/r/CAHk-=whb=XuU=LGKnJWaa7LOYQz9VwHs8SLfgLbT5sf2VAbX1A@xxxxxxxxxxxxxx
