On Wed, Jan 24, 2024 at 01:32:02PM -0800, Kees Cook wrote: > On Wed, Jan 24, 2024 at 12:47:34PM -0800, Linus Torvalds wrote: > > On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > > > Hmpf, and frustratingly Ubuntu (and Debian) still builds with > > > CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago. > > For completeness, Fedora hasn't had CONFIG_USELIB for a while now. > > > Well, we could just remove the __FMODE_EXEC from uselib. > > > > It's kind of wrong anyway. > > Yeah. > > > So I think just removing __FMODE_EXEC would just do the > > RightThing(tm), and changes nothing for any sane situation. > > Agreed about these: > > - fs/fcntl.c is just doing a bitfield sanity check. > > - nfs_open_permission_mask(), as you say, is only checking for > unreadable case. > > - fsnotify would also see uselib() as a read, but afaict, > that's what it would see for an mmap(), so this should > be functionally safe. > > This one, though, I need some more time to examine: > > - AppArmor, TOMOYO, and LandLock will see uselib() as an > open-for-read, so that might still be a problem? As you > say, it's more of a mmap() call, but that would mean > adding something a call like security_mmap_file() into > uselib()... If user space can emulate uselib() without opening a file with __FMODE_EXEC, then there is no security reason to keep __FMODE_EXEC for uselib(). Removing __FMODE_EXEC from uselib() looks OK for Landlock. We use __FMODE_EXEC to infer if a file is being open for execution i.e., by execve(2). If __FMODE_EXEC is removed from uselib(), I think it should also be backported to all stable kernels for consistency though. > > The issue isn't an insane "support uselib() under AppArmor" case, but > rather "Can uselib() be used to bypass exec/mmap checks?" > > This totally untested patch might give appropriate coverage: > > diff --git a/fs/exec.c b/fs/exec.c > index d179abb78a1c..0c9265312c8d 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -143,6 +143,10 @@ SYSCALL_DEFINE1(uselib, const char __user *, library) > if (IS_ERR(file)) > goto out; > > + error = security_mmap_file(file, PROT_READ | PROT_EXEC, MAP_FIXED | MAP_SHARED); > + if (error) > + goto exit; > + > /* > * may_open() has already checked for this, so it should be > * impossible to trip now. But we need to be extra cautious > > > Of course, as you say, not having CONFIG_USELIB enabled at all is the > > _truly_ sane thing, but the only thing that used the FMODE_EXEC bit > > were landlock and some special-case nfs stuff. > > Do we want to attempt deprecation again? This was suggested last time: > https://lore.kernel.org/lkml/20200518130251.zih2s32q2rxhxg6f@wittgenstein/ > > -Kees > > -- > Kees Cook >
