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()... 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
