On Wed, Jan 24, 2024 at 10:40:49PM +0100, Jann Horn wrote: > On Wed, Jan 24, 2024 at 10:32 PM Kees Cook <keescook@xxxxxxxxxxxx> 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()... > > > > 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; > > Call path from here is: > > sys_uselib -> load_elf_library -> elf_load -> elf_map -> vm_mmap -> > vm_mmap_pgoff > > Call path for normal mmap is: > > sys_mmap_pgoff -> ksys_mmap_pgoff -> vm_mmap_pgoff > > So I think the call paths converge before any real security checks > happen, and the check you're suggesting should be superfluous. (There > is some weird audit call in ksys_mmap_pgoff() but that's just to > record the FD number, so I guess that doesn't matter.) Yeah, I was just noticing this. I was over thinking. :) It does look like all that is needed is to remove __FMODE_EXEC. -- Kees Cook
