On Fri, Jul 19, 2024 at 07:16:55AM -0700, Jeff Xu wrote: > On Fri, Jul 19, 2024 at 1:45 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > On Thu, Jul 18, 2024 at 06:29:54PM -0700, Jeff Xu wrote: > > > On Thu, Jul 18, 2024 at 5:24 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > > > > > On Wed, Jul 17, 2024 at 07:08:17PM -0700, Jeff Xu wrote: > > > > > On Wed, Jul 17, 2024 at 3:01 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > > > > > > > > > On Tue, Jul 16, 2024 at 11:33:55PM -0700, Jeff Xu wrote: > > > > > > > On Thu, Jul 4, 2024 at 12:02 PM Mickaël Salaün <mic@xxxxxxxxxxx> 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. > > > > > > > > > > > > > > > So we concluded that execveat(AT_CHECK) will be used to check the > > > > > > > exec, shared object, script and config file (such as seccomp config), > > > > > > > > > I think binfmt_elf.c in the kernel needs to check the ld.so to make > > > > > > > sure it passes AT_CHECK, before loading it into memory. > > > > > > > > > > > > All ELF dependencies are opened and checked with open_exec(), which > > > > > > perform the main executability checks (with the __FMODE_EXEC flag). > > > > > > Did I miss something? > > > > > > > > > > > I mean the ld-linux-x86-64.so.2 which is loaded by binfmt in the kernel. > > > > > The app can choose its own dynamic linker path during build, (maybe > > > > > even statically link one ?) This is another reason that relying on a > > > > > userspace only is not enough. > > > > > > > > The kernel calls open_exec() on all dependencies, including > > > > ld-linux-x86-64.so.2, so these files are checked for executability too. > > > > > > > This might not be entirely true. iiuc, kernel calls open_exec for > > > open_exec for interpreter, but not all its dependency (e.g. libc.so.6) > > > > Correct, the dynamic linker is in charge of that, which is why it must > > be enlighten with execveat+AT_CHECK and securebits checks. > > > > > load_elf_binary() { > > > interpreter = open_exec(elf_interpreter); > > > } > > > > > > libc.so.6 is opened and mapped by dynamic linker. > > > so the call sequence is: > > > execve(a.out) > > > - open exec(a.out) > > > - security_bprm_creds(a.out) > > > - open the exec(ld.so) > > > - call open_exec() for interruptor (ld.so) > > > - call execveat(AT_CHECK, ld.so) <-- do we want ld.so going through > > > the same check and code path as libc.so below ? > > > > open_exec() checks are enough. LSMs can use this information (open + > > __FMODE_EXEC) if needed. execveat+AT_CHECK is only a user space > > request. > > > Then the ld.so doesn't go through the same security_bprm_creds() check > as other .so. Indeed, but... > > As my previous email, the ChromeOS LSM restricts executable mfd > through security_bprm_creds(), the end result is that ld.so can still > be executable memfd, but not other .so. The chromeOS LSM can check that with the security_file_open() hook and the __FMODE_EXEC flag, see Landlock's implementation. I think this should be the only hook implementation that chromeOS LSM needs to add. > > One way to address this is to refactor the necessary code from > execveat() code patch, and make it available to call from both kernel > and execveat() code paths., but if we do that, we might as well use > faccessat2(AT_CHECK) That's why I think it makes sense to rely on the existing __FMODE_EXEC information. > > > > > - transfer the control to ld.so) > > > - ld.so open (libc.so) > > > - ld.so call execveat(AT_CHECK,libc.so) <-- proposed by this patch, > > > require dynamic linker change. > > > - ld.so mmap(libc.so,rx) > > > > Explaining these steps is useful. I'll include that in the next patch > > series.