On Thu, Jul 18, 2024 at 09:02:56AM +0800, Andy Lutomirski wrote: > > On Jul 17, 2024, at 6:01 PM, Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > On Wed, Jul 17, 2024 at 09:26:22AM +0100, Steve Dower wrote: > >>> On 17/07/2024 07:33, Jeff Xu wrote: > >>> Consider those cases: I think: > >>> a> relying purely on userspace for enforcement does't seem to be > >>> effective, e.g. it is trivial to call open(), then mmap() it into > >>> executable memory. > >> > >> If there's a way to do this without running executable code that had to pass > >> a previous execveat() check, then yeah, it's not effective (e.g. a Python > >> interpreter that *doesn't* enforce execveat() is a trivial way to do it). > >> > >> Once arbitrary code is running, all bets are off. So long as all arbitrary > >> code is being checked itself, it's allowed to do things that would bypass > >> later checks (and it's up to whoever audited it in the first place to > >> prevent this by not giving it the special mark that allows it to pass the > >> check). > > > > Exactly. As explained in the patches, one crucial prerequisite is that > > the executable code is trusted, and the system must provide integrity > > guarantees. We cannot do anything without that. This patches series is > > a building block to fix a blind spot on Linux systems to be able to > > fully control executability. > > Circling back to my previous comment (did that ever get noticed?), I Yes, I replied to your comments. Did I miss something? > don’t think this is quite right: > > https://lore.kernel.org/all/CALCETrWYu=PYJSgyJ-vaa+3BGAry8Jo8xErZLiGR3U5h6+U0tA@xxxxxxxxxxxxxx/ > > On a basic system configuration, a given path either may or may not be > executed. And maybe that path has some integrity check (dm-verity, > etc). So the kernel should tell the interpreter/loader whether the > target may be executed. All fine. > > But I think the more complex cases are more interesting, and the > “execute a program” process IS NOT BINARY. An attempt to execute can > be rejected outright, or it can be allowed *with a change to creds or > security context*. It would be entirely reasonable to have a policy > that allows execution of non-integrity-checked files but in a very > locked down context only. I guess you mean to transition to a sandbox when executing an untrusted file. This is a good idea. I talked about role transition in the patch's description: With the information that a script interpreter is about to interpret a script, an LSM security policy can adjust caller's access rights or log execution request as for native script execution (e.g. role transition). This is possible thanks to the call to security_bprm_creds_for_exec(). > > So… shouldn’t a patch series to this effect actually support this? > This patch series brings the minimal building blocks to have a consistent execution environment. Role transitions for script execution are left to LSMs. For instance, we could extend Landlock to automatically sandbox untrusted scripts.
