On Wed, Jul 17, 2024 at 06:51:11PM -0700, Jeff Xu wrote: > On Wed, Jul 17, 2024 at 3:00 AM 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). > > > We will want to define what is considered as "arbitrary code is running" > > Using an example of ROP, attackers change the return address in stack, > e.g. direct the execution flow to a gauge to call "ld.so /tmp/a.out", > do you consider "arbitrary code is running" when stack is overwritten > ? or after execve() is called. Yes, ROP is arbitrary code execution (which can be mitigated with CFI). ROP could be enough to interpret custom commands and create a small interpreter/VM. > If it is later, this patch can prevent "ld.so /tmp/a.out". > > > 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. > > Even trusted executable can have a bug. Definitely, but this patch series is dedicated to script execution control. > > I'm thinking in the context of ChromeOS, where all its system services > are from trusted partitions, and legit code won't load .so from a > non-exec mount. But we want to sandbox those services, so even under > some kind of ROP attack, the service still won't be able to load .so > from /tmp. Of course, if an attacker can already write arbitrary > length of data into the stack, it is probably already a game over. > OK, you want to tie executable file permission to mmap. That makes sense if you have a consistent execution model. This can be enforced by LSMs. Contrary to script interpretation which is a full user space implementation (and then controlled by user space), mmap restrictions should indeed be enforced by the kernel.
