On Thu, Jun 10, 2021 at 11:28:12AM -0500, Jeremy Linton via Libc-alpha wrote: > Hi, > > On 6/4/21 6:24 AM, Mark Brown wrote: > >Deployments of BTI on arm64 have run into issues interacting with > >systemd's MemoryDenyWriteExecute feature. Currently for dynamically > >linked executables the kernel will only handle architecture specific > >properties like BTI for the interpreter, the expectation is that the > >interpreter will then handle any properties on the main executable. > >For BTI this means remapping the executable segments PROT_EXEC | > >PROT_BTI. > > > >This interacts poorly with MemoryDenyWriteExecute since that is > >implemented using a seccomp filter which prevents setting PROT_EXEC on > >already mapped memory and lacks the context to be able to detect that > >memory is already mapped with PROT_EXEC. This series resolves this by > >handling the BTI property for both the interpreter and the main > >executable. > > I've got a Fedora34 system booting in qemu or a model with BTI enabled. On > that system I took the systemd-resolved executable, which is one of the > services with MDWE enabled, and replaced a number of the bti's with nops. I > expect the service to continue to work with the fedora or mainline 5.13 > kernel and it does. If instead I boot with MDWE=no for the service, it > should fail to start given either of those kernels, and it does. > > Thus, I expect that with his patch applied to 5.13 the service will fail to > start regardless of the state of MDWE, but it seems to continue starting > when I set MDWE=yes. Same behavior with v1 FWTW. > > Of course, there is a good chance I've messed something up or i'm missing > something. I should really validate the /lib/ld-linux behavior itself too. I > guess this could just as well be a glibc issue (f34 has glibc 2.33-5 which > appears to have the re-mmap on failure patch). Either way, systemd-resolved > is a LSB PIE, with /lib/ld-linux as its interpreter. I've not dug too deep > into debugging this, cause I've got a couple other things I need to deal > with in the next couple days, and I strongly dislike booting a full > debug+system on the model. chuckle, sorry... [...] If the failure we're trying to detect is that BTI is undesirably left off for the main executable, surely replacing BTIs with NOPs will make no differenece? The behaviour with PROT_BTI clear is strictly more permissive than with PROT_BTI set, so I'm not sure we can test the behaviour this way. Maybe I'm missing sometihng / confused myself somewhere. Looking at /proc/<pid>/maps after the process starts up may be a more reliable approach, so see what the actual prot value is on the main executable's text pages. Cheers ---Dave
