Hi Szabolcs, On Fri, Nov 27, 2020 at 01:19:16PM +0000, Szabolcs Nagy wrote: > This is v2 of > https://sourceware.org/pipermail/libc-alpha/2020-November/119305.html > > To enable BTI support, re-mmap executable segments instead of > mprotecting them in case mprotect is seccomp filtered. > > I would like linux to change to map the main exe with PROT_BTI when > that is marked as BTI compatible. From the linux side i heard the > following concerns about this: > - it's an ABI change so requires some ABI bump. (this is fine with > me, i think glibc does not care about backward compat as nothing > can reasonably rely on the current behaviour, but if we have a > new bit in auxv or similar then we can save one mprotect call.) I'm not concerned about the ABI change but there are workarounds like a new auxv bit. > - in case we discover compatibility issues with user binaries it's > better if userspace can easily disable BTI (e.g. removing the > mprotect based on some env var, but if kernel adds PROT_BTI and > mprotect is filtered then we have no reliable way to remove that > from executables. this problem already exists for static linked > exes, although admittedly those are less of a compat concern.) This is our main concern. For static binaries, the linker could detect, in theory, potential issues when linking and not set the corresponding ELF information. At runtime, a dynamic linker could detect issues and avoid enabling BTI. In both cases, it's a (static or dynamic) linker decision that belongs in user-space. > - ideally PROT_BTI would be added via a new syscall that does not > interfere with PROT_EXEC filtering. (this does not conflict with > the current patches: even with a new syscall we need a fallback.) This can be discussed as a long term solution. > - solve it in systemd (e.g. turn off the filter, use better filter): > i would prefer not to have aarch64 (or BTI) specific policy in > user code. and there was no satisfying way to do this portably. I agree. I think the best for now (as a back-portable glibc fix) is to ignore the mprotect(PROT_EXEC|PROT_BTI) error that the dynamic loader gets. BTI will be disabled if MDWX is enabled. In the meantime, we should start (continue) looking at a solution that works for both systemd and the kernel and be generic enough for other architectures. The stateless nature of the current SECCOMP approach is not suitable for this W^X policy. Kees had some suggestions here but the thread seems to have died: https://lore.kernel.org/kernel-hardening/202010221256.A4F95FD11@keescook/ -- Catalin
