Date: Thu, 30 Jul 2020 18:01:55 +0200 From: Florian Weimer <fweimer@...hat.com> To: oss-security@...ts.openwall.com, x86-64-abi@...glegroups.com, kernel-hardening@...ts.openwall.com Cc: Szabolcs Nagy <szabolcs.nagy@....com> Subject: Alternative CET ABI CET (and Arm BTI) restrict targets for indirect jumps and calls to landing pads which start with specially-formatted NOP instruction dedicated to this purpose (endrb64 in the x86-64 case). The traditional way of implementing ELF on top of this is to have every global function start with that NOP, and also use these NOPs in PLT stubs in the main program (which may provide the canonical address of functions, i.e. there address may be taken). The downside of this approach is that all functions in the process become available for execution, whether they are used in the original program or not. (In principle, control flow integrity provides reasonably efficient ways to counteract that, by keeping track of symbol resolution and verifying flags at the start of critical functions, but we do not have automated support for that today, and there are some open issues about complex call graphs.) CET has a NOTRACK prefix for indirect jumps/and calls. It asserts that the jump target address is trusted and disables the control flow integrity check. It is expected to be used with jump tables and the like, in conjunction with RELRO (so that the address has been loaded from read-only memory). I think this also provides support for a completely different ABI, where global functions are not automatically addressable. It depends on BIND_NOW and RELRO, for a read-only GOT. First of all, it needs new relocation types that tell the static link editor which symbol references are address-significant. Generally, function addresses which end up in RELRO data only are not address-significant if they are used immediately in call instructions (without indirection of any form through writable memory). This means that direct calls do not have address significance. For vtables, it depends on how they are used; their function addresses probably need to be treated conservatively as address-significant (because the vtable pointer is in writable memory; at least for C++ vtables, the address of a virtual member function is not significant). Functions no longer start with the ENDBR64 prefix. Instead, the link editor produces a PLT entry with an ENDBR64 prefix if it detects any address-significant relocation for it. The PLT entry performs a NOTRACK jump to the target address. This assumes that the target address is subject to RELRO, of course, so that redirection is not possible. Without address-significant relocations, the link editor produces a PLT entry without the ENDBR64 prefix (but still with the NOTRACK jump), or perhaps no PLT entry at all. The net effect is that only functions which have their address taken in the original program can be called through indirect function calls. For example, this means that the system function in libc is usually dormant, and cannot be reached, even if an attacker can cause the process to call arbitrary functions with an arbitrary string argument. The reason is that the system function lacks the ENDBR64 prefix, and all PLT entries calling it also lack it. dlopen'ing a shared object which has a address-significant relocation against a function is not a problem under this model. Either there already was an address-significant relocation before, then the function already has a canonical address, and that can be used. Or there was not, then the just-loaded PLT entry (which as an ENDBR64 prefix) provides the canonical address function. To support dlsym, each global function definition would have a separate ENDBR64-enabled PLT/GOT slot for that, with the GOT slot only filled in at the time of the dlsym call (with mprotect calls around that, with some hand-waving required these can never fail). This is probably the most awkward part about all this. Alternatively, these stubs could also be generated at run time, from a pre-computed code page. Obviously, it is too late for that now for x86-64, but maybe someone else gets a chance to try this. Thanks, Florian
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