linux-hardening - Re: Fine-grained Forward CFI on top of Intel CET / IBT

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Date:   Fri, 19 Mar 2021 15:51:04 -0700
From:   Joao Moreira <joao@...rdrivepizza.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     x86-64-abi@...glegroups.com, kernel-hardening@...ts.openwall.com,
        samitolvanen@...gle.com, hjl.tools@...il.com,
        linux-hardening@...r.kernel.org
Subject: Re: Fine-grained Forward CFI on top of Intel CET / IBT


>> That is a good point about R11 availability. Have you examined kernel
>> images for unintended gadgets? It seems like it'd be rare to find an 
>> arbitrary R11 load
>> followed by an indirect call together, but stranger gadgets show up, and
>> before the BPF JIT obfuscation happened, it was possible for attackers
>> (with sufficient access) to construct a series of immediates that would
>> contain the needed gadgets. (And not all systems run with BPF JIT
>> hardening enabled.)
>
> I haven't. On a CET-enabled environment, these unintended gadgets 
> would need to be preceded with an endbr instruction, otherwise they 
> won't be reachable indirectly. I assume that these cases can still 
> exist (specially in the presence of things like vulnerable BPF JIT or 
> if you consider full non-fineibt-instrumented functions working as 
> gadgets), but that this is a raised bar. Besides that, there are 
> patches like this one (which unfortunately was abandoned) that could 
> come handy:
>
> https://reviews.llvm.org/D88194
>
Actually (as clear in the end of the patch review) this was replaced by 
a different patch, which got in :)

review: https://reviews.llvm.org/D89178

commit: https://reviews.llvm.org/rGf385823e04f300c92ec03dbd660d621cc618a271


o/

Joao