Add RELRO build knob, default to enabled
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Authored by emaste on Jun 22 2022, 1:39 PM.

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imp
markj

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rGee71c37bc123: Add RELRO build knob, default to enabled
rG2f3a961487c9: Add RELRO build knob, default to enabled

Summary

Note that lld enables relro by default, so that we already had either partial or full RELRO, depending on the state of the BIND_NOW knob.

Add a RELRO knob so that the option can be disabled if desired, and so that builds using the GNU toolchain are equivalent to those using the standard Clang/LLVM toolchain.

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emaste created this revision.Jun 22 2022, 1:39 PM

Herald added a subscriber: bdrewery. · View Herald TranscriptJun 22 2022, 1:39 PM

emaste requested review of this revision.Jun 22 2022, 1:39 PM

markj added inline comments.Jun 22 2022, 2:23 PM

share/mk/bsd.prog.mk
48	What about bsd.lib.mk?
tools/build/options/WITH_RELRO
2	Do you plan to add documentation anywhere explaining what the mitigation does? In particular, you need to also set WITH_BIND_NOW to get "full" RELRO.

emaste added inline comments.Jun 22 2022, 2:30 PM

share/mk/bsd.prog.mk
48	should be applicable only to linking binaries, i.e. .plt and .got.plt
tools/build/options/WITH_RELRO
2	Not quite sure where it should go - this WITH_RELRO won't actually be used with the option on by default. Maybe describe partial/full RELRO in the BIND_NOW option?

emaste added inline comments.Jun 22 2022, 2:37 PM

tools/build/options/WITH_RELRO
2	Or add here Binaries will not have a `GNU_RELRO` segment and the .plt and .got.plt sections will not be switched to read-only after applying relocations. See also BIND_NOW. Or maybe src.conf isn't the right place to try to explain this, and we should have text in security(7) or a man page on elf hardening?

emaste added inline comments.Jun 22 2022, 2:47 PM

tools/build/options/WITH_RELRO
2	Perhaps in BIND_NOW The combination of the .Va BIND_NOW and .Va RELRO options provide "full" Relocation Read-Only (RELRO) support. With full RELRO the entire GOT is made read-only after performing relocation at startup, avoiding GOT overwrite attacks.

markj added inline comments.Jun 22 2022, 2:58 PM

share/mk/bsd.prog.mk
48	I don't really follow. RELRO applies to shared libs too.
tools/build/options/WITH_RELRO
2	I think it'd make the most sense to describe the mitigation in security.7 (or anywhere that's not src.conf.5), but mention briefly the relationship between RELRO and BIND_NOW here. Your last suggestion seems fine to me. I'd also add "see the WITH_BIND_NOW option" to the description of WITH_RELRO.

emaste added inline comments.Jun 22 2022, 3:01 PM

share/mk/bsd.prog.mk
48	Yes of course, just a braino.
tools/build/options/WITH_RELRO
2	security.7 needs a more major overhaul, I should document it more fully there in the future. For now I think the last suggestion along with a xref is fine. The xref would have to be in both WITH_RELRO and WITHOUT_RELRO.