From: Catalin Marinas <catalin.marinas@arm.com>
To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: libc-alpha@sourceware.org, Mark Rutland <mark.rutland@arm.com>,
Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org,
Jeremy Linton <jeremy.linton@arm.com>,
Mark Brown <broonie@kernel.org>,
kernel-hardening@lists.openwall.com,
Topi Miettinen <toiwoton@gmail.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831]
Date: Thu, 3 Dec 2020 17:30:06 +0000 [thread overview]
Message-ID: <20201203173006.GH2830@gaia> (raw)
In-Reply-To: <cover.1606319495.git.szabolcs.nagy@arm.com>
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
next prev parent reply other threads:[~2020-12-03 17:30 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-27 13:19 [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831] Szabolcs Nagy
2020-11-27 13:19 ` [PATCH v2 1/6] aarch64: Fix missing BTI protection from dependencies [BZ #26926] Szabolcs Nagy
2020-12-10 17:51 ` Adhemerval Zanella
2020-12-11 15:33 ` Szabolcs Nagy
2020-11-27 13:20 ` [PATCH v2 2/6] elf: lose is closely tied to _dl_map_object_from_fd Szabolcs Nagy
2020-11-27 13:20 ` [PATCH v2 3/6] elf: Fix failure handling in _dl_map_object_from_fd Szabolcs Nagy
2020-12-10 18:25 ` Adhemerval Zanella
2020-12-11 9:32 ` Szabolcs Nagy
2020-11-27 13:20 ` [PATCH v2 4/6] elf: Move note processing after l_phdr is updated Szabolcs Nagy
2020-11-27 13:21 ` [PATCH v2 5/6] elf: Pass the fd to note processing Szabolcs Nagy
2020-12-10 18:35 ` Adhemerval Zanella
2020-11-27 13:21 ` [PATCH v2 6/6] aarch64: Use mmap to add PROT_BTI instead of mprotect [BZ #26831] Szabolcs Nagy
2020-12-02 8:55 ` [PATCH v3 1/2] aarch64: align address for BTI protection [BZ #26988] Szabolcs Nagy
2020-12-10 18:49 ` Adhemerval Zanella
2020-12-02 8:55 ` [PATCH v3 2/2] aarch64: Use mmap to add PROT_BTI instead of mprotect [BZ #26831] Szabolcs Nagy
2020-12-10 19:12 ` Adhemerval Zanella
2020-11-30 15:56 ` [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) " Szabolcs Nagy
2020-12-03 17:30 ` Catalin Marinas [this message]
2020-12-07 20:03 ` Szabolcs Nagy
2020-12-11 17:46 ` Catalin Marinas
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