linux-kernel - Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT

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Date:   Tue, 3 Nov 2020 23:41:42 -0600
From:   Jeremy Linton <jeremy.linton@....com>
To:     Mark Brown <broonie@...nel.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>
Cc:     libc-alpha@...rceware.org,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Topi Miettinen <toiwoton@...il.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ
 #26831]

Hi,

On 11/3/20 11:34 AM, Mark Brown wrote:
> On Tue, Nov 03, 2020 at 10:25:37AM +0000, Szabolcs Nagy wrote:
> 
>> Re-mmap executable segments instead of mprotecting them in
>> case mprotect is seccomp filtered.
> 
>> For the kernel mapped main executable we don't have the fd
>> for re-mmap so linux needs to be updated to add BTI. (In the
>> presence of seccomp filters for mprotect(PROT_EXEC) the libc
>> cannot change BTI protection at runtime based on user space
>> policy so it is better if the kernel maps BTI compatible
>> binaries with PROT_BTI by default.)
> 
> Given that there were still some ongoing discussions on a more robust
> kernel interface here and there seem to be a few concerns with this
> series should we perhaps just take a step back and disable this seccomp
> filter in systemd on arm64, at least for the time being?  That seems
> safer than rolling out things that set ABI quickly, a big part of the

So, that's a bigger hammer than I think is needed and punishes !BTI 
machines. I'm going to suggest that if we need to carry a temp patch its 
more like the glibc patch I mentioned in the Fedora defect. That patch 
simply logs a message, on the mprotect failures rather than aborting. 
Its fairly non-intrusive.

That leaves seccomp functional, and BTI generally functional except when 
seccomp is restricting it. I've also been asked that if a patch like 
that is needed, its (temporary?) merged to the glibc trunk, rather than 
just being carried by the distro's.

Thanks,

> reason we went with having the dynamic linker enable PROT_BTI in the
> first place was to give us more flexibility to handle any unforseen
> consequences of enabling BTI that we run into.  We are going to have
> similar issues with other features like MTE so we need to make sure that
> whatever we're doing works with them too.
> 
> Also updated to Will's current e-mail address - Will, do you have
> thoughts on what we should do here?
>