Re: BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

From: Szabolcs Nagy <szabolcs.nagy@arm.com>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	systemd-devel@lists.freedesktop.org,
	Kees Cook <keescook@chromium.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Jeremy Linton <jeremy.linton@arm.com>,
	Mark Brown <broonie@kernel.org>,
	toiwoton@gmail.com, libc-alpha@sourceware.org,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures
Date: Mon, 26 Oct 2020 16:57:55 +0000	[thread overview]
Message-ID: <20201026165755.GV3819@arm.com> (raw)
In-Reply-To: <20201026162410.GB27285@arm.com>

The 10/26/2020 16:24, Dave Martin via Libc-alpha wrote:
> Unrolling this discussion a bit, this problem comes from a few sources:
> 
> 1) systemd is trying to implement a policy that doesn't fit SECCOMP
> syscall filtering very well.
> 
> 2) The program is trying to do something not expressible through the
> syscall interface: really the intent is to set PROT_BTI on the page,
> with no intent to set PROT_EXEC on any page that didn't already have it
> set.
> 
> 
> This limitation of mprotect() was known when I originally added PROT_BTI,
> but at that time we weren't aware of a clear use case that would fail.
> 
> 
> Would it now help to add something like:
> 
> int mchangeprot(void *addr, size_t len, int old_flags, int new_flags)
> {
> 	int ret = -EINVAL;
> 	mmap_write_lock(current->mm);
> 	if (all vmas in [addr .. addr + len) have
> 			their mprotect flags set to old_flags) {
> 
> 		ret = mprotect(addr, len, new_flags);
> 	}
> 	
> 	mmap_write_unlock(current->mm);
> 	return ret;
> }

if more prot flags are introduced then the exact
match for old_flags may be restrictive and currently
there is no way to query these flags to figure out
how to toggle one prot flag in a future proof way,
so i don't think this solves the issue completely.

i think we might need a new api, given that aarch64
now has PROT_BTI and PROT_MTE while existing code
expects RWX only, but i don't know what api is best.

> libc would now be able to do
> 
> 	mchangeprot(addr, len, PROT_EXEC | PROT_READ,
> 		PROT_EXEC | PROT_READ | PROT_BTI);
> 
> while systemd's MDWX filter would reject the call if
> 
> 	(new_flags & PROT_EXEC) &&
> 		(!(old_flags & PROT_EXEC) || (new_flags & PROT_WRITE)
> 
> 
> 
> This won't magically fix current code, but something along these lines
> might be better going forward.
> 
> 
> Thoughts?
> 
> ---Dave

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