linux-kernel - Re: [PATCH] arm64: mm: force write fault for atomic RMW instructions

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Date: Tue, 14 May 2024 11:53:37 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Yang Shi <yang@...amperecomputing.com>
Cc: will@...nel.org, scott@...amperecomputing.com, cl@...two.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: mm: force write fault for atomic RMW instructions

On Mon, May 13, 2024 at 09:19:39PM -0600, Yang Shi wrote:
> > That said, I'm not keen on this kernel workaround. If openjdk decides to
> > improve some security and goes for PROT_EXEC-only mappings of its text
> > sections, the above trick will no longer work.
> 
> I noticed futex does replace insns. IIUC, the below sequence should
> can do the trick for exec-only, right?
> 
> disable privileged
> read insn with ldxr
> enable privileged

Do you mean not using the unprivileged LDTR as in get_user()? You don't
even need an LDXR, just plain LDR but with the extable entry etc.

However, with PIE we got proper execute-only permission (not the kind of
fake one where we disabled the PTE_USER bit while keeping PTE_UXN as 0).
So the futex-style approach won't work unless we changed the PIE_E1
entry for _PAGE_EXECONLY to be PIE_R by the kernel.

-- 
Catalin