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

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Date: Fri, 17 May 2024 09:10:40 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Catalin Marinas <catalin.marinas@....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 5/14/24 3:53 AM, Catalin Marinas wrote:
> 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.

I see. Thanks. Yes, I did see this works without PIE. As you said in the 
earlier email, exec-only is not that popular yet. I think we can just 
ignore it for now.

>