linux-kernel - Re: [RFC PATCH 1/1] mm/vmalloc: Modify permission reset procedure to avoid invalid access

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Date: Tue, 11 Jun 2024 14:21:42 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "lstoakes@...il.com" <lstoakes@...il.com>, "akpm@...ux-foundation.org"
	<akpm@...ux-foundation.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hch@...radead.org" <hch@...radead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
	"urezki@...il.com" <urezki@...il.com>, "ycliang@...estech.com"
	<ycliang@...estech.com>
CC: "patrick@...estech.com" <patrick@...estech.com>
Subject: Re: [RFC PATCH 1/1] mm/vmalloc: Modify permission reset procedure to
 avoid invalid access

On Tue, 2024-06-11 at 21:13 +0800, Leo Yu-Chi Liang wrote:
> The previous reset procedure is
> 1. Set direct map attribute to invalid
> 2. Flush TLB
> 3. Reset direct map attribute to default
> 
> It is possible that kernel forks another process
> on another core that access the invalid mappings after
> sync_kernel_mappings.
> 
> We could reproduce this scenario by running LTP/bpf_prog
> multiple times on RV32 kernel on QEMU.
> 
> Therefore, the following procedure is proposed
> to avoid mappings being invalid.
> 1. Reset direct map attribute to default
> 2. Flush TLB

Can you explain more about what is happening in this scenario? Looking briefly,
riscv is doing something unique around sync_kernel_mappings(). If a RO mapping
is copied instead of a NP/invalid mapping, how is the problem avoided?