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

[Edgecombe, Rick P]

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?