[PATCH V2] arm64: fix oops in concurrently setting insn_emulation sysctls

[Will Deacon]

On Wed, Jul 20, 2022 at 01:23:24PM +0100, Lee Jones wrote:
> On Sat, 02 Jul 2022, haibinzhang(张海斌) wrote:
> 
> > How to reproduce:
> >     launch two shell executions:
> >        #!/bin/bash
> >        while [ 1 ];
> >        do
> >            echo 1 > /proc/sys/abi/swp
> >        done
> > 
> > Oops info:
> >     Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> >     Internal error: Oops: 96000006 [#1] SMP
> >     Call trace:
> >     update_insn_emulation_mode+0xc0/0x148
> >     emulation_proc_handler+0x64/0xb8
> >     proc_sys_call_handler+0x9c/0xf8
> >     proc_sys_write+0x18/0x20
> >     __vfs_write+0x20/0x48
> >     vfs_write+0xe4/0x1d0
> >     ksys_write+0x70/0xf8
> >     __arm64_sys_write+0x20/0x28
> >     el0_svc_common.constprop.0+0x7c/0x1c0
> >     el0_svc_handler+0x2c/0xa0
> >     el0_svc+0x8/0x200
> > 
> > emulation_proc_handler changes table->data for proc_dointvec_minmax
> > and so it isn't allowed to reenter before restoring table->data,
> > which isn't right now.
> > To fix this issue, keep the table->data as &insn->current_mode and
> > use container_of() to retrieve the insn pointer. Another mutex is
> > used to protect against the current_mode update but not for retrieving
> > insn_emulation as table->data is no longer changing.
> 
> Looks as though this lost its Fixes tag during the rework.
> 
>   Fixes: 587064b610c7 ("arm64: Add framework for legacy instruction emulation")
> 
> Will, are you able to add this retroactively?

Sadly, this is now buried under some other patches so I'd have to rebase the
branch if I were to add this and I don't think it's worth it just to add a
tag. On the plus side, the patch has a Link: tag to this thread, so the
Fixes tag is retrievable if you're determined enough.

If somebody wants this for stable, then I suppose they'll have to send
a backport to make sure it doesn't get missed.

Will