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

[Lee Jones]

On Wed, 20 Jul 2022, Will Deacon wrote:

> 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.

No worries.  Just thought I'd ask.

> 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.

I'll add it to my TODO.

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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