linux-kernel - Re: [PATCH v7 3/6] mce: fix set_mce_nospec to always unmap the whole page

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Message-ID: <YlVPcrK4SSXyPx+Y@zn.tnic>
Date:   Tue, 12 Apr 2022 12:07:46 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     david@...morbit.com, djwong@...nel.org, dan.j.williams@...el.com,
        hch@...radead.org, vishal.l.verma@...el.com, dave.jiang@...el.com,
        agk@...hat.com, snitzer@...hat.com, dm-devel@...hat.com,
        ira.weiny@...el.com, willy@...radead.org, vgoyal@...hat.com,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v7 3/6] mce: fix set_mce_nospec to always unmap the whole
 page

On Tue, Apr 05, 2022 at 01:47:44PM -0600, Jane Chu wrote:
> The set_memory_uc() approach doesn't work well in all cases.
> For example, when "The VMM unmapped the bad page from guest
> physical space and passed the machine check to the guest."
> "The guest gets virtual #MC on an access to that page.
>  When the guest tries to do set_memory_uc() and instructs
>  cpa_flush() to do clean caches that results in taking another
>  fault / exception perhaps because the VMM unmapped the page
>  from the guest."

I presume this is quoting someone...

> Since the driver has special knowledge to handle NP or UC,
> let's mark the poisoned page with NP and let driver handle it

s/let's mark/mark/

> when it comes down to repair.
> 
> Please refer to discussions here for more details.
> https://lore.kernel.org/all/CAPcyv4hrXPb1tASBZUg-GgdVs0OOFKXMXLiHmktg_kFi7YBMyQ@mail.gmail.com/
> 
> Now since poisoned page is marked as not-present, in order to
> avoid writing to a 'np' page and trigger kernel Oops, also fix

You can write it out: "non-present page..."

> pmem_do_write().
> 
> Fixes: 284ce4011ba6 ("x86/memory_failure: Introduce {set, clear}_mce_nospec()")
> Signed-off-by: Jane Chu <jane.chu@...cle.com>
> ---
>  arch/x86/kernel/cpu/mce/core.c |  6 +++---
>  arch/x86/mm/pat/set_memory.c   | 18 ++++++------------
>  drivers/nvdimm/pmem.c          | 31 +++++++------------------------
>  include/linux/set_memory.h     |  4 ++--
>  4 files changed, 18 insertions(+), 41 deletions(-)

For such mixed subsystem patches we probably should talk about who picks
them up, eventually...

> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index 981496e6bc0e..fa67bb9d1afe 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -579,7 +579,7 @@ static int uc_decode_notifier(struct notifier_block *nb, unsigned long val,
>  
>  	pfn = mce->addr >> PAGE_SHIFT;
>  	if (!memory_failure(pfn, 0)) {
> -		set_mce_nospec(pfn, whole_page(mce));
> +		set_mce_nospec(pfn);
>  		mce->kflags |= MCE_HANDLED_UC;
>  	}
>  
> @@ -1316,7 +1316,7 @@ static void kill_me_maybe(struct callback_head *cb)
>  
>  	ret = memory_failure(p->mce_addr >> PAGE_SHIFT, flags);
>  	if (!ret) {
> -		set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
> +		set_mce_nospec(p->mce_addr >> PAGE_SHIFT);
>  		sync_core();
>  		return;
>  	}
> @@ -1342,7 +1342,7 @@ static void kill_me_never(struct callback_head *cb)
>  	p->mce_count = 0;
>  	pr_err("Kernel accessed poison in user space at %llx\n", p->mce_addr);
>  	if (!memory_failure(p->mce_addr >> PAGE_SHIFT, 0))
> -		set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
> +		set_mce_nospec(p->mce_addr >> PAGE_SHIFT);
>  }

Both that ->mce_whole_page and whole_page() look unused after this.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette