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Date: Sat, 19 Sep 2026 16:02:51 -0700
From: Wei Liu <wei.liu@kernel.org>
To: Naman Jain <namjain@linux.microsoft.com>
Cc: Hardik Garg <hargar@linux.microsoft.com>, kys@microsoft.com,
	haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com,
	mhklinux@outlook.com, ssengar@linux.microsoft.com,
	romank@linux.microsoft.com, linux-hyperv@vger.kernel.org,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] mshv_vtl: Check per-CPU register page before mmap
Message-ID: <20260919230251.GB1008213@liuwe-devbox-debian-v2.local>
References: <20260911175146.1370148-1-hargar@linux.microsoft.com>
 <3f771293-f5df-4e74-aca7-5d34966d00e0@linux.microsoft.com>
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In-Reply-To: <3f771293-f5df-4e74-aca7-5d34966d00e0@linux.microsoft.com>

On Tue, Sep 15, 2026 at 10:11:36AM +0530, Naman Jain wrote:
> 
> 
> On 9/11/2026 11:21 PM, Hardik Garg wrote:
> > Register-page setup is optional for each CPU. If allocation or
> > registration fails, mshv_vtl_configure_reg_page() warns and leaves that
> > CPU's reg_page NULL, but initial context setup continues successfully.
> > Meanwhile, successful setup on another CPU sets the global
> > mshv_has_reg_page flag.
> > 
> > mshv_vtl_fault() checks this global flag before selecting the requested
> > CPU's register page. With mixed setup results across online CPUs, the
> > check passes even for a CPU with no register page, reaching
> > get_page(NULL) when userspace faults in that mapping.
> > 
> > Check the selected per-CPU register page before taking its reference and
> > return VM_FAULT_SIGBUS if it is absent.
> > 
> > Fixes: 7bfe3b8ea6e3 ("Drivers: hv: Introduce mshv_vtl driver")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Hardik Garg <hargar@linux.microsoft.com>
> > ---
> >   drivers/hv/mshv_vtl_main.c | 3 +++
> >   1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/hv/mshv_vtl_main.c b/drivers/hv/mshv_vtl_main.c
> > index fc993560a45c..3296ab5c0243 100644
> > --- a/drivers/hv/mshv_vtl_main.c
> > +++ b/drivers/hv/mshv_vtl_main.c
> > @@ -912,6 +912,9 @@ static vm_fault_t mshv_vtl_fault(struct vm_fault *vmf)
> >   		return VM_FAULT_NOPAGE;
> >   	}
> > +	if (!page)
> > +		return VM_FAULT_SIGBUS;
> > +
> >   	get_page(page);
> >   	vmf->page = page;
> 
> 
> LGTM. Thanks.
> 
> Reviewed-by: Naman Jain <namjain@linux.microsoft.com>

Applied. Thanks.