linux-kernel - Re: [PATCH 02/15] cxl/core: Check physical address before mapping it in devm_cxl_iomap

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Date:   Fri, 16 Sep 2022 11:04:01 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Robert Richter <rrichter@....com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 02/15] cxl/core: Check physical address before mapping it
 in devm_cxl_iomap_block()

Robert Richter wrote:
> On 07.09.22 22:48:57, Dan Williams wrote:
> > Robert Richter wrote:
> > > The physical base address of a CXL range can be invalid and is then
> > > set to CXL_RESOURCE_NONE. Early check this case before mapping a
> > > memory block in devm_cxl_iomap_block().
> > > 
> > > Signed-off-by: Robert Richter <rrichter@....com>
> > > ---
> > >  drivers/cxl/core/regs.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/cxl/core/regs.c b/drivers/cxl/core/regs.c
> > > index 39a129c57d40..f216c017a474 100644
> > > --- a/drivers/cxl/core/regs.c
> > > +++ b/drivers/cxl/core/regs.c
> > > @@ -165,6 +165,9 @@ void __iomem *devm_cxl_iomap_block(struct device *dev, resource_size_t addr,
> > >  	void __iomem *ret_val;
> > >  	struct resource *res;
> > >  
> > > +	if (addr == CXL_RESOURCE_NONE)
> > > +		return NULL;
> > > +
> > >  	res = devm_request_mem_region(dev, addr, length, dev_name(dev));
> > >  	if (!res) {
> > >  		resource_size_t end = addr + length - 1;
> > > -- 
> > > 2.30.2
> > > 
> > 
> > devm_request_mem_region() succeeds for you when this happens? More
> > details about the failure scenario please.
> 
> No, CXL_RESOURCE_NONE (all FFs) is used as address. A broken range is
> calculated that even overflows. I only vaguely remember the exact
> error message.
> 
> This may happen e.g. if the Component Register Block is missing in the
> DVSEC. cxl_find_regblock() may fail then and returns
> CXL_RESOURCE_NONE. There are a couple of code paths there
> component_reg_phys is set to CXL_RESOURCE_NONE without exiting
> immediately.
> 
> I saw it during code development, when I pre-inititalized a port with
> component_reg_phys set to CXL_RESOURCE_NONE. Since that case can
> generally happen, I think it must be checked.

I think Jonathan had it right when we posited that the code should
probably have failed before getting to this point. For example, the
scenarios where the driver looks for a component register block via the
register locator DVSEC are not valid for RCDs in the first instance.