Re: [PATCH] PCI: dwc: keystone: Fix potential NULL dereference

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Mar 29, 2024 at 08:19:47AM +0300, Aleksandr Mishin wrote:
> In ks_pcie_setup_rc_app_regs() resource_list_first_type() may return
> NULL which is later dereferenced. Fix this bug by adding NULL check.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: 0f71c60ffd26 ("PCI: dwc: Remove storing of PCI resources")
> Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
> ---
>  drivers/pci/controller/dwc/pci-keystone.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/dwc/pci-keystone.c b/drivers/pci/controller/dwc/pci-keystone.c
> index 844de4418724..00d616654171 100644
> --- a/drivers/pci/controller/dwc/pci-keystone.c
> +++ b/drivers/pci/controller/dwc/pci-keystone.c
> @@ -392,7 +392,11 @@ static void ks_pcie_setup_rc_app_regs(struct keystone_pcie *ks_pcie)
>  	struct resource *mem;
>  	int i;
>  
> -	mem = resource_list_first_type(&pp->bridge->windows, IORESOURCE_MEM)->res;
> +	struct resource_entry *ft = resource_list_first_type(&pp->bridge->windows, IORESOURCE_MEM);
> +	if (!ft)
> +		return;

Like the other one
(https://lore.kernel.org/all/20240328180126.23574-1-amishin@t-argos.ru/),
I think this potentially avoids a NULL pointer dereference (though I
didn't do the analysis in this case to see whether that's actually
possible), but fails to consider the implication of simply skipping
the rest of ks_pcie_setup_rc_app_regs().

"start" and "end" are used only for the loop about "Using Direct 1:1
mapping of RC <-> PCI memory space".  If there's no IORESOURCE_MEM
resource, obviously that loop makes no sense.  The function would
probably be improved by moving the resource_list_first_type() so it's
next to the loop that uses the result.

But the rest of the function doesn't depend on that IORESOURCE_MEM
resource, and it's not at all clear that the rest of the function
should be skipped.

> +	mem = ft->res;
>  	start = mem->start;
>  	end = mem->end;
>  
> -- 
> 2.30.2
>