Re: [PATCH] PCI: Fix unmasked value in pci_generic_config_write32()

On Mon, Jul 27, 2026 at 01:36:43PM +0530, Mohamad Raizudeen wrote:
> When writing less than 4 bytes, pci_generic_config_write32() uses a
> read-modify-write sequence to protect adjacent bytes. The code comment
> states the goal is to "merge in the <size> bits we intend to write".
> 
> However the code does not mask the incoming 'val' before shifting it.
> Since the function 'pci_generic_config_write32()' accepts a 'u32', a
> caller could pass a value with extra bits set outside the intended write
> size, those extra bits will shift into the adjacent bytes and corrupt them.
> 
> Fix this my masking 'val' before the shift so only the target bytes are
> written. This makes the code do what the comment says and matches the
> read function, pci_generic_config_read32().
> 
> Fixes: 1f94a94f67e10 ("PCI: Add generic config accessors")
> Signed-off-by: Mohamad Raizudeen <[email protected]>
> ---
>  drivers/pci/access.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index b123da16b63b..47b38d01694c 100644
> --- a/drivers/pci/access.c
> +++ b/drivers/pci/access.c
> @@ -176,7 +176,7 @@ int pci_generic_config_write32(struct pci_bus *bus, unsigned int devfn,
>  
>  	mask = ~(((1 << (size * 8)) - 1) << ((where & 0x3) * 8));
>  	tmp = readl(addr) & mask;
> -	tmp |= val << ((where & 0x3) * 8);
> +	tmp |= (val & ((1 << (size * 8)) - 1)) << ((where & 0x3) * 8);
>  	writel(tmp, addr);
>  
>  	return PCIBIOS_SUCCESSFUL;
> -- 
> 2.53.0
>

Hi Bjorn,

Just following up on this patch. Please let me know if you have any
feedback or if further changes are needed.

Thanks & regards,
Mohamad Raizudeen