Re: [PATCH] mm: use PFN_PHYS() in phys_mem_access_prot()

[Mike Rapoport <rppt@kernel.org>]

On Fri, Nov 13, 2026 at 04:38:12PM -0300, Ivan Ezequiel Rodriguez wrote:
> From: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
> 
> phys_mem_access_prot() stores "pfn << PAGE_SHIFT" in a phys_addr_t.
> The shift is evaluated as unsigned long, so on a 32-bit kernel with
> CONFIG_PHYS_ADDR_T_64BIT an address above 4 GiB is truncated before
> uncached_access() compares it with __pa(high_memory).
> 
> mmap_mem_prepare() already shifts in phys_addr_t, so the mapping is
> accepted and then left cacheable. PFN_PHYS() casts before the shift.
> 
> On a MIPS32 XPA Malta guest, mmap of /dev/mem at 0xf00000000 without
> O_DSYNC used the boot CCA. The same mapping with O_DSYNC was uncached.
> With PFN_PHYS() both are uncached.
> 
> Fixes: 7e6735c3578e ("/dev/mem: use phys_addr_t for physical addresses")
> Cc: stable@vger.kernel.org
> Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  mm/char-mem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/char-mem.c b/mm/char-mem.c
> index c5761ddedbd7..53ce0034a4c1 100644
> --- a/mm/char-mem.c
> +++ b/mm/char-mem.c
> @@ -272,7 +272,7 @@ static pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  				     unsigned long size, pgprot_t vma_prot)
>  {
>  #ifdef pgprot_noncached
> -	phys_addr_t offset = pfn << PAGE_SHIFT;
> +	phys_addr_t offset = PFN_PHYS(pfn);
>  
>  	if (uncached_access(file, offset))
>  		return pgprot_noncached(vma_prot);
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.