Re: [PATCH v2] arm64: io: Reject non-user protection in ioremap_prot()

[Will Deacon]

On Fri, Sep 11, 2026 at 09:58:59AM +0800, Zeng Heng wrote:
> From: Zeng Heng <zengheng4@xxxxxxxxxx>
> 
> Mapping a stack-top page via /dev/mem with PROT_NONE and then
> reading that process's /proc/<pid>/cmdline triggers a spurious WARN
> in ioremap_prot() through generic_access_phys():
> 
>   WARNING: ./arch/arm64/include/asm/io.h:275 at generic_access_phys
>   Call trace:
>     generic_access_phys+0x1c8/0x228 (P)
>     __access_remote_vm+0x2b4/0x398
>     access_remote_vm+0x14/0x30
>     get_mm_cmdline+0xf8/0x2a0
>     proc_pid_cmdline_read+0x68/0x120
> 
> generic_access_phys() passes the protection derived from the user PTE
> to ioremap_prot(). On arm64, a PROT_NONE mapping is represented by a
> present-invalid PTE, so pte_present() still returns true and the
> protection reaches ioremap_prot().
> 
> A PROT_NONE mapping does not have PTE_USER, causing the existing
> WARN_ON_ONCE() in ioremap_prot() to fire even though this is a valid
> user mapping. Execute-only mappings have the same issue and must not
> be readable through this path either.
> 
> ioremap_prot() should therefore reject protection values without
> PTE_USER without warning. This makes the access fail cleanly for
> PROT_NONE and execute-only mappings while retaining the existing
> user-protection contract.
> 
> Fixes: 8f098037139b ("arm64: io: Extract user memory type in ioremap_prot()")
> Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/io.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> index 21c8e400107c..31e67f6bd4a7 100644
> --- a/arch/arm64/include/asm/io.h
> +++ b/arch/arm64/include/asm/io.h
> @@ -272,7 +272,8 @@ static inline void __iomem *ioremap_prot(phys_addr_t phys, size_t size,
>  	pgprot_t prot;
>  	ptval_t user_prot_val = pgprot_val(user_prot);
>  
> -	if (WARN_ON_ONCE(!(user_prot_val & PTE_USER)))
> +	/* Reject PROT_NONE and exec-only */
> +	if (!(user_prot_val & PTE_USER))
>  		return NULL;

I was initially worried about getting an invalid PTE here which happened
to have PTE_USER set (e.g. as part of a swap entry), but it looks like
follow_pfnmap_start() returns -EINVAL if the PTE isn't present.

So I think this is ok, but I'll wait to see what Catalin thinks.

Will