Re: [PATCH] arm64: io: Reject present-invalid user prot in ioremap_prot()

[Will Deacon]

On Wed, Sep 09, 2026 at 01:50:09PM +0100, Catalin Marinas wrote:
> On Sat, Sep 05, 2026 at 11:31:48AM +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 full pgprot derived from the user PTE
> > to ioremap_prot(). A PROT_NONE /dev/mem mapping is encoded as PAGE_NONE,
> > which clears PTE_VALID and sets the software PTE_PRESENT_INVALID bit.
> > On arm64 such an entry is still pte_present(), so follow_pfnmap_start()
> > reports the pfn and generic_access_phys() reaches ioremap_prot().
> > The PTE_USER assertion, which is meant to catch kernel prots being
> > passed by mistake, then fires for a PROT_NONE user mapping
> > that legitimately lacks PTE_USER, producing the spurious WARN.
> > 
> > Reject a user prot encoding a present-invalid (i.e. PROT_NONE) entry up
> > front so that generic_access_phys() cleanly fails the access instead
> > of warning. Note that PTE_PRESENT_INVALID aliases the PTE_NG bit and
> > is only meaningful when PTE_VALID is clear, so both bits must be
> > checked together.
> > 
> > Fixes: 8f098037139b ("arm64: io: Extract user memory type in ioremap_prot()")
> > Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/io.h | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> > index 21c8e400107c..bbfbc4682639 100644
> > --- a/arch/arm64/include/asm/io.h
> > +++ b/arch/arm64/include/asm/io.h
> > @@ -272,6 +272,10 @@ 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 ((user_prot_val & (PTE_VALID | PTE_PRESENT_INVALID)) ==
> > +	     PTE_PRESENT_INVALID)
> > +		return NULL;
> > +
> >  	if (WARN_ON_ONCE(!(user_prot_val & PTE_USER)))
> >  		return NULL;
> 
> I wonder whether we should just drop the warning and return NULL if
> !PTE_USER && PTE_UXN. The latter check would also catch execute-only
> mappings (Sashiko pointed out this case still trips the warning). The
> PROT_NONE case would be covered automatically as well since PTE_USER is
> cleared, PTE_UXN set. Maybe add a comment that that pte_protnone()
> relies on !PTE_USER && PTE_UXN, so it's not that we avoid the PROT_NONE
> issue by chance.

Hmm, do we want exec-only mappings to be readable via /dev/mem?

Will