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

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

[Zeng Heng]

From: Zeng Heng <zengheng4@huawei.com>

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@huawei.com>
---
 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;
 
-- 
2.43.0

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

[Zeng Heng]

Hi all,

    Kindly ping.


On 2026/9/5 11:31, Zeng Heng wrote:
> From: Zeng Heng <zengheng4@huawei.com>
> 
> 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@huawei.com>
> ---
>   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;
>   

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

[Catalin Marinas]

On Sat, Sep 05, 2026 at 11:31:48AM +0800, Zeng Heng wrote:
> From: Zeng Heng <zengheng4@huawei.com>
> 
> 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@huawei.com>
> ---
>  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.

Unrelated to your patch, also spotted by Sashiko, we only test a single
pte but the size can span two. The fix should be at the higher level in
generic_access_phys().

-- 
Catalin

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@huawei.com>
> > 
> > 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@huawei.com>
> > ---
> >  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

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

[Catalin Marinas]

On Wed, Sep 09, 2026 at 02:04:50PM +0100, Will Deacon wrote:
> 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@huawei.com>
> > > 
> > > 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@huawei.com>
> > > ---
> > >  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?

Ah, yes, got confused on how it reaches this path. It doesn't make sense
to allow exec-only to be readable. Also if we mmap(PROT_EXEC) /dev/mem,
PTE_UXN ends up set anyway via pgprot_noncached(). But it does have
PTE_VALID, so the above won't catch it.

Checking !PTE_USER should be sufficient here and return NULL. For the
warning, I think we can check PTE_NG first but it only works without
kpti.

-- 
Catalin

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

[Zeng Heng]

Hi Catalin,

On 2026/9/10 1:07, Catalin Marinas wrote:
> On Wed, Sep 09, 2026 at 02:04:50PM +0100, Will Deacon wrote:
>> 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@huawei.com>
>>>>
>>>> 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@huawei.com>
>>>> ---
>>>>   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?
> Ah, yes, got confused on how it reaches this path. It doesn't make sense
> to allow exec-only to be readable. Also if we mmap(PROT_EXEC) /dev/mem,
> PTE_UXN ends up set anyway via pgprot_noncached(). But it does have
> PTE_VALID, so the above won't catch it.
>
> Checking !PTE_USER should be sufficient here and return NULL. For the
> warning, I think we can check PTE_NG first but it only works without
> kpti.

Thanks for your reply. I agree that checking PTE_USER is sufficient here.

I'll drop the WARN_ON_ONCE() and return NULL directly when PTE_USER
is not set. And I won't add a PTE_NG check for the warning since, as you
pointed out, that would not work reliably with KPTI.


Best regards,
Zeng Heng