powerpc: fix alignment bug whithin the init sections

This is a bug that can cause early crashes in configurations with a
.exit.text section smaller than a page and a .init.text section that
ends in the beginning of a physical page (this is kinda random, which
might explain why this wasn't really encountered before).

The init sections are ordered like this:
	.init.text
	.exit.text
	.init.data

Currently, these sections aren't page aligned.

Because the init code is mapped read-only at runtime and because the
.init.text section can potentially reside on the same physical page as
.init.data, the beginning of .init.data might be mapped read-only along
with .init.text.

Then when the kernel tries to modify a variable in .init.data (like
kthreadd_done, used in kernel_init()) the kernel panics.

To avoid this, I made these sections page aligned.

Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com>
---
 arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
 1 file changed, 7 insertions(+)


base-commit: 1398820fee515873379809a6415930ad0764b2f6

Le 13/12/2020 à 19:35, Ariel Marcovitch a écrit :
> This is a bug that can cause early crashes in configurations with a
> .exit.text section smaller than a page and a .init.text section that
> ends in the beginning of a physical page (this is kinda random, which
> might explain why this wasn't really encountered before).

It can cause, or it causes ? Did you encounter the issue ?

> 
> The init sections are ordered like this:
> 	.init.text
> 	.exit.text
> 	.init.data
> 
> Currently, these sections aren't page aligned.
> 
> Because the init code is mapped read-only at runtime and because the
> .init.text section can potentially reside on the same physical page as
> .init.data, the beginning of .init.data might be mapped read-only along
> with .init.text.

init code is mapped PAGE_KERNEL_TEXT.

Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected options.

> 
> Then when the kernel tries to modify a variable in .init.data (like
> kthreadd_done, used in kernel_init()) the kernel panics.
> 
> To avoid this, I made these sections page aligned.

Should write this unpersonal, something like "To avoid this, make these sections page aligned"

> 
> Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
> Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com>
> ---
>   arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 326e113d2e45..e3a7c90c03f4 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -179,6 +179,11 @@ SECTIONS
>   #endif
>   	} :text
>   
> +	/* .init.text is made RO and .exit.text is not, so we must
> +	 * ensure these sections reside in separate physical pages.
> +	 */
> +	. = ALIGN(PAGE_SIZE);
> +

In principle, as it is text, it should be made RO as well. But what happens at the begining doesn't 
really matter, anyway .exit.text should never be executed and is discarded together with init text.
So, I think it is OK the live with it as is for the time being.

Making it page aligned makes sense anyway.

Should we make _einittext page aligned instead, just like _etext ?

>   	/* .exit.text is discarded at runtime, not link time,
>   	 * to deal with references from __bug_table
>   	 */
> @@ -186,6 +191,8 @@ SECTIONS
>   		EXIT_TEXT
>   	}
>   
> +	. = ALIGN(PAGE_SIZE);
> +

Here for sure, as you explain in the coming log, this needs to be separated from init text. So 
making it aligned is a must.

>   	.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
>   		INIT_DATA
>   	}
> 
> base-commit: 1398820fee515873379809a6415930ad0764b2f6
> 

Christophe

On Fri, Dec 18, 2020 at 5:39 PM Christophe Leroy <
christophe.leroy@csgroup.eu> wrote:

> It can cause, or it causes ? Did you encounter the issue ?
>
Yes, in configs that result in the section layout I described, the crush is
consistent.

>
> > The init sections are ordered like this:
> >       .init.text
> >       .exit.text
> >       .init.data
> >
> > Currently, these sections aren't page aligned.
> >
> > Because the init code is mapped read-only at runtime and because the
> > .init.text section can potentially reside on the same physical page as
> > .init.data, the beginning of .init.data might be mapped read-only along
> > with .init.text.
>
> init code is mapped PAGE_KERNEL_TEXT.
>
> Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected
> options.
>
You are right, of course. Should I change the commit message to 'might be
mapped' or something?

>
> > Then when the kernel tries to modify a variable in .init.data (like
> > kthreadd_done, used in kernel_init()) the kernel panics.
> >
> > To avoid this, I made these sections page aligned.
>
> Should write this unpersonal, something like "To avoid this, make these
> sections page aligned"
>
Noted, thanks.


>
> >
> > Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
> > Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com>
> > ---
> >   arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/powerpc/kernel/vmlinux.lds.S
> b/arch/powerpc/kernel/vmlinux.lds.S
> > index 326e113d2e45..e3a7c90c03f4 100644
> > --- a/arch/powerpc/kernel/vmlinux.lds.S
> > +++ b/arch/powerpc/kernel/vmlinux.lds.S
> > @@ -179,6 +179,11 @@ SECTIONS
> >   #endif
> >       } :text
> >
> > +     /* .init.text is made RO and .exit.text is not, so we must
> > +      * ensure these sections reside in separate physical pages.
> > +      */
> > +     . = ALIGN(PAGE_SIZE);
> > +
>
> In principle, as it is text, it should be made RO as well. But what
> happens at the begining doesn't
> really matter, anyway .exit.text should never be executed and is discarded
> together with init text.
> So, I think it is OK the live with it as is for the time being.


> Making it page aligned makes sense anyway.
>
> Should we make _einittext page aligned instead, just like _etext ?

Yes, this will probably be better (because when _einittext is not aligned,
the part of the page after _einittext is mapped RO implicitly, and it's
hard to notice from the code). I suppose you mean something like this:
                _sinittext = .;
                INIT_TEXT
+
+               . = ALIGN(.);
                _einittext = .;

>       /* .exit.text is discarded at runtime, not link time,
> >        * to deal with references from __bug_table
> >        */
> > @@ -186,6 +191,8 @@ SECTIONS
> >               EXIT_TEXT
> >       }
> >
> > +     . = ALIGN(PAGE_SIZE);
> > +
>
> Here for sure, as you explain in the coming log, this needs to be
> separated from init text. So
> making it aligned is a must.


> >       .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
> >               INIT_DATA
> >       }
> >
> > base-commit: 1398820fee515873379809a6415930ad0764b2f6
> >
>
> Christophe
>
Thanks for your time,
        Ariel Marcovitch

On Fri, Dec 18, 2020 at 5:39 PM Christophe Leroy <
christophe.leroy@csgroup.eu> wrote:

> It can cause, or it causes ? Did you encounter the issue ?
>
Yes, in configs that result in the section layout I described, the crush is
consistent.

>
> > The init sections are ordered like this:
> >       .init.text
> >       .exit.text
> >       .init.data
> >
> > Currently, these sections aren't page aligned.
> >
> > Because the init code is mapped read-only at runtime and because the
> > .init.text section can potentially reside on the same physical page as
> > .init.data, the beginning of .init.data might be mapped read-only along
> > with .init.text.
>
> init code is mapped PAGE_KERNEL_TEXT.
>
> Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected
> options.
>
You are right, of course. Should I change the commit message to 'might be
mapped' or something?

>
> > Then when the kernel tries to modify a variable in .init.data (like
> > kthreadd_done, used in kernel_init()) the kernel panics.
> >
> > To avoid this, I made these sections page aligned.
>
> Should write this unpersonal, something like "To avoid this, make these
> sections page aligned"
>
Noted, thanks.


>
> >
> > Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
> > Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com>
> > ---
> >   arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/powerpc/kernel/vmlinux.lds.S
> b/arch/powerpc/kernel/vmlinux.lds.S
> > index 326e113d2e45..e3a7c90c03f4 100644
> > --- a/arch/powerpc/kernel/vmlinux.lds.S
> > +++ b/arch/powerpc/kernel/vmlinux.lds.S
> > @@ -179,6 +179,11 @@ SECTIONS
> >   #endif
> >       } :text
> >
> > +     /* .init.text is made RO and .exit.text is not, so we must
> > +      * ensure these sections reside in separate physical pages.
> > +      */
> > +     . = ALIGN(PAGE_SIZE);
> > +
>
> In principle, as it is text, it should be made RO as well. But what
> happens at the begining doesn't
> really matter, anyway .exit.text should never be executed and is discarded
> together with init text.
> So, I think it is OK the live with it as is for the time being.


> Making it page aligned makes sense anyway.
>
> Should we make _einittext page aligned instead, just like _etext ?

Yes, this will probably be better (because when _einittext is not aligned,
the part of the page after _einittext is mapped RO implicitly, and it's
hard to notice from the code). I suppose you mean something like this:
                _sinittext = .;
                INIT_TEXT
+
+               . = ALIGN(.);
                _einittext = .;

>       /* .exit.text is discarded at runtime, not link time,
> >        * to deal with references from __bug_table
> >        */
> > @@ -186,6 +191,8 @@ SECTIONS
> >               EXIT_TEXT
> >       }
> >
> > +     . = ALIGN(PAGE_SIZE);
> > +
>
> Here for sure, as you explain in the coming log, this needs to be
> separated from init text. So
> making it aligned is a must.


> >       .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
> >               INIT_DATA
> >       }
> >
> > base-commit: 1398820fee515873379809a6415930ad0764b2f6
> >
>
> Christophe
>
Thanks for your time,
        Ariel Marcovitch