Re: [PATCH v2 3/3] riscv: Fix crash when flushing executable ioremap regions

[Jan Kiszka]

On 16.02.20 15:41, Alex Ghiti wrote:
> Hi Jan,
>
> On 2/15/20 6:49 AM, Jan Kiszka wrote:
> > From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> >
> > Those are not backed by page structs, and pte_page is returning an
> > invalid pointer.
> >
> > Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> > =2D--
> >   arch/riscv/mm/cacheflush.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> > index 8930ab7278e6..9ee2c1a387cc 100644
> > =2D-- a/arch/riscv/mm/cacheflush.c
> > +++ b/arch/riscv/mm/cacheflush.c
> > @@ -84,7 +84,8 @@ void flush_icache_pte(pte_t pte)
> >   {
> >       struct page *page =3D pte_page(pte);
> >
> > -    if (!test_and_set_bit(PG_dcache_clean, &page->flags))
> > +    if (!pfn_valid(pte_pfn(pte)) ||
> > +        !test_and_set_bit(PG_dcache_clean, &page->flags))
> >           flush_icache_all();
> >   }
> >   #endif /* CONFIG_MMU */
> > =2D-
> > 2.16.4
>
> When did you encounter such a situation ? i.e. executable code that is
> not backed by struct page ?
>
> Riscv uses the generic implementation of ioremap and the way
> _PAGE_IOREMAP is defined does not allow to map executable memory region
> using ioremap, so I'm interested to understand how we end up in
> flush_icache_pte for an executable region not backed by any struct page.

You can create executable mappings of memory that Linux does not
initially consider as RAM via ioremap_prot or ioremap_page_range. We are
using that in Jailhouse to load the hypervisor code into reserved memory
that is ioremapped for the purpose. Works fine on x86, arm and arm64.

Jan