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SubjectRe: [PATCH] arm64/mm: fold check for KFENCE into can_set_direct_map()
Hi Anshuman,

On Wed, Sep 21, 2022 at 05:09:19PM +0530, Anshuman Khandual wrote:
>
>
> On 9/21/22 13:18, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@linux.ibm.com>
> >
> > KFENCE requires linear map to be mapped at page granularity, so that it
> > is possible to protect/unprotect single pages, just like with
> > rodata_full and DEBUG_PAGEALLOC.
> >
> > Instead of repating
> >
> > can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)
> >
> > make can_set_direct_map() handle the KFENCE case.
> >
> > This also prevents potential false positives in kernel_page_present()
> > that may return true for non-present page if CONFIG_KFENCE is enabled.
> >
> > Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> > ---
> > arch/arm64/mm/mmu.c | 8 ++------
> > arch/arm64/mm/pageattr.c | 8 +++++++-
> > 2 files changed, 9 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index e7ad44585f40..c5065abec55a 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -535,7 +535,7 @@ static void __init map_mem(pgd_t *pgdp)
> > */
> > BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
> >
> > - if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> > + if (can_set_direct_map())
> > flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >
> > /*
> > @@ -1547,11 +1547,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >
> > VM_BUG_ON(!mhp_range_allowed(start, size, true));
> >
> > - /*
> > - * KFENCE requires linear map to be mapped at page granularity, so that
> > - * it is possible to protect/unprotect single pages in the KFENCE pool.
> > - */
> > - if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> > + if (can_set_direct_map())
> > flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >
> > __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> > diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> > index 64e985eaa52d..d107c3d434e2 100644
> > --- a/arch/arm64/mm/pageattr.c
> > +++ b/arch/arm64/mm/pageattr.c
> > @@ -21,7 +21,13 @@ bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED
> >
> > bool can_set_direct_map(void)
> > {
> > - return rodata_full || debug_pagealloc_enabled();
> > + /*
> > + * rodata_full, DEBUG_PAGEALLOC and KFENCE require linear map to be
> > + * mapped at page granularity, so that it is possible to
> > + * protect/unprotect single pages.
> > + */
> > + return rodata_full || debug_pagealloc_enabled() ||
> > + IS_ENABLED(CONFIG_KFENCE);
> > }
>
> Changing can_set_direct_map() also changes behaviour for other functions such as
>
> set_direct_map_default_noflush()
> set_direct_map_invalid_noflush()
> __kernel_map_pages()
>
> Is that okay ?

Yes. Since KFENCE disables block mappings, these will actually change the
page tables.
Actually, before this change the test for can_set_direct_map() in these
functions was false negative when CONFIG_KFENCE=y

> > static int change_page_range(pte_t *ptep, unsigned long addr, void *data)

--
Sincerely yours,
Mike.

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