On Mon, Aug 08, 2022 at 10:56:46PM +0800, Aaron Lu wrote:
> For configs that don't have PTI enabled or cpus that don't need
> meltdown mitigation, current kernel can lose GLOBAL bit after a page
> goes through a cycle of present -> not present -> present.
>
> It happened like this(__vunmap() does this in vm_remove_mappings()):
> original page protection: 0x8000000000000163 (NX/G/D/A/RW/P)
> set_memory_np(page, 1): 0x8000000000000062 (NX/D/A/RW) lose G and P
> set_memory_p(pagem 1): 0x8000000000000063 (NX/D/A/RW/P) restored P
>
> In the end, this page's protection no longer has Global bit set and this
> would create problem for this merge small mapping feature.
>
> For this reason, restore Global bit for systems that do not have PTI
> enabled if page is present.
>
> (pgprot_clear_protnone_bits() deserves a better name if this patch is
> acceptible but first, I would like to get some feedback if this is the
> right way to solve this so I didn't bother with the name yet)
>
> Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> ---
> arch/x86/mm/pat/set_memory.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 1abd5438f126..33657a54670a 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -758,6 +758,8 @@ static pgprot_t pgprot_clear_protnone_bits(pgprot_t prot)
> */
> if (!(pgprot_val(prot) & _PAGE_PRESENT))
> pgprot_val(prot) &= ~_PAGE_GLOBAL;
> + else
> + pgprot_val(prot) |= _PAGE_GLOBAL & __default_kernel_pte_mask;
>
> return prot;
> }
IIUC It makes it unable to set _PAGE_GLOBL when PTI is on.
Maybe it would be less intrusive to make
set_direct_map_default_noflush() replace protection bits
with PAGE_KENREL as it's only called for direct map, and the function
is to reset permission to default:
diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 1abd5438f126..0dd4433c1382 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -2250,7 +2250,16 @@ int set_direct_map_invalid_noflush(struct page *page)
int set_direct_map_default_noflush(struct page *page)
{
- return __set_pages_p(page, 1);
+ unsigned long tempaddr = (unsigned long) page_address(page);
+ struct cpa_data cpa = {
+ .vaddr = &tempaddr,
+ .pgd = NULL,
+ .numpages = 1,
+ .mask_set = PAGE_KERNEL,
+ .mask_clr = __pgprot(~0),
+ .flags = 0};
+
+ return __change_page_attr_set_clr(&cpa, 0);
}
set_direct_map_{invalid,default}_noflush() is the exact reason
why direct map become split after vmalloc/vfree with special
permissions.
> --
> 2.37.1
>
>
