On 5/12/22 22:33, Hyeonggon Yoo wrote: > Thanks Mel, and IIUC nor does do_kern_addr_fault() in arch/x86/mm/fault.c > expect _PAGE_PROTNONE instead of _PAGE_GLOBAL. I want to make it clear > in the code that _PAGE_PROTNONE is only used for user mappings. > > How does below look? > > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h > index 40497a9020c6..f8d02b91a90c 100644 > --- a/arch/x86/include/asm/pgtable_types.h > +++ b/arch/x86/include/asm/pgtable_types.h > @@ -35,7 +35,10 @@ > #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4 > > /* If _PAGE_BIT_PRESENT is clear, we use these: */ > -/* - if the user mapped it with PROT_NONE; pte_present gives true */ > +/* > + * if the user mapped it with PROT_NONE; pte_present gives true > + * this is only used for user mappings (with _PAGE_BIT_USER) > + */ > #define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL > > #define _PAGE_PRESENT (_AT(pteval_t, 1) << _PAGE_BIT_PRESENT) > @@ -115,7 +118,8 @@ > #define _PAGE_DEVMAP (_AT(pteval_t, 0)) > #endif > > -#define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE) > +#define _PAGE_PROTNONE ((_AT(pteval_t, 1) << _PAGE_BIT_USER) | \ > + (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)) > > /* > * Set of bits not changed in pte_modify. The pte's I don't like the idea of _PAGE_BIT_USER being so implicit. It is something kernel users should know explicitly that they are messing with. I was thinking of something more along the lines of taking the set_memory.c code and ensuring that it never sets (or even observes) _PAGE_BIT_GLOBAL on a _PAGE_USER mapping. There was also a question of if set_memory.c is ever used on userspace mappings. It would be good to validate whether it's possible in-tree today and if not, enforce that _PAGE_USER PTEs should never even be observed with set_memory.c. The arch/x86/mm/dump_pagetables.c code is also a reasonable place to put assumptions about the page tables since it walks *everything* when asked.
