powerpc/hugetlb: fix page rights verification in gup_hugepte()

gup_hugepte() checks if pages are present and readable, and
when  'write' is set, also checks if the pages are writable.

Initially this was done by checking if _PAGE_PRESENT and
_PAGE_READ were set. In addition, _PAGE_WRITE was verified for write
accesses.

The problem is that we have to handle the three following cases:
1/ The target defines __PAGE_READ and __PAGE_WRITE
2/ The target defines __PAGE_RW
3/ The target defines __PAGE_RO

In case 1/, this is obvious
In case 2/, __PAGE_READ is defined as 0 and __PAGE_WRITE as __PAGE_RW
so it works as well.
But in case 3, __PAGE_RW is defined as 0, which means __PAGE_WRITE is 0
and then the test returns true (page writable) in all cases.

A first correction was attempted in commit 6b8cb66a6a ("powerpc: Fix
usage of _PAGE_RO in hugepage"), but that fix is wrong:
instead of checking that the page is writable when write is requested,
it checks that the page is NOT writable when write is NOT requested.

This patch adds a new pte_read() helper to check whether a page is
readable or not. This avoids handling all possible cases in
gup_hugepte().

Then gup_hugepte() is modified to use pte_present(), pte_read()
and pte_write() instead of the raw flags.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/hugetlbpage.c

@@ -977,7 +977,6 @@ EXPORT_SYMBOL_GPL(__find_linux_pte_or_hugepte);
 int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 		unsigned long end, int write, struct page **pages, int *nr)
 {
-	unsigned long mask;
 	unsigned long pte_end;
 	struct page *head, *page;
 	pte_t pte;
@@ -988,18 +987,10 @@ int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 		end = pte_end;
 
 	pte = READ_ONCE(*ptep);
-	mask = _PAGE_PRESENT | _PAGE_READ;
 
-	/*
-	 * On some CPUs like the 8xx, _PAGE_RW hence _PAGE_WRITE is defined
-	 * as 0 and _PAGE_RO has to be set when a page is not writable
-	 */
-	if (write)
-		mask |= _PAGE_WRITE;
-	else
-		mask |= _PAGE_RO;
-
-	if ((pte_val(pte) & mask) != mask)
+	if (!pte_present(pte) || !pte_read(pte))
+		return 0;
+	if (write && !pte_write(pte))
 		return 0;
 
 	/* hugepages are never "special" */