powerpc: Fix random segfault when freeing hugetlb range

The following random segfault is observed from time to time with
map_hugetlb selftest:

root@localhost:~# ./map_hugetlb 1 19
524288 kB hugepages
Mapping 1 Mbytes
Segmentation fault

[   31.219972] map_hugetlb[365]: segfault (11) at 117 nip 77974f8c lr 779a6834 code 1 in ld-2.23.so[77966000+21000]
[   31.220192] map_hugetlb[365]: code: 9421ffc0 480318d1 93410028 90010044 9361002c 93810030 93a10034 93c10038
[   31.220307] map_hugetlb[365]: code: 93e1003c 93210024 8123007c 81430038 <80e90004> 814a0004 7f443a14 813a0004
[   31.221911] BUG: Bad rss-counter state mm:(ptrval) type:MM_FILEPAGES val:33
[   31.229362] BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:5

This fault is due to hugetlb_free_pgd_range() freeing page tables
that are also used by regular pages.

As explain in the comment at the beginning of
hugetlb_free_pgd_range(), the verification done in free_pgd_range()
on floor and ceiling is not done here, which means
hugetlb_free_pte_range() can free outside the expected range.

As the verification cannot be done in hugetlb_free_pgd_range(), it
must be done in hugetlb_free_pte_range().

Fixes: b250c8c0 ("powerpc/8xx: Manage 512k huge pages as standard pages.")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f0cb2a5477cd87d1eaadb128042e20aeb2bc2859.1598860677.git.christophe.leroy@csgroup.eu

arch/powerpc/mm/hugetlbpage.c

@@ -330,10 +330,24 @@ static void free_hugepd_range(struct mmu_gather *tlb, hugepd_t *hpdp, int pdshif
 				 get_hugepd_cache_index(pdshift - shift));
 }
 
-static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd, unsigned long addr)
+static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd,
+				   unsigned long addr, unsigned long end,
+				   unsigned long floor, unsigned long ceiling)
 {
+	unsigned long start = addr;
 	pgtable_t token = pmd_pgtable(*pmd);
 
+	start &= PMD_MASK;
+	if (start < floor)
+		return;
+	if (ceiling) {
+		ceiling &= PMD_MASK;
+		if (!ceiling)
+			return;
+	}
+	if (end - 1 > ceiling - 1)
+		return;
+
 	pmd_clear(pmd);
 	pte_free_tlb(tlb, token, addr);
 	mm_dec_nr_ptes(tlb->mm);
@@ -363,7 +377,7 @@ static void hugetlb_free_pmd_range(struct mmu_gather *tlb, pud_t *pud,
 			 */
 			WARN_ON(!IS_ENABLED(CONFIG_PPC_8xx));
 
-			hugetlb_free_pte_range(tlb, pmd, addr);
+			hugetlb_free_pte_range(tlb, pmd, addr, end, floor, ceiling);
 
 			continue;
 		}