[PATCH 02/12] mm/pgtable: add PAE safety to __pte_offset_map()

[Hugh Dickins <hughd@google.com>]

There is a faint risk that __pte_offset_map(), on a 32-bit architecture
with a 64-bit pmd_t e.g. x86-32 with CONFIG_X86_PAE=y, would succeed on
a pmdval assembled from a pmd_low and a pmd_high which never belonged
together: their combination not pointing to a page table at all, perhaps
not even a valid pfn.  pmdp_get_lockless() is not enough to prevent that.

Guard against that (on such configs) by local_irq_save() blocking TLB
flush between present updates, as linux/pgtable.h suggests.  It's only
needed around the pmdp_get_lockless() in __pte_offset_map(): a race when
__pte_offset_map_lock() repeats the pmdp_get_lockless() after getting the
lock, would just send it back to __pte_offset_map() again.

CONFIG_GUP_GET_PXX_LOW_HIGH is enabled when required by mips, sh and x86.
It is not enabled by arm-32 CONFIG_ARM_LPAE: my understanding is that
Will Deacon's 2020 enhancements to READ_ONCE() are sufficient for arm.
It is not enabled by arc, but its pmd_t is 32-bit even when pte_t 64-bit.

Limit the IRQ disablement to CONFIG_HIGHPTE?  Perhaps, but would need a
little more work, to retry if pmd_low good for page table, but pmd_high
non-zero from THP (and that might be making x86-specific assumptions).

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/pgtable-generic.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index 674671835631..d28b63386cef 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -232,12 +232,32 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address,
 #endif
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
+#if defined(CONFIG_GUP_GET_PXX_LOW_HIGH) && \
+	(defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RCU))
+/*
+ * See the comment above ptep_get_lockless() in include/linux/pgtable.h:
+ * the barriers in pmdp_get_lockless() cannot guarantee that the value in
+ * pmd_high actually belongs with the value in pmd_low; but holding interrupts
+ * off blocks the TLB flush between present updates, which guarantees that a
+ * successful __pte_offset_map() points to a page from matched halves.
+ */
+#define config_might_irq_save(flags)	local_irq_save(flags)
+#define config_might_irq_restore(flags)	local_irq_restore(flags)
+#else
+#define config_might_irq_save(flags)
+#define config_might_irq_restore(flags)
+#endif
+
 pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
 {
+	unsigned long __maybe_unused flags;
 	pmd_t pmdval;
 
 	rcu_read_lock();
+	config_might_irq_save(flags);
 	pmdval = pmdp_get_lockless(pmd);
+	config_might_irq_restore(flags);
+
 	if (pmdvalp)
 		*pmdvalp = pmdval;
 	if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
-- 
2.35.3