Merge tag 'v5.8' into drm-next

I need to backmerge 5.8 as I've got a bunch of fixes sitting
on an rc7 base that I want to land.

Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
index 468169e33c86..9b9f92ad0e7a 100644
--- a/arch/powerpc/mm/book3s64/hash_utils.c
+++ b/arch/powerpc/mm/book3s64/hash_utils.c
@@ -1559,6 +1559,7 @@ static void hash_preload(struct mm_struct *mm, pte_t *ptep, unsigned long ea,
 	pgd_t *pgdir;
 	int rc, ssize, update_flags = 0;
 	unsigned long access = _PAGE_PRESENT | _PAGE_READ | (is_exec ? _PAGE_EXEC : 0);
+	unsigned long flags;
 
 	BUG_ON(get_region_id(ea) != USER_REGION_ID);
 
@@ -1592,6 +1593,28 @@ static void hash_preload(struct mm_struct *mm, pte_t *ptep, unsigned long ea,
 		return;
 #endif /* CONFIG_PPC_64K_PAGES */
 
+	/*
+	 * __hash_page_* must run with interrupts off, as it sets the
+	 * H_PAGE_BUSY bit. It's possible for perf interrupts to hit at any
+	 * time and may take a hash fault reading the user stack, see
+	 * read_user_stack_slow() in the powerpc/perf code.
+	 *
+	 * If that takes a hash fault on the same page as we lock here, it
+	 * will bail out when seeing H_PAGE_BUSY set, and retry the access
+	 * leading to an infinite loop.
+	 *
+	 * Disabling interrupts here does not prevent perf interrupts, but it
+	 * will prevent them taking hash faults (see the NMI test in
+	 * do_hash_page), then read_user_stack's copy_from_user_nofault will
+	 * fail and perf will fall back to read_user_stack_slow(), which
+	 * walks the Linux page tables.
+	 *
+	 * Interrupts must also be off for the duration of the
+	 * mm_is_thread_local test and update, to prevent preempt running the
+	 * mm on another CPU (XXX: this may be racy vs kthread_use_mm).
+	 */
+	local_irq_save(flags);
+
 	/* Is that local to this CPU ? */
 	if (mm_is_thread_local(mm))
 		update_flags |= HPTE_LOCAL_UPDATE;
@@ -1614,6 +1637,8 @@ static void hash_preload(struct mm_struct *mm, pte_t *ptep, unsigned long ea,
 				   mm_ctx_user_psize(&mm->context),
 				   mm_ctx_user_psize(&mm->context),
 				   pte_val(*ptep));
+
+	local_irq_restore(flags);
 }
 
 /*