Subject: [PATCH AUTOSEL 4.19 557/671] powerpc/mm/mce: Keep irqs disabled during lockless page table walk

From: Sasha Levin <[email-masked]>
Date: Thu, 16 Jan 2020 12:03:15 -0500

From: "Aneesh Kumar K.V" <[email-masked]>

[ Upstream commit d9101bfa6adc831bda8836c4d774820553c14942 ]

__find_linux_mm_pte() returns a page table entry pointer after walking
the page table without holding locks. To make it safe against a THP
split and/or collapse, we disable interrupts around the lockless page
table walk. However we need to keep interrupts disabled as long as we
use the page table entry pointer that is returned.

Fix addr_to_pfn() to do that.

Fixes: ba41e1e1ccb9 ("powerpc/mce: Hookup derror (load/store) UE errors")
Signed-off-by: Aneesh Kumar K.V <[email-masked]>
[mpe: Rearrange code slightly and tweak change log wording]
Signed-off-by: Michael Ellerman <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sasha Levin <[email-masked]>
---
 arch/powerpc/kernel/mce_power.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/kernel/mce_power.c b/arch/powerpc/kernel/mce_power.c
index 37a110b8e7e1..ecb375040637 100644
--- a/arch/powerpc/kernel/mce_power.c
+++ b/arch/powerpc/kernel/mce_power.c
@@ -40,7 +40,7 @@ static unsigned long addr_to_pfn(struct pt_regs *regs, unsigned long addr)
 {
 	pte_t *ptep;
 	unsigned int shift;
-	unsigned long flags;
+	unsigned long pfn, flags;
 	struct mm_struct *mm;
 
 	if (user_mode(regs))
@@ -50,18 +50,22 @@ static unsigned long addr_to_pfn(struct pt_regs *regs, unsigned long addr)
 
 	local_irq_save(flags);
 	ptep = __find_linux_pte(mm->pgd, addr, NULL, &shift);
-	local_irq_restore(flags);
 
-	if (!ptep || pte_special(*ptep))
-		return ULONG_MAX;
+	if (!ptep || pte_special(*ptep)) {
+		pfn = ULONG_MAX;
+		goto out;
+	}
 
-	if (shift > PAGE_SHIFT) {
+	if (shift <= PAGE_SHIFT)
+		pfn = pte_pfn(*ptep);
+	else {
 		unsigned long rpnmask = (1ul << shift) - PAGE_SIZE;
-
-		return pte_pfn(__pte(pte_val(*ptep) | (addr & rpnmask)));
+		pfn = pte_pfn(__pte(pte_val(*ptep) | (addr & rpnmask)));
 	}
 
-	return pte_pfn(*ptep);
+out:
+	local_irq_restore(flags);
+	return pfn;
 }
 
 /* flush SLBs and reload */
-- 
2.20.1