[PATCH v2] powerpc, mm: Fix mprotect on book3s 32-bit

[Dave Vasilevsky]

On 32-bit book3s with hash-MMUs, tlb_flush() was a no-op. This was
unnoticed because all uses until recently were for unmaps, and thus
handled by __tlb_remove_tlb_entry().

After commit 4a18419f71cd ("mm/mprotect: use mmu_gather") in kernel 5.19,
tlb_gather_mmu() started being used for mprotect as well. This caused
mprotect to simply not work on these machines:

  int *ptr = mmap(NULL, 4096, PROT_READ|PROT_WRITE,
                  MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  *ptr = 1; // force HPTE to be created
  mprotect(ptr, 4096, PROT_READ);
  *ptr = 2; // should segfault, but succeeds

Fixed by making tlb_flush() actually flush TLB pages. This finally
agrees with the behaviour of boot3s64's tlb_flush().

Fixes: 4a18419f71cd ("mm/mprotect: use mmu_gather")
Signed-off-by: Dave Vasilevsky <dave at vasilevsky.ca>
Cc: stable at vger.kernel.org
---
Changes in v2:
- Flush entire TLB if full mm is requested.
- Link to v1: https://lore.kernel.org/r/20251027-vasi-mprotect-g3-v1-1-3c5187085f9a@vasilevsky.ca
---
 arch/powerpc/include/asm/book3s/32/tlbflush.h | 8 ++++++--
 arch/powerpc/mm/book3s32/tlb.c                | 9 +++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/32/tlbflush.h b/arch/powerpc/include/asm/book3s/32/tlbflush.h
index e43534da5207aa3b0cb3c07b78e29b833c141f3f..b8c587ad2ea954f179246a57d6e86e45e91dcfdc 100644
--- a/arch/powerpc/include/asm/book3s/32/tlbflush.h
+++ b/arch/powerpc/include/asm/book3s/32/tlbflush.h
@@ -11,6 +11,7 @@
 void hash__flush_tlb_mm(struct mm_struct *mm);
 void hash__flush_tlb_page(struct vm_area_struct *vma, unsigned long vmaddr);
 void hash__flush_range(struct mm_struct *mm, unsigned long start, unsigned long end);
+void hash__flush_gather(struct mmu_gather *tlb);
 
 #ifdef CONFIG_SMP
 void _tlbie(unsigned long address);
@@ -28,9 +29,12 @@ void _tlbia(void);
  */
 static inline void tlb_flush(struct mmu_gather *tlb)
 {
-	/* 603 needs to flush the whole TLB here since it doesn't use a hash table. */
-	if (!mmu_has_feature(MMU_FTR_HPTE_TABLE))
+	if (mmu_has_feature(MMU_FTR_HPTE_TABLE)) {
+		hash__flush_gather(tlb);
+	} else {
+		/* 603 needs to flush the whole TLB here since it doesn't use a hash table. */
 		_tlbia();
+	}
 }
 
 static inline void flush_range(struct mm_struct *mm, unsigned long start, unsigned long end)
diff --git a/arch/powerpc/mm/book3s32/tlb.c b/arch/powerpc/mm/book3s32/tlb.c
index 9ad6b56bfec96e989b96f027d075ad5812500854..e54a7b0112322e5818d80facd2e3c7722e6dd520 100644
--- a/arch/powerpc/mm/book3s32/tlb.c
+++ b/arch/powerpc/mm/book3s32/tlb.c
@@ -105,3 +105,12 @@ void hash__flush_tlb_page(struct vm_area_struct *vma, unsigned long vmaddr)
 		flush_hash_pages(mm->context.id, vmaddr, pmd_val(*pmd), 1);
 }
 EXPORT_SYMBOL(hash__flush_tlb_page);
+
+void hash__flush_gather(struct mmu_gather *tlb)
+{
+	if (tlb->fullmm || tlb->need_flush_all)
+		hash__flush_tlb_mm(tlb->mm);
+	else
+		hash__flush_range(tlb->mm, tlb->start, tlb->end);
+}
+EXPORT_SYMBOL(hash__flush_gather);

---
base-commit: 24172e0d79900908cf5ebf366600616d29c9b417
change-id: 20251027-vasi-mprotect-g3-f8f5278d4140

Best regards,
-- 
Dave Vasilevsky <dave at vasilevsky.ca>