powerpc/pkeys: Fix handling of pkey state across fork()

Protection key tracking information is not copied over to the
mm_struct of the child during fork(). This can cause the child to
erroneously allocate keys that were already allocated. Any allocated
execute-only key is lost aswell.

Add code; called by dup_mmap(), to copy the pkey state from parent to
child explicitly.

This problem was originally found by Dave Hansen on x86, which turns
out to be a problem on powerpc aswell.

Fixes: cf43d3b264 ("powerpc: Enable pkey subsystem")
Cc: stable@vger.kernel.org # v4.16+
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/mmu_context.h

@@ -217,12 +217,6 @@ static inline void enter_lazy_tlb(struct mm_struct *mm,
 #endif
 }
 
-static inline int arch_dup_mmap(struct mm_struct *oldmm,
-				struct mm_struct *mm)
-{
-	return 0;
-}
-
 #ifdef CONFIG_PPC_BOOK3E_64
 static inline void arch_exit_mmap(struct mm_struct *mm)
 {
@@ -247,6 +241,7 @@ static inline void arch_bprm_mm_init(struct mm_struct *mm,
 #ifdef CONFIG_PPC_MEM_KEYS
 bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write,
 			       bool execute, bool foreign);
+void arch_dup_pkeys(struct mm_struct *oldmm, struct mm_struct *mm);
 #else /* CONFIG_PPC_MEM_KEYS */
 static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
 		bool write, bool execute, bool foreign)
@@ -259,6 +254,7 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
 #define thread_pkey_regs_save(thread)
 #define thread_pkey_regs_restore(new_thread, old_thread)
 #define thread_pkey_regs_init(thread)
+#define arch_dup_pkeys(oldmm, mm)
 
 static inline u64 pte_to_hpte_pkey_bits(u64 pteflags)
 {
@@ -267,5 +263,12 @@ static inline u64 pte_to_hpte_pkey_bits(u64 pteflags)
 
 #endif /* CONFIG_PPC_MEM_KEYS */
 
+static inline int arch_dup_mmap(struct mm_struct *oldmm,
+				struct mm_struct *mm)
+{
+	arch_dup_pkeys(oldmm, mm);
+	return 0;
+}
+
 #endif /* __KERNEL__ */
 #endif /* __ASM_POWERPC_MMU_CONTEXT_H */

arch/powerpc/mm/pkeys.c

@@ -415,3 +415,13 @@ bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write,
 
 	return pkey_access_permitted(vma_pkey(vma), write, execute);
 }
+
+void arch_dup_pkeys(struct mm_struct *oldmm, struct mm_struct *mm)
+{
+	if (static_branch_likely(&pkey_disabled))
+		return;
+
+	/* Duplicate the oldmm pkey state in mm: */
+	mm_pkey_allocation_map(mm) = mm_pkey_allocation_map(oldmm);
+	mm->context.execute_only_pkey = oldmm->context.execute_only_pkey;
+}