[PATCH 4/9] powerpc/dexcr: Support userspace ROP protection

[Benjamin Gray]

The ISA 3.1B hashst and hashchk instructions use a per-cpu SPR HASHKEYR
to hold a key used in the hash calculation. This key should be different
for each process to make it harder for a malicious process to recreate
valid hash values for a victim process.

Add support for storing a per-thread hash key, and setting/clearing
HASHKEYR appropriately.

Signed-off-by: Benjamin Gray <bg...@linux.ibm.com>

---

v1:     * Guard HASHKEYR update behind change check
        * HASHKEYR reset moved earlier to patch 2
---
 arch/powerpc/include/asm/processor.h |  1 +
 arch/powerpc/kernel/process.c        | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/arch/powerpc/include/asm/processor.h 
b/arch/powerpc/include/asm/processor.h
index bad64d6a5d36..666d4e9804a8 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -264,6 +264,7 @@ struct thread_struct {
        unsigned long   mmcr3;
        unsigned long   sier2;
        unsigned long   sier3;
+       unsigned long   hashkeyr;
 
 #endif
 };
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 4b29ac5ddac6..62762795e24e 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1182,6 +1182,9 @@ static inline void save_sprs(struct thread_struct *t)
                 */
                t->tar = mfspr(SPRN_TAR);
        }
+
+       if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE))
+               t->hashkeyr = mfspr(SPRN_HASHKEYR);
 #endif
 }
 
@@ -1260,6 +1263,10 @@ static inline void restore_sprs(struct thread_struct 
*old_thread,
        if (cpu_has_feature(CPU_FTR_P9_TIDR) &&
            old_thread->tidr != new_thread->tidr)
                mtspr(SPRN_TIDR, new_thread->tidr);
+
+       if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE) &&
+           old_thread->hashkeyr != new_thread->hashkeyr)
+               mtspr(SPRN_HASHKEYR, new_thread->hashkeyr);
 #endif
 
 }
@@ -1844,6 +1851,10 @@ int copy_thread(struct task_struct *p, const struct 
kernel_clone_args *args)
                childregs->ppr = DEFAULT_PPR;
 
        p->thread.tidr = 0;
+#endif
+#ifdef CONFIG_PPC_BOOK3S_64
+       if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE))
+               p->thread.hashkeyr = current->thread.hashkeyr;
 #endif
        /*
         * Run with the current AMR value of the kernel
@@ -1972,6 +1983,12 @@ void start_thread(struct pt_regs *regs, unsigned long 
start, unsigned long sp)
        current->thread.tm_tfiar = 0;
        current->thread.load_tm = 0;
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
+#ifdef CONFIG_PPC_BOOK3S_64
+       if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE)) {
+               current->thread.hashkeyr = get_random_long();
+               mtspr(SPRN_HASHKEYR, current->thread.hashkeyr);
+       }
+#endif /* CONFIG_PPC_BOOK3S_64 */
 }
 EXPORT_SYMBOL(start_thread);
 
-- 
2.39.2