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

[Michael Ellerman]

Benjamin Gray  writes:
> 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 
>
> ---
>
> 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;
  
hashkeyr is part of the thread state, so we should save it in core dumps.

The DEXCR also influences the threads behaviour, at least by
enabling/disabling hashst/chk, so I think we should also include it in
core dumps.

Adding regs to the core dump is done by adding eg. a new NT_PPC_HASHKEY
entry in include/uapi/linux/elf.h and wiring it up in ptrace.

But those are a non-renewable resource, so if we're going to add
HASHKEYR and DEXCR it would be better to group them as a single note. I
think given that HASHKEYR doesn't exist without the DEXCR, grouping them
is OK. Could be called NT_PPC_DEXCF (F for facility) ?

See NT_PPC_PKEY for an example.

I know HASHKEYR is security sensitive, but I think the existing ptrace
checks should be sufficient. A ptracer has more or less full control of
the tracee anyway.

To support checkpoint/restore we'd need to support setting NPHIE in the
DEXCR via ptrace. I think for starters we can just fail the ->set() if
the DEXCR doesn't match the current SPR value.

cheers

[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 

---

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