arm: Implement thread_struct whitelist for hardened usercopy

While ARM32 carries FPU state in the thread structure that is saved and
restored during signal handling, it doesn't need to declare a usercopy
whitelist, since existing accessors are all either using a bounce buffer
(for which whitelisting isn't checking the slab), are statically sized
(which will bypass the hardened usercopy check), or both.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>

2 files changed, 11 insertions, 0 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 51c8df561077..3ea00d65f35d 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -50,6 +50,7 @@ config ARM
         select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
         select HAVE_ARCH_MMAP_RND_BITS if MMU
         select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
+        select HAVE_ARCH_THREAD_STRUCT_WHITELIST
         select HAVE_ARCH_TRACEHOOK
         select HAVE_ARM_SMCCC if CPU_V7
         select HAVE_EBPF_JIT if !CPU_ENDIAN_BE32
diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
index 338cbe0a18ef..1bf65b47808a 100644
--- a/arch/arm/include/asm/processor.h
+++ b/arch/arm/include/asm/processor.h
@@ -45,6 +45,16 @@ struct thread_struct {
         struct debug_info       debug;
 };
 
+/*
+ * Everything usercopied to/from thread_struct is statically-sized, so
+ * no hardened usercopy whitelist is needed.
+ */
+static inline void arch_thread_struct_whitelist(unsigned long *offset,
+                                                unsigned long *size)
+{
+        *offset = *size = 0;
+}
+
 #define INIT_THREAD  {  }
 
 #define start_thread(regs,pc,sp)                                        \