arm64/ptrace: Preserve previous registers for short regset write

Ensure that if userspace supplies insufficient data to
PTRACE_SETREGSET to fill all the registers, the thread's old
registers are preserved.

Cc: <stable@vger.kernel.org> # 4.3.x-
Fixes: 5d220ff9420f ("arm64: Better native ptrace support for compat tasks")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <Will.Deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index ead36d5..91c4719 100644 (file)
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -950,7 +950,7 @@ static int compat_tls_set(struct task_struct *target,
                          const void __user *ubuf)
 {
        int ret;
-       compat_ulong_t tls;
+       compat_ulong_t tls = target->thread.tp_value;
 
        ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &tls, 0, -1);
        if (ret)