[PATCH v2] x86: avoid copying dynamic FP state from init_task

[Benjamin Berg <benjamin@sipsolutions.net>]

From: Benjamin Berg <benjamin.berg@intel.com>

The init_task instance of struct task_struct is statically allocated and
may not contain the full FP state for userspace. As such, limit the copy
to the valid area of both init_task and dst and ensure all memory is
initialized.

Note that the FP state is only needed for userspace, and as such it is
entirely reasonable for init_task to not contain parts of it.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Fixes: 5aaeb5c01c5b ("x86/fpu, sched: Introduce CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT and use it on x86")

----

v2:
- Fix code if arch_task_struct_size < sizeof(init_task) by using
  memcpy_and_pad.
---
 arch/x86/kernel/process.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 6da6769d7254..a8f65c17df10 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -93,7 +93,13 @@ EXPORT_PER_CPU_SYMBOL_GPL(__tss_limit_invalid);
  */
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
-	memcpy(dst, src, arch_task_struct_size);
+	/* init_task is not dynamically sized (incomplete FPU state) */
+	if (unlikely(src == &init_task))
+		memcpy_and_pad(dst, arch_task_struct_size,
+			       src, sizeof(init_task), 0);
+	else
+		memcpy(dst, src, arch_task_struct_size);
+
 #ifdef CONFIG_VM86
 	dst->thread.vm86 = NULL;
 #endif
-- 
2.48.1