[PATCH 3/3] 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 init_task and fill the rest with zero.

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")
---
 arch/x86/kernel/process.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index f63f8fd00a91..1be45fe70cad 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -92,7 +92,15 @@ 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(dst, src, sizeof(init_task));
+		memset((void *)dst + sizeof(init_task), 0,
+		       arch_task_struct_size - sizeof(init_task));
+	} else {
+		memcpy(dst, src, arch_task_struct_size);
+	}
+
 #ifdef CONFIG_VM86
 	dst->thread.vm86 = NULL;
 #endif
-- 
2.47.1