um: avoid copying FP state from init_task

The init_task instance of struct task_struct is statically allocated and
does not contain the dynamic area for the userspace FP registers. 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 it.

Reported-by: Brian Norris <briannorris@chromium.org>
Closes: https://lore.kernel.org/Z1ySXmjZm-xOqk90@google.com
Fixes: 3f17fed21491 ("um: switch to regset API and depend on XSTATE")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241217202745.1402932-3-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index e5a2d4d897e0c7211c425fbd9fe7c4d4f4a8a32c..0cd6fad3d908d43e84ebc821676e05377ec641e9 100644 (file)
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -191,7 +191,15 @@ void initial_thread_cb(void (*proc)(void *), void *arg)
 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 (missing 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);
+       }
+
        return 0;
 }