[PATCH 2/3] um: avoid copying FP state from init_task

From: Benjamin Berg
Date: Tue Dec 17 2024 - 15:30:50 EST


From: Benjamin Berg <benjamin.berg@xxxxxxxxx>

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@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/Z1ySXmjZm-xOqk90@xxxxxxxxxx
Fixes: 3f17fed21491 ("um: switch to regset API and depend on XSTATE")
Signed-off-by: Benjamin Berg <benjamin.berg@xxxxxxxxx>
---
arch/um/kernel/process.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index 30bdc0a87dc8..3a67ba8aa62d 100644
--- 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;
}

--
2.47.1