[PATCH 1/1] arm64/sve: Fix wrong free for task->thread.sve_state

[Masayoshi Mizuma]

From: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>

The system which has SVE feature crashed because of
the memory pointed by task->thread.sve_state was destroyed
by someone.

That is because sve_state is freed while the forking the
child process. The child process has the pointer of sve_state
which is same as the parent's because the child's task_struct
is copied from the parent's one. If the copy_process()
fails as an error on somewhere, for example, copy_creds(),
then the sve_state is freed even if the parent is alive.
The flow is as follows.

copy_process
        p = dup_task_struct
            => arch_dup_task_struct
                *dst = *src;  // copy the entire region.
:
        retval = copy_creds
        if (retval < 0)
                goto bad_fork_free;
:
bad_fork_free:
...
        delayed_free_task(p);
          => free_task
             => arch_release_task_struct
                => fpsimd_release_task
                   => __sve_free
                      => kfree(task->thread.sve_state);
                         // free the parent's sve_state

Add a flag in task->thread which shows the fork is in progress.
If the fork is in progress, that means the child has the pointer
to the parent's sve_state, doesn't free the sve_state.

Signed-off-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
Reported-by: Hidetoshi Seto <seto.hidetoshi@xxxxxxxxxxxxxx>
---
 arch/arm64/include/asm/processor.h | 1 +
 arch/arm64/kernel/fpsimd.c         | 6 ++++--
 arch/arm64/kernel/process.c        | 2 ++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 5623685c7d13..3ca3e350145a 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -143,6 +143,7 @@ struct thread_struct {
 	unsigned long		fault_address;	/* fault info */
 	unsigned long		fault_code;	/* ESR_EL1 value */
 	struct debug_info	debug;		/* debugging */
+	unsigned int		fork_in_progress;
 #ifdef CONFIG_ARM64_PTR_AUTH
 	struct ptrauth_keys	keys_user;
 #endif
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 37d3912cfe06..8641db4cb062 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -202,8 +202,10 @@ static bool have_cpu_fpsimd_context(void)
  */
 static void __sve_free(struct task_struct *task)
 {
-	kfree(task->thread.sve_state);
-	task->thread.sve_state = NULL;
+	if (!task->thread.fork_in_progress) {
+		kfree(task->thread.sve_state);
+		task->thread.sve_state = NULL;
+	}
 }
 
 static void sve_free(struct task_struct *task)
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index a47462def04b..8ac0ee4e5f76 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -347,6 +347,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	if (current->mm)
 		fpsimd_preserve_current_state();
 	*dst = *src;
+	dst->thread.fork_in_progress = 1;
 
 	return 0;
 }
@@ -365,6 +366,7 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 	 * and disable discard SVE state for p:
 	 */
 	clear_tsk_thread_flag(p, TIF_SVE);
+	p->thread.fork_in_progress = 0;
 	p->thread.sve_state = NULL;
 
 	/*
-- 
2.18.1