arm64/sve: Fix wrong free for task->thread.sve_state

BugLink: https://bugs.launchpad.net/bugs/1850541
commit 4585fc59c0e813188d6a4c5de1f6976fce461fc2 upstream.

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

Move child's sve_state = NULL and clearing TIF_SVE flag
to arch_dup_task_struct() so that the child doesn't free the
parent's one.
There is no need to wait until copy_process() to clear TIF_SVE for
dst, because the thread flags for dst are initialized already by
copying the src task_struct.
This change simplifies the code, so get rid of comments that are no
longer needed.

As a note, arm64 used to have thread_info on the stack. So it
would not be possible to clear TIF_SVE until the stack is initialized.
From commit c02433dd6de3 ("arm64: split thread_info from task stack"),
the thread_info is part of the task, so it should be valid to modify
the flag from arch_dup_task_struct().

Cc: stable@vger.kernel.org # 4.15.x-
Fixes: bc0ee4760364 ("arm64/sve: Core task context handling")
Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Suggested-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index d6ca5fccb2298d43a70e819edef9205562e739a6..80b3286c75a460334697b31ec5f2f31f74e6727c 100644 (file)
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -286,22 +286,27 @@ void arch_release_task_struct(struct task_struct *tsk)
        fpsimd_release_task(tsk);
 }
 
-/*
- * src and dst may temporarily have aliased sve_state after task_struct
- * is copied.  We cannot fix this properly here, because src may have
- * live SVE state and dst's thread_info may not exist yet, so tweaking
- * either src's or dst's TIF_SVE is not safe.
- *
- * The unaliasing is done in copy_thread() instead.  This works because
- * dst is not schedulable or traceable until both of these functions
- * have been called.
- */
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
        if (current->mm)
                fpsimd_preserve_current_state();
        *dst = *src;
 
+       /* We rely on the above assignment to initialize dst's thread_flags: */
+       BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));
+
+       /*
+        * Detach src's sve_state (if any) from dst so that it does not
+        * get erroneously used or freed prematurely.  dst's sve_state
+        * will be allocated on demand later on if dst uses SVE.
+        * For consistency, also clear TIF_SVE here: this could be done
+        * later in copy_process(), but to avoid tripping up future
+        * maintainers it is best not to leave TIF_SVE and sve_state in
+        * an inconsistent state, even temporarily.
+        */
+       dst->thread.sve_state = NULL;
+       clear_tsk_thread_flag(dst, TIF_SVE);
+
        return 0;
 }
 
@@ -314,13 +319,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 
        memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context));
 
-       /*
-        * Unalias p->thread.sve_state (if any) from the parent task
-        * and disable discard SVE state for p:
-        */
-       clear_tsk_thread_flag(p, TIF_SVE);
-       p->thread.sve_state = NULL;
-
        /*
         * In case p was allocated the same task_struct pointer as some
         * other recently-exited task, make sure p is disassociated from