From: Masayoshi Mizuma m.mizuma@jp.fujitsu.com
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
--- arch/arm64/kernel/process.c | 32 +++++++++++++++----------------- 1 file changed, 15 insertions(+), 17 deletions(-)
--- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -285,22 +285,27 @@ void arch_release_task_struct(struct tas 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_flag 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 * any cpu that may have run that now-exited task recently.