On Mon, Sep 30, 2019 at 04:56:00PM -0400, Masayoshi Mizuma wrote: > From: Masayoshi Mizuma <[email protected]> > > 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: [email protected] # 4.15.x- > Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") > Signed-off-by: Masayoshi Mizuma <[email protected]> > Reported-by: Hidetoshi Seto <[email protected]> > Suggested-by: Dave Martin <[email protected]> > Tested-by: Julien Grall <[email protected]>
Looks OK to me: Reviewed-by: Dave Martin <[email protected]> Cheers ---Dave > --- > arch/arm64/kernel/process.c | 32 +++++++++++++++----------------- > 1 file changed, 15 insertions(+), 17 deletions(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index a47462def04b..ef7aa909bfda 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -332,22 +332,27 @@ 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; > } > > @@ -360,13 +365,6 @@ 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 > -- > 2.18.1 > > > _______________________________________________ > linux-arm-kernel mailing list > [email protected] > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
