On Fri, Sep 27, 2019 at 11:39:49AM -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.
You could also add: --8<-- There is no need to wait until copy_process() to clear TIF_SVE for dst, becuase 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. -->8-- > > Cc: [email protected] Since SVE only exists from v4.15, it may be helpful to specify that, i.e., replace that Cc line with: Cc: <[email protected]> # 4.15.x- Otherwise, I'm happy to see this applied, but I'd like somebody to confirm that this change definitely fixes the bug. Cheers ---Dave [...] > 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]> > --- > arch/arm64/kernel/process.c | 21 ++++----------------- > 1 file changed, 4 insertions(+), 17 deletions(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index f674f28df..6937f5935 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -323,22 +323,16 @@ 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; > > + BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); > + dst->thread.sve_state = NULL; > + clear_tsk_thread_flag(dst, TIF_SVE); > + > return 0; > } > > @@ -351,13 +345,6 @@ 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
