Re: [PATCH] tty: fix data race in tty_buffer_flush

[Peter Hurley]

On Tue, Sep 8, 2015 at 8:48 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> tty_buffer_flush frees not acquired buffers.
> As the result, for example, read of b->size in tty_buffer_free
> can return garbage value which will lead to a huge buffer
> hanging in the freelist. This is just the benignest
> manifestation of freeing of a not acquired object.
> If the object is passed to kfree, heap can be corrupted.
>
> Acquire visibility over the buffer before freeing it.
>
> The data race was found with KernelThreadSanitizer (KTSAN).

Dmitry,

Looks good. Thanks for splitting this from the other fix.

Like Greg said, the patch needs revision info to help maintainers, et al
track which is most current/relevant/etc.

Typical format for $subject is something like:

[PATCH v4] tty: fix data race in tty_buffer_flush


Notes re: changes from other patch versions are added below the
git changelog separator (example below).

> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> ---

v4: Corrected commit log and patch revision notes

v3: Added code comment re: paired smp barrier

v2: Split from 'tty: fix data races on tty_buffer.commit'

>  drivers/tty/tty_buffer.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 5a3fa89..a2a8cd0 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -242,7 +242,10 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
>         atomic_inc(&buf->priority);
>
>         mutex_lock(&buf->lock);
> -       while ((next = buf->head->next) != NULL) {
> +       /* paired w/ release in __tty_buffer_request_room; ensures there are
> +        * no pending memory accesses to the freed buffer
> +        */
> +       while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
>                 tty_buffer_free(port, buf->head);
>                 buf->head = next;
>         }
> --
> 2.5.0.457.gab17608
>
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