linux-kernel - Re: [PATCH v2 1/1] cxl/core: Fix initialization of mbox_cmd.size

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Date: Thu, 4 Apr 2024 14:30:49 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Dan Williams <dan.j.williams@...el.com>
CC: Kwangjin Ko <kwangjin.ko@...com>, <dave@...olabs.net>,
	<dave.jiang@...el.com>, <alison.schofield@...el.com>,
	<vishal.l.verma@...el.com>, <ira.weiny@...el.com>,
	<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<kernel_team@...ynix.com>
Subject: Re: [PATCH v2 1/1] cxl/core: Fix initialization of
 mbox_cmd.size_out in get event

On Wed, 3 Apr 2024 07:53:24 -0700
Dan Williams <dan.j.williams@...el.com> wrote:

> Kwangjin Ko wrote:
> > Since mbox_cmd.size_out is overwritten with the actual output size in
> > the function below, it needs to be initialized every time.
> > 
> > cxl_internal_send_cmd -> __cxl_pci_mbox_send_cmd
> > 
> > Problem scenario:
> > 
> > 1) The size_out variable is initially set to the size of the mailbox.
> > 2) Read an event.
> >    - size_out is set to 160 bytes(header 32B + one event 128B).
> >    - Two event are created while reading.
> > 3) Read the new *two* events.
> >    - size_out is still set to 160 bytes.
> >    - Although the value of out_len is 288 bytes, only 160 bytes are
> >      copied from the mailbox register to the local variable.
> >    - record_count is set to 2.
> >    - Accessing records[1] will result in reading incorrect data.
> > 
> > Signed-off-by: Kwangjin Ko <kwangjin.ko@...com>
> > ---
> >  drivers/cxl/core/mbox.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
> > index 9adda4795eb7..a38531a055c8 100644
> > --- a/drivers/cxl/core/mbox.c
> > +++ b/drivers/cxl/core/mbox.c
> > @@ -958,13 +958,14 @@ static void cxl_mem_get_records_log(struct cxl_memdev_state *mds,
> >  		.payload_in = &log_type,
> >  		.size_in = sizeof(log_type),
> >  		.payload_out = payload,
> > -		.size_out = mds->payload_size,
> >  		.min_out = struct_size(payload, records, 0),
> >  	};
> >  
> >  	do {
> >  		int rc, i;
> >  
> > +		mbox_cmd.size_out = mds->payload_size;
> > +  
> 
> Fix looks correct, but I am concerned it is a band-aid for a more
> general problem. For example, if I am not mistaken, we have a similar
> bug in cxl_mem_get_poison().
> 
> So perhaps a convention to always define @mbox_cmd immediately before
> cxl_internal_send_cmd() like this:

Makes sense to me.  These aren't hot paths, so safe code is worth the
possible extra writes.

> 
> diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
> index 9adda4795eb7..5d44b5c095b7 100644
> --- a/drivers/cxl/core/mbox.c
> +++ b/drivers/cxl/core/mbox.c
> @@ -946,24 +946,22 @@ static void cxl_mem_get_records_log(struct cxl_memdev_state *mds,
>         struct cxl_memdev *cxlmd = mds->cxlds.cxlmd;
>         struct device *dev = mds->cxlds.dev;
>         struct cxl_get_event_payload *payload;
> -       struct cxl_mbox_cmd mbox_cmd;
>         u8 log_type = type;
>         u16 nr_rec;
>  
>         mutex_lock(&mds->event.log_lock);
>         payload = mds->event.buf;
>  
> -       mbox_cmd = (struct cxl_mbox_cmd) {
> -               .opcode = CXL_MBOX_OP_GET_EVENT_RECORD,
> -               .payload_in = &log_type,
> -               .size_in = sizeof(log_type),
> -               .payload_out = payload,
> -               .size_out = mds->payload_size,
> -               .min_out = struct_size(payload, records, 0),
> -       };
> -
>         do {
>                 int rc, i;
> +               struct cxl_mbox_cmd mbox_cmd = (struct cxl_mbox_cmd){
> +                       .opcode = CXL_MBOX_OP_GET_EVENT_RECORD,
> +                       .payload_in = &log_type,
> +                       .size_in = sizeof(log_type),
> +                       .payload_out = payload,
> +                       .size_out = mds->payload_size,
> +                       .min_out = struct_size(payload, records, 0),
> +               };
>  
>                 rc = cxl_internal_send_cmd(mds, &mbox_cmd);
>                 if (rc) {
> @@ -1296,7 +1294,6 @@ int cxl_mem_get_poison(struct cxl_memdev *cxlmd, u64 offset, u64 len,
>         struct cxl_memdev_state *mds = to_cxl_memdev_state(cxlmd->cxlds);
>         struct cxl_mbox_poison_out *po;
>         struct cxl_mbox_poison_in pi;
> -       struct cxl_mbox_cmd mbox_cmd;
>         int nr_records = 0;
>         int rc;
>  
> @@ -1308,16 +1305,16 @@ int cxl_mem_get_poison(struct cxl_memdev *cxlmd, u64 offset, u64 len,
>         pi.offset = cpu_to_le64(offset);
>         pi.length = cpu_to_le64(len / CXL_POISON_LEN_MULT);
>  
> -       mbox_cmd = (struct cxl_mbox_cmd) {
> -               .opcode = CXL_MBOX_OP_GET_POISON,
> -               .size_in = sizeof(pi),
> -               .payload_in = &pi,
> -               .size_out = mds->payload_size,
> -               .payload_out = po,
> -               .min_out = struct_size(po, record, 0),
> -       };
> -
>         do {
> +               struct cxl_mbox_cmd mbox_cmd = (struct cxl_mbox_cmd){
> +                       .opcode = CXL_MBOX_OP_GET_POISON,
> +                       .size_in = sizeof(pi),
> +                       .payload_in = &pi,
> +                       .size_out = mds->payload_size,
> +                       .payload_out = po,
> +                       .min_out = struct_size(po, record, 0),
> +               };
> +
>                 rc = cxl_internal_send_cmd(mds, &mbox_cmd);
>                 if (rc)
>                         break;
>