RE: [PATCH v2 1/1] cxl/core: Fix initialization of mbox_cmd.size_out in get event

[Dan Williams <dan.j.williams@intel.com>]

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@sk.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:

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;