While connecting to a Linux host with CDC_NCM_NTB_DEF_SIZE_TX
set to 65536, it has been observed that we receive short packets,
which come at interval of 5-10 seconds sometimes and have block
length zero but still contain 1-2 valid datagrams present.
According to the NCM spec:
"If wBlockLength = 0x0000, the block is terminated by a
short packet. In this case, the USB transfer must still
be shorter than dwNtbInMaxSize or dwNtbOutMaxSize. If
exactly dwNtbInMaxSize or dwNtbOutMaxSize bytes are sent,
and the size is a multiple of wMaxPacketSize for the
given pipe, then no ZLP shall be sent.
wBlockLength= 0x0000 must be used with extreme care, because
of the possibility that the host and device may get out of
sync, and because of test issues.
wBlockLength = 0x0000 allows the sender to reduce latency by
starting to send a very large NTB, and then shortening it when
the sender discovers that there’s not sufficient data to justify
sending a large NTB"
However, there is a potential issue with the current implementation,
as it checks for the occurrence of multiple NTBs in a single
giveback by verifying if the leftover bytes to be processed is zero
or not. If the block length reads zero, we would process the same
NTB infintely because the leftover bytes is never zero and it leads
to a crash. Fix this by bailing out if block length reads zero.
Cc: <[email protected]>
Fixes: 427694cfaafa ("usb: gadget: ncm: Handle decoding of multiple NTB's in unwrap call")
Signed-off-by: Krishna Kurapati <[email protected]>
---
Changes in v2:
Removed goto label
Link to v1:
https://lore.kernel.org/all/[email protected]/
drivers/usb/gadget/function/f_ncm.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
index e2a059cfda2c..28f4e6552e84 100644
--- a/drivers/usb/gadget/function/f_ncm.c
+++ b/drivers/usb/gadget/function/f_ncm.c
@@ -1346,7 +1346,7 @@ static int ncm_unwrap_ntb(struct gether *port,
if (to_process == 1 &&
(*(unsigned char *)(ntb_ptr + block_len) == 0x00)) {
to_process--;
- } else if (to_process > 0) {
+ } else if ((to_process > 0) && (block_len != 0)) {
ntb_ptr = (unsigned char *)(ntb_ptr + block_len);
goto parse_ntb;
}
--
2.34.1
On Wed, Feb 28, 2024 at 3:54 AM Krishna Kurapati
<[email protected]> wrote:
>
> While connecting to a Linux host with CDC_NCM_NTB_DEF_SIZE_TX
> set to 65536, it has been observed that we receive short packets,
> which come at interval of 5-10 seconds sometimes and have block
> length zero but still contain 1-2 valid datagrams present.
>
> According to the NCM spec:
>
> "If wBlockLength = 0x0000, the block is terminated by a
> short packet. In this case, the USB transfer must still
> be shorter than dwNtbInMaxSize or dwNtbOutMaxSize. If
> exactly dwNtbInMaxSize or dwNtbOutMaxSize bytes are sent,
> and the size is a multiple of wMaxPacketSize for the
> given pipe, then no ZLP shall be sent.
>
> wBlockLength= 0x0000 must be used with extreme care, because
> of the possibility that the host and device may get out of
> sync, and because of test issues.
>
> wBlockLength = 0x0000 allows the sender to reduce latency by
> starting to send a very large NTB, and then shortening it when
> the sender discovers that there’s not sufficient data to justify
> sending a large NTB"
>
> However, there is a potential issue with the current implementation,
> as it checks for the occurrence of multiple NTBs in a single
> giveback by verifying if the leftover bytes to be processed is zero
> or not. If the block length reads zero, we would process the same
> NTB infintely because the leftover bytes is never zero and it leads
> to a crash. Fix this by bailing out if block length reads zero.
>
> Cc: <[email protected]>
> Fixes: 427694cfaafa ("usb: gadget: ncm: Handle decoding of multiple NTB's in unwrap call")
> Signed-off-by: Krishna Kurapati <[email protected]>
> ---
> Changes in v2:
> Removed goto label
>
> Link to v1:
> https://lore.kernel.org/all/[email protected]/
>
> drivers/usb/gadget/function/f_ncm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
> index e2a059cfda2c..28f4e6552e84 100644
> --- a/drivers/usb/gadget/function/f_ncm.c
> +++ b/drivers/usb/gadget/function/f_ncm.c
> @@ -1346,7 +1346,7 @@ static int ncm_unwrap_ntb(struct gether *port,
> if (to_process == 1 &&
> (*(unsigned char *)(ntb_ptr + block_len) == 0x00)) {
> to_process--;
> - } else if (to_process > 0) {
> + } else if ((to_process > 0) && (block_len != 0)) {
> ntb_ptr = (unsigned char *)(ntb_ptr + block_len);
> goto parse_ntb;
> }
> --
> 2.34.1
Reviewed-by: Maciej Żenczykowski <[email protected]>