Re: [PATCH v2] usb: storage: Fix memory leak in USB bulk transport

[Alan Stern <stern@rowland.harvard.edu>]

On Thu, Oct 30, 2025 at 06:48:33PM -0300, Desnes Nunes wrote:
> A kernel memory leak was identified by the 'ioctl_sg01' test from Linux
> Test Project (LTP). The following bytes were mainly observed: 0x53425355.
> 
> When USB storage devices incorrectly skip the data phase with status data,
> the code extracts/validates the CSW from the sg buffer, but fails to clear
> it afterwards. This leaves status protocol data in srb's transfer buffer,
> such as the US_BULK_CS_SIGN 'USBS' signature observed here. Thus, this can
> lead to USB protocols leaks to user space through SCSI generic (/dev/sg*)
> interfaces, such as the one seen here when the LTP test requested 512 KiB.
> 
> Fix the leak by zeroing the CSW data in srb's transfer buffer immediately
> after the validation of devices that skip data phase.
> 
> Note: Differently from CVE-2018-1000204, which fixed a big leak by zero-
> ing pages at allocation time, this leak occurs after allocation, when USB
> protocol data is written to already-allocated sg pages.
> 
> v2: Use the same code style found on usb_stor_Bulk_transport()

Minor nit: The version information is supposed to go below the "---" 
line.  It's not really part of the patch; when people in the future see 
this patch in the git history, they won't care how many previous 
versions it went through or what the changes were.

> 
> Fixes: a45b599ad808 ("scsi: sg: allocate with __GFP_ZERO in sg_build_indirect()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Desnes Nunes <desnesn@redhat.com>
> ---
>  drivers/usb/storage/transport.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/usb/storage/transport.c b/drivers/usb/storage/transport.c
> index 1aa1bd26c81f..ee6b89f7f9ac 100644
> --- a/drivers/usb/storage/transport.c
> +++ b/drivers/usb/storage/transport.c
> @@ -1200,7 +1200,19 @@ int usb_stor_Bulk_transport(struct scsi_cmnd *srb, struct us_data *us)
>  						US_BULK_CS_WRAP_LEN &&
>  					bcs->Signature ==
>  						cpu_to_le32(US_BULK_CS_SIGN)) {
> +				unsigned char buf[US_BULK_CS_WRAP_LEN];
> +
> +				sg = NULL;
> +				offset = 0;
> +				memset(buf, 0, US_BULK_CS_WRAP_LEN);
>  				usb_stor_dbg(us, "Device skipped data phase\n");

Another nit: Logically the comment belongs before the three new lines, 
because it notes that there was a problem whereas the new lines are part 
of the scheme to then mitigate the problem.  It might also be worthwhile 
to add a comment explaining the reason for overwriting the CSW data, 
namely, to avoid leaking protocol information to userspace.  This point 
is not immediately obvious.

> +
> +				if (usb_stor_access_xfer_buf(buf,
> +						US_BULK_CS_WRAP_LEN, srb, &sg,
> +						&offset, TO_XFER_BUF) !=
> +							US_BULK_CS_WRAP_LEN)

Yet another nit: Don't people recommend using sizeof(buf) instead of 
US_BULK_CS_WRAP_LEN in places like these?  Particularly in memset()?

> +					usb_stor_dbg(us, "Failed to clear CSW data\n");
> +
>  				scsi_set_resid(srb, transfer_length);
>  				goto skipped_data_phase;
>  			}

Regardless of the nits:

Reviewed-by: Alan Stern <stern@rowland.harvard.edu>

Alan Stern