[PATCH 6.18 217/396] ALSA: usb-audio: Fix DMA buffer out-of-bounds write when fill_max is set

6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sonali Pradhan <[email protected]>

commit d0199ae1666ff9ae2d1d568d64c3430d4c47f0e5 upstream.

When a USB audio endpoint requests full packet transfers via the fill_max
descriptor flag, data_ep_set_params() promotes ep->curpacksize to
ep->maxpacksize. However, maxsize is left at the original sample-rate
derived value.

Since u->buffer_size is allocated as maxsize * packets, the resulting
DMA buffer is far too small for the requested transfer length. When the
USB host controller streams up to curpacksize bytes per packet, it writes
past the end of the buffer via DMA, corrupting kernel heap memory.

Update maxsize to curpacksize when fill_max is set so that the allocated
DMA buffer size matches the actual transfer request size.

[ changed to reassign maxsize only when ep->fill_max is set -- tiwai ]

Fixes: 8fdff6a319e7 ("ALSA: snd-usb: implement new endpoint streaming model")
Cc: [email protected]
Assisted-by: Jetski:Gemini-3.6-Flash
Signed-off-by: Sonali Pradhan <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 sound/usb/endpoint.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -1164,10 +1164,12 @@ static int data_ep_set_params(struct snd
 				<< (16 - ep->datainterval);
 	}
 
-	if (ep->fill_max)
+	if (ep->fill_max) {
 		ep->curpacksize = ep->maxpacksize;
-	else
+		maxsize = ep->curpacksize;
+	} else {
 		ep->curpacksize = maxsize;
+	}
 
 	if (snd_usb_get_speed(chip->dev) != USB_SPEED_FULL) {
 		packs_per_ms = 8 >> ep->datainterval;