[PATCH 1/2] ALSA: usb-audio: qcom: reject stream disable with no active interface

[Michael Bommarito <michael.bommarito@gmail.com>]

handle_uaudio_stream_req() resolves an interface index with
info_idx_from_ifnum(), which returns -EINVAL when no interface matches.
The enable branch and the response: cleanup label both guard against a
negative index, but the disable branch does not: it forms
info = &uadev[pcm_card_num].info[info_idx] and dereferences it.

uadev[].info is a pointer allocated only when a stream is first enabled,
so a negative info_idx on the disable path is unsafe in two ways:

 - If the card was never enabled, .info is NULL and &info[-EINVAL] is a
   wild pointer; reading info->data_ep_pipe faults (kernel oops).

 - If the card was enabled at least once (.info allocated) and the
   disable names an interface that does not match, &info[-EINVAL] points
   before the allocation; info->data_ep_pipe / info->sync_ep_pipe are an
   out-of-bounds slab read and, when non-zero, an out-of-bounds 4-byte
   write (both pipe fields are cleared to 0). That is memory corruption,
   not just a NULL dereference.

The request is reachable from unprivileged local userspace over
AF_QIPCRTR. Reject a disable request with no resolved interface, matching
the guard the enable path already has.

Fixes: 326bbc348298a ("ALSA: usb-audio: qcom: Introduce QC USB SND offloading support")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
Not reproduced: static analysis against current mainline only (no Qualcomm
hardware; CONFIG_SND_USB_AUDIO_QMI does not build on x86). The enable
branch and the response: cleanup label already guard info_idx; this adds
the same guard to the disable branch. Please confirm on a Qualcomm build
whether a disable request can reach this branch with info_idx < 0, and
whether the out-of-bounds write in the .info-allocated case is reachable.

 sound/usb/qcom/qc_audio_offload.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/sound/usb/qcom/qc_audio_offload.c b/sound/usb/qcom/qc_audio_offload.c
index a3f90cc7c6cad..852a91e4b8686 100644
--- a/sound/usb/qcom/qc_audio_offload.c
+++ b/sound/usb/qcom/qc_audio_offload.c
@@ -1640,6 +1640,11 @@ static void handle_uaudio_stream_req(struct qmi_handle *handle,
 			subs->opened = 0;
 		}
 	} else {
+		if (info_idx < 0) {
+			ret = -EINVAL;
+			goto response;
+		}
+
 		info = &uadev[pcm_card_num].info[info_idx];
 		if (info->data_ep_pipe) {
 			ep = usb_pipe_endpoint(uadev[pcm_card_num].udev,
-- 
2.53.0