ALSA: FCP: Fix NULL pointer dereference in interface lookup

JIRA: https://issues.redhat.com/browse/RHEL-193253

commit e1e31e0ec8a609e17fd2e86b77bc00d9cbb24d7c
Author: Jiaming Zhang <r772577952@gmail.com>
Date: Thu Jun 25 21:49:33 2026 +0800

    ALSA: FCP: Fix NULL pointer dereference in interface lookup

    A malformed USB device can provide a vendor-specific interface without
    any endpoint descriptors. fcp_find_fc_interface() currently selects the
    first vendor-specific interface and reads endpoint 0 from it, without
    checking whether the interface actually has any endpoints.

    When bNumEndpoints is zero, no endpoint array is allocated for the parsed
    alternate setting, so get_endpoint(..., 0) yields an invalid endpoint
    descriptor pointer. Dereferencing it through usb_endpoint_num() then
    triggers a NULL pointer dereference.

    Skip vendor-specific interfaces that do not have any endpoints.

    Fixes: 46757a3e7d50 ("ALSA: FCP: Add Focusrite Control Protocol driver")
    Reported-by: Jiaming Zhang <r772577952@gmail.com>
    Closes: https://lore.kernel.org/lkml/CANypQFb1EHj0xX8bA1WxSOSK-5xca6ZNKzOQcp12=s=puY7VFw@mail.gmail.com/
    Signed-off-by: Jiaming Zhang <r772577952@gmail.com>
    Link: https://patch.msgid.link/20260625134933.425785-1-r772577952@gmail.com
    Signed-off-by: Takashi Iwai <tiwai@suse.de>

Signed-off-by: Jaroslav Kysela <jkysela@redhat.com>

sound/usb/fcp.c

@@ -1083,6 +1083,8 @@ static int fcp_find_fc_interface(struct usb_mixer_interface *mixer)
 
 		if (desc->bInterfaceClass != 255)
 			continue;
+		if (desc->bNumEndpoints < 1)
+			continue;
 
 		epd = get_endpoint(intf->altsetting, 0);
 		private->bInterfaceNumber = desc->bInterfaceNumber;