The usb support for asyncio encoded one of it's values in the wrong
field.  It should have used si_value but instead used si_addr which is
not present in the _rt union member of struct siginfo.

The practical result of this is that on a 64bit big endian kernel
when delivering a signal to a 32bit process the si_addr field
is set to NULL, instead of the expected pointer value.

This issue can not be fixed in copy_siginfo_to_user32 as the usb
usage of the the _sigfault (aka si_addr) member of the siginfo
union when SI_ASYNCIO is set is incompatible with the POSIX and
glibc usage of the _rt member of the siginfo union.

Therefore replace kill_pid_info_as_cred with kill_pid_usb_asyncio a
dedicated function for this one specific case.  There are no other
users of kill_pid_info_as_cred so this specialization should have no
impact on the amount of code in the kernel.  Have kill_pid_usb_asyncio
take instead of a siginfo_t which is difficult and error prone, 3
arguments, a signal number, an errno value, and an address enconded as
a sigval_t.  The encoding of the address as a sigval_t allows the
code that reads the userspace request for a signal to handle this
compat issue along with all of the other compat issues.

Add BUILD_BUG_ONs in kernel/signal.c to ensure that we can now place
the pointer value at the in si_pid (instead of si_addr).  That is the
code now verifies that si_pid and si_addr always occur at the same
location.  Further the code veries that for native structures a value
placed in si_pid and spilling into si_uid will appear in userspace in
si_addr (on a byte by byte copy of siginfo or a field by field copy of
siginfo).  The code also verifies that for a 64bit kernel and a 32bit
userspace the 32bit pointer will fit in si_pid.

I have used the usbsig.c program below written by Alan Stern and
slightly tweaked by me to run on a big endian machine to verify the
issue exists (on sparc64) and to confirm the patch below fixes the issue.

 /* usbsig.c -- test USB async signal delivery */

 #define _GNU_SOURCE
 #include <stdio.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <string.h>
 #include <sys/ioctl.h>
 #include <unistd.h>
 #include <endian.h>
 #include <linux/usb/ch9.h>
 #include <linux/usbdevice_fs.h>

 static struct usbdevfs_urb urb;
 static struct usbdevfs_disconnectsignal ds;
 static volatile sig_atomic_t done = 0;

 void urb_handler(int sig, siginfo_t *info , void *ucontext)
 {
 	printf("Got signal %d, signo %d errno %d code %d addr: %p urb: %p\n",
 	       sig, info->si_signo, info->si_errno, info->si_code,
 	       info->si_addr, &urb);

 	printf("%s\n", (info->si_addr == &urb) ? "Good" : "Bad");
 }

 void ds_handler(int sig, siginfo_t *info , void *ucontext)
 {
 	printf("Got signal %d, signo %d errno %d code %d addr: %p ds: %p\n",
 	       sig, info->si_signo, info->si_errno, info->si_code,
 	       info->si_addr, &ds);

 	printf("%s\n", (info->si_addr == &ds) ? "Good" : "Bad");
 	done = 1;
 }

 int main(int argc, char **argv)
 {
 	char *devfilename;
 	int fd;
 	int rc;
 	struct sigaction act;
 	struct usb_ctrlrequest *req;
 	void *ptr;
 	char buf[80];

 	if (argc != 2) {
 		fprintf(stderr, "Usage: usbsig device-file-name\n");
 		return 1;
 	}

 	devfilename = argv[1];
 	fd = open(devfilename, O_RDWR);
 	if (fd == -1) {
 		perror("Error opening device file");
 		return 1;
 	}

 	act.sa_sigaction = urb_handler;
 	sigemptyset(&act.sa_mask);
 	act.sa_flags = SA_SIGINFO;

 	rc = sigaction(SIGUSR1, &act, NULL);
 	if (rc == -1) {
 		perror("Error in sigaction");
 		return 1;
 	}

 	act.sa_sigaction = ds_handler;
 	sigemptyset(&act.sa_mask);
 	act.sa_flags = SA_SIGINFO;

 	rc = sigaction(SIGUSR2, &act, NULL);
 	if (rc == -1) {
 		perror("Error in sigaction");
 		return 1;
 	}

 	memset(&urb, 0, sizeof(urb));
 	urb.type = USBDEVFS_URB_TYPE_CONTROL;
 	urb.endpoint = USB_DIR_IN | 0;
 	urb.buffer = buf;
 	urb.buffer_length = sizeof(buf);
 	urb.signr = SIGUSR1;

 	req = (struct usb_ctrlrequest *) buf;
 	req->bRequestType = USB_DIR_IN | USB_TYPE_STANDARD | USB_RECIP_DEVICE;
 	req->bRequest = USB_REQ_GET_DESCRIPTOR;
 	req->wValue = htole16(USB_DT_DEVICE << 8);
 	req->wIndex = htole16(0);
 	req->wLength = htole16(sizeof(buf) - sizeof(*req));

 	rc = ioctl(fd, USBDEVFS_SUBMITURB, &urb);
 	if (rc == -1) {
 		perror("Error in SUBMITURB ioctl");
 		return 1;
 	}

 	rc = ioctl(fd, USBDEVFS_REAPURB, &ptr);
 	if (rc == -1) {
 		perror("Error in REAPURB ioctl");
 		return 1;
 	}

 	memset(&ds, 0, sizeof(ds));
 	ds.signr = SIGUSR2;
 	ds.context = &ds;
 	rc = ioctl(fd, USBDEVFS_DISCSIGNAL, &ds);
 	if (rc == -1) {
 		perror("Error in DISCSIGNAL ioctl");
 		return 1;
 	}

 	printf("Waiting for usb disconnect\n");
 	while (!done) {
 		sleep(1);
 	}

 	close(fd);
 	return 0;
 }

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oneukum@suse.com>
Fixes: v2.3.39
Cc: stable@vger.kernel.org
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>