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SubjectRe: [PATCH] signal/usb: Replace kill_pid_info_as_cred with kill_pid_usb_asyncio
Alan Stern <stern@rowland.harvard.edu> writes:

> On Tue, 21 May 2019, Eric W. Biederman wrote:
>
>> 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 */

Sigh git commit ate the includes...

>> 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));
>
> In fact, these values are supposed to be in host-endian order, not
> necessarily little-endian. The USB core converts them if necessary.

Please look again. In include/uapi/linux/ch9.h those fields are
explicitly defined as little endian and the code in devio.c for
USBDEVFS_URB_TYPE_CONTROL treats them as little endian. Perhaps there
is a mismatch here but I haven't seen it and I needed this change to get
the code to work on big endian.

>> 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
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>>
>> I managed to wrestle a sparc64 qemu to the ground so I could verify this
>> bug exists and the patch below fixes it.
>>
>> Can I get an Ack from the usb side of things?
>
> Give me some time to review the description and the changes.

Please, it always helps when more people understand these things.

Eric

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