floppy: use a statically allocated error counter

Interrupt handler bad_flp_intr() may cause a UAF on the recently freed
request just to increment the error count.  There's no point keeping
that one in the request anyway, and since the interrupt handler uses a
static pointer to the error which cannot be kept in sync with the
pending request, better make it use a static error counter that's reset
for each new request.  This reset now happens when entering
redo_fd_request() for a new request via set_next_request().

One initial concern about a single error counter was that errors on one
floppy drive could be reported on another one, but this problem is not
real given that the driver uses a single drive at a time, as that
PC-compatible controllers also have this limitation by using shared
signals.  As such the error count is always for the "current" drive.
Reported-by: Minh Yuan <yuanmingbuaa@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Tested-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/floppy.c

@@ -509,8 +509,8 @@ static unsigned long fdc_busy;
 static DECLARE_WAIT_QUEUE_HEAD(fdc_wait);
 static DECLARE_WAIT_QUEUE_HEAD(command_done);
 
-/* Errors during formatting are counted here. */
-static int format_errors;
+/* errors encountered on the current (or last) request */
+static int floppy_errors;
 
 /* Format request descriptor. */
 static struct format_descr format_req;
@@ -530,7 +530,6 @@ static struct format_descr format_req;
 static char *floppy_track_buffer;
 static int max_buffer_sectors;
 
-static int *errors;
 typedef void (*done_f)(int);
 static const struct cont_t {
 	void (*interrupt)(void);
@@ -1455,7 +1454,7 @@ static int interpret_errors(void)
 			if (drive_params[current_drive].flags & FTD_MSG)
 				DPRINT("Over/Underrun - retrying\n");
 			bad = 0;
-		} else if (*errors >= drive_params[current_drive].max_errors.reporting) {
+		} else if (floppy_errors >= drive_params[current_drive].max_errors.reporting) {
 			print_errors();
 		}
 		if (reply_buffer[ST2] & ST2_WC || reply_buffer[ST2] & ST2_BC)
@@ -2095,7 +2094,7 @@ static void bad_flp_intr(void)
 		if (!next_valid_format(current_drive))
 			return;
 	}
-	err_count = ++(*errors);
+	err_count = ++floppy_errors;
 	INFBOUND(write_errors[current_drive].badness, err_count);
 	if (err_count > drive_params[current_drive].max_errors.abort)
 		cont->done(0);
@@ -2241,9 +2240,8 @@ static int do_format(int drive, struct format_descr *tmp_format_req)
 		return -EINVAL;
 	}
 	format_req = *tmp_format_req;
-	format_errors = 0;
 	cont = &format_cont;
-	errors = &format_errors;
+	floppy_errors = 0;
 	ret = wait_til_done(redo_format, true);
 	if (ret == -EINTR)
 		return -EINTR;
@@ -2759,10 +2757,11 @@ static int set_next_request(void)
 	current_req = list_first_entry_or_null(&floppy_reqs, struct request,
 					       queuelist);
 	if (current_req) {
-		current_req->error_count = 0;
+		floppy_errors = 0;
 		list_del_init(&current_req->queuelist);
+		return 1;
 	}
-	return current_req != NULL;
+	return 0;
 }
 
 /* Starts or continues processing request. Will automatically unlock the
@@ -2821,7 +2820,6 @@ static void redo_fd_request(void)
 		_floppy = floppy_type + drive_params[current_drive].autodetect[drive_state[current_drive].probed_format];
 	} else
 		probing = 0;
-	errors = &(current_req->error_count);
 	tmp = make_raw_rw_request();
 	if (tmp < 2) {
 		request_done(tmp);