[SRU Bionic 1/1] floppy: use a statically allocated error counter

[Thadeu Lima de Souza Cascardo]

From: Willy Tarreau <w at 1wt.eu>

commit f71f01394f742fc4558b3f9f4c7ef4c4cf3b07c8 upstream.

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 at gmail.com>
Suggested-by: Linus Torvalds <torvalds at linuxfoundation.org>
Tested-by: Denis Efremov <efremov at linux.com>
Signed-off-by: Willy Tarreau <w at 1wt.eu>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Denis Efremov <efremov at linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit 3392d8711ad9e5b688999c948fd36d798c0d075d linux-4.19.y)
CVE-2022-1652
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 drivers/block/floppy.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
index c6404cc9b2f9..fcc173ad229c 100644
--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -520,8 +520,8 @@ 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;
@@ -541,7 +541,6 @@ 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);
@@ -1434,7 +1433,7 @@ static int interpret_errors(void)
 			if (DP->flags & FTD_MSG)
 				DPRINT("Over/Underrun - retrying\n");
 			bad = 0;
-		} else if (*errors >= DP->max_errors.reporting) {
+		} else if (floppy_errors >= DP->max_errors.reporting) {
 			print_errors();
 		}
 		if (ST2 & ST2_WC || ST2 & ST2_BC)
@@ -2057,7 +2056,7 @@ static void bad_flp_intr(void)
 		if (!next_valid_format())
 			return;
 	}
-	err_count = ++(*errors);
+	err_count = ++floppy_errors;
 	INFBOUND(DRWE->badness, err_count);
 	if (err_count > DP->max_errors.abort)
 		cont->done(0);
@@ -2202,9 +2201,8 @@ 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;
@@ -2687,7 +2685,7 @@ static int make_raw_rw_request(void)
 		 */
 		if (!direct ||
 		    (indirect * 2 > direct * 3 &&
-		     *errors < DP->max_errors.read_track &&
+		     floppy_errors < DP->max_errors.read_track &&
 		     ((!probing ||
 		       (DP->read_track & (1 << DRS->probed_format)))))) {
 			max_size = blk_rq_sectors(current_req);
@@ -2821,7 +2819,7 @@ static int set_next_request(void)
 		if (q) {
 			current_req = blk_fetch_request(q);
 			if (current_req) {
-				current_req->error_count = 0;
+				floppy_errors = 0;
 				break;
 			}
 		}
@@ -2883,7 +2881,6 @@ static void redo_fd_request(void)
 		_floppy = floppy_type + DP->autodetect[DRS->probed_format];
 	} else
 		probing = 0;
-	errors = &(current_req->error_count);
 	tmp = make_raw_rw_request();
 	if (tmp < 2) {
 		request_done(tmp);
-- 
2.34.1