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Subject[PATCH 3.16 138/157] floppy: fix div-by-zero in setup_format_params
3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Denis Efremov <efremov@ispras.ru>

commit f3554aeb991214cbfafd17d55e2bfddb50282e32 upstream.

This fixes a divide by zero error in the setup_format_params function of
the floppy driver.

Two consecutive ioctls can trigger the bug: The first one should set the
drive geometry with such .sect and .rate values for the F_SECT_PER_TRACK
to become zero. Next, the floppy format operation should be called.

A floppy disk is not required to be inserted. An unprivileged user
could trigger the bug if the device is accessible.

The patch checks F_SECT_PER_TRACK for a non-zero value in the
set_geometry function. The proper check should involve a reasonable
upper limit for the .sect and .rate fields, but it could change the
UAPI.

The patch also checks F_SECT_PER_TRACK in the setup_format_params, and
cancels the formatting operation in case of zero.

The bug was found by syzkaller.

Signed-off-by: Denis Efremov <efremov@ispras.ru>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/block/floppy.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -2113,6 +2113,9 @@ static void setup_format_params(int trac
raw_cmd->kernel_data = floppy_track_buffer;
raw_cmd->length = 4 * F_SECT_PER_TRACK;

+ if (!F_SECT_PER_TRACK)
+ return;
+
/* allow for about 30ms for data transport per track */
head_shift = (F_SECT_PER_TRACK + 5) / 6;

@@ -3235,6 +3238,8 @@ static int set_geometry(unsigned int cmd
/* sanity checking for parameters. */
if (g->sect <= 0 ||
g->head <= 0 ||
+ /* check for zero in F_SECT_PER_TRACK */
+ (unsigned char)((g->sect << 2) >> FD_SIZECODE(g)) == 0 ||
g->track <= 0 || g->track > UDP->tracks >> STRETCH(g) ||
/* check if reserved bits are set */
(g->stretch & ~(FD_STRETCH | FD_SWAPSIDES | FD_SECTBASEMASK)) != 0)
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