Commit f3554aeb authored Jul 12, 2019 by Denis Efremov Committed by Linus Torvalds Jul 17, 2019

floppy: fix div-by-zero in setup_format_params



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>

parent 0ecfebd2

drivers/block/floppy.c

+5 −0

Original line number	Diff line number	Diff line
		@@ -2120,6 +2120,9 @@ static void setup_format_params(int track)
		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;

		@@ -3232,6 +3235,8 @@ static int set_geometry(unsigned int cmd, struct floppy_struct *g,
		/* 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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