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floppy: fix div-by-zero in setup_format_params

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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>
This commit is contained in:
Denis Efremov 2019-07-12 21:55:20 +03:00 committed by Linus Torvalds
parent 0ecfebd2b5
commit f3554aeb99
1 changed files with 5 additions and 0 deletions
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5
drivers/block/floppy.c
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@ -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)