Messages in this thread | | | Date | Wed, 18 Mar 2020 11:06:12 +0100 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console |
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On Mon, Mar 02, 2020 at 01:23:06PM -0800, Eric Biggers wrote: > On Mon, Feb 24, 2020 at 12:19:13AM -0800, Eric Biggers wrote: > > On Mon, Feb 24, 2020 at 09:04:33AM +0100, Jiri Slaby wrote: > > > > KASAN report: > > > > BUG: KASAN: use-after-free in con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278 > > > > Write of size 8 at addr ffff88806a4ec108 by task syz_vt/129 > > > > > > > > CPU: 0 PID: 129 Comm: syz_vt Not tainted 5.6.0-rc2 #11 > > > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014 > > > > Call Trace: > > > > [...] > > > > con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278 > > > > release_tty+0xa8/0x410 drivers/tty/tty_io.c:1514 > > > > tty_release_struct+0x34/0x50 drivers/tty/tty_io.c:1629 > > > > tty_release+0x984/0xed0 drivers/tty/tty_io.c:1789 > > > > [...] > > > > > > > > Allocated by task 129: > > > > [...] > > > > kzalloc include/linux/slab.h:669 [inline] > > > > vc_allocate drivers/tty/vt/vt.c:1085 [inline] > > > > vc_allocate+0x1ac/0x680 drivers/tty/vt/vt.c:1066 > > > > con_install+0x4d/0x3f0 drivers/tty/vt/vt.c:3229 > > > > tty_driver_install_tty drivers/tty/tty_io.c:1228 [inline] > > > > tty_init_dev+0x94/0x350 drivers/tty/tty_io.c:1341 > > > > tty_open_by_driver drivers/tty/tty_io.c:1987 [inline] > > > > tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035 > > > > [...] > > > > > > > > Freed by task 130: > > > > [...] > > > > kfree+0xbf/0x1e0 mm/slab.c:3757 > > > > vt_disallocate drivers/tty/vt/vt_ioctl.c:300 [inline] > > > > vt_ioctl+0x16dc/0x1e30 drivers/tty/vt/vt_ioctl.c:818 > > > > tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660 > > > > > > That means the associated tty_port is destroyed while the tty layer > > > still has a tty on the top of it. That is a BUG anyway. > > > > > > > Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle") > > > > Cc: <stable@vger.kernel.org> # v3.4+ > > > > Reported-by: syzbot+522643ab5729b0421998@syzkaller.appspotmail.com > > > > Signed-off-by: Eric Biggers <ebiggers@google.com> > > > > --- > > > > drivers/tty/vt/vt_ioctl.c | 6 +++++- > > > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c > > > > index ee6c91ef1f6cf..57d681706fa85 100644 > > > > --- a/drivers/tty/vt/vt_ioctl.c > > > > +++ b/drivers/tty/vt/vt_ioctl.c > > > > @@ -42,7 +42,7 @@ > > > > char vt_dont_switch; > > > > extern struct tty_driver *console_driver; > > > > > > > > -#define VT_IS_IN_USE(i) (console_driver->ttys[i] && console_driver->ttys[i]->count) > > > > +#define VT_IS_IN_USE(i) (console_driver->ttys[i] != NULL) > > > > #define VT_BUSY(i) (VT_IS_IN_USE(i) || i == fg_console || vc_cons[i].d == sel_cons) > > > > > > > > /* > > > > @@ -288,12 +288,14 @@ static int vt_disallocate(unsigned int vc_num) > > > > struct vc_data *vc = NULL; > > > > int ret = 0; > > > > > > > > + mutex_lock(&tty_mutex); /* synchronize with release_tty() */ > > > > console_lock(); > > > > > > Is this lock dependency new or pre-existing? > > > > It's the same locking order used during release_tty(). > > > > > > > > Locking tty_mutex here does not sound quite right. What about switching > > > vc_data to proper refcounting based on tty_port? (Instead of doing > > > tty_port_destroy and kfree in vt_disallocate*.) > > > > > > > How would that work? We could make struct vc_data refcounted such that > > VT_DISALLOCATE doesn't free it right away but rather it's freed in the next > > con_shutdown(). But release_tty() still accesses tty->port afterwards, which is > > part of the 'struct vc_data' that would have just been freed. > > > > Jiri, can you explain what you meant here? I don't see how your suggestion > would solve the problem. > > Greg, any opinion?
I'll defer to Jiri here :)
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