Fixes a bug reported by syzbot. A null pointer dereference can
happen when attempting to access the "gsm->receive()" function in
gsmld_receive_buf(). Currently, the "gsm->receive()" function is only set
after a call to the GSMIO_SETCONF ioctl. Since the gsmld_receive_buf()
function can be accessed without the need to call the line discipline
ioctl (GSMIO_SETCONF), the gsm->receive() function will not be set and a
NULL pointer dereference will occur.
Fix this by setting the gsm->receive() function when the line discipline
is being attached to the terminal device, inside gsmld_attach_gsm(). This
will guarantee that the function is assigned and a call to TIOCSTI,
which calls gsmld_receive_buf(), will not reference a null pointer.
Call Trace:
<TASK>
gsmld_receive_buf+0x1c2/0x2f0 drivers/tty/n_gsm.c:2861
tiocsti drivers/tty/tty_io.c:2293 [inline]
tty_ioctl+0xa75/0x15d0 drivers/tty/tty_io.c:2692
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:870 [inline]
__se_sys_ioctl fs/ioctl.c:856 [inline]
__x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
Fixes: 01aecd917114 ("tty: n_gsm: fix tty registration before control channel open")
Reported-and-tested-by: [email protected]
Signed-off-by: Mazin Al Haddad <[email protected]>
---
drivers/tty/n_gsm.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index caa5c14ed57f..1d74dd9d70d0 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2825,6 +2825,11 @@ static void gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
/* Turn off tty XON/XOFF handling to handle it explicitly. */
gsm->old_c_iflag = tty->termios.c_iflag;
tty->termios.c_iflag &= (IXON | IXOFF);
+
+ if (gsm->encoding == 0)
+ gsm->receive = gsm0_receive;
+ else
+ gsm->receive = gsm1_receive;
}
/**
--
2.37.1
On Wed, Aug 10, 2022 at 06:12:52AM +0300, Mazin Al Haddad wrote:
> Fixes a bug reported by syzbot.
I do not understand this sentence :(
> A null pointer dereference can
> happen when attempting to access the "gsm->receive()" function in
> gsmld_receive_buf(). Currently, the "gsm->receive()" function is only set
> after a call to the GSMIO_SETCONF ioctl. Since the gsmld_receive_buf()
> function can be accessed without the need to call the line discipline
> ioctl (GSMIO_SETCONF), the gsm->receive() function will not be set and a
> NULL pointer dereference will occur.
>
> Fix this by setting the gsm->receive() function when the line discipline
> is being attached to the terminal device, inside gsmld_attach_gsm(). This
> will guarantee that the function is assigned and a call to TIOCSTI,
> which calls gsmld_receive_buf(), will not reference a null pointer.
>
> Call Trace:
> <TASK>
> gsmld_receive_buf+0x1c2/0x2f0 drivers/tty/n_gsm.c:2861
> tiocsti drivers/tty/tty_io.c:2293 [inline]
> tty_ioctl+0xa75/0x15d0 drivers/tty/tty_io.c:2692
> vfs_ioctl fs/ioctl.c:51 [inline]
> __do_sys_ioctl fs/ioctl.c:870 [inline]
> __se_sys_ioctl fs/ioctl.c:856 [inline]
> __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
>
> Fixes: 01aecd917114 ("tty: n_gsm: fix tty registration before control channel open")
> Reported-and-tested-by: [email protected]
> Signed-off-by: Mazin Al Haddad <[email protected]>
What commit does this fix?
thanks,
greg k-h
> Fix this by setting the gsm->receive() function when the line
> discipline is being attached to the terminal device, inside
> gsmld_attach_gsm(). This will guarantee that the function is assigned
> and a call to TIOCSTI, which calls gsmld_receive_buf(), will not
> reference a null pointer.
In my opinion there are only two possible ways to fix this:
a) Move the gsm->receive initialization from gsm_activate_mux() to
gsmld_attach_gsm().
b) Avoid calling gsm->receive in gsmld_receive_buf() if not initialized.
The current code might assume that gsm->receive is only called after MUX
activation. Therefore, variant a) may break the code in other places.
I see no need to initialize gsm->receive in gsm_activate_mux() and
gsmld_attach_gsm().
Best regards,
Daniel Starke
On Wed Aug 10, 2022 at 12:08 PM +03, Starke, Daniel wrote:
> > Fix this by setting the gsm->receive() function when the line
> > discipline is being attached to the terminal device, inside
> > gsmld_attach_gsm(). This will guarantee that the function is assigned
> > and a call to TIOCSTI, which calls gsmld_receive_buf(), will not
> > reference a null pointer.
>
> In my opinion there are only two possible ways to fix this:
> a) Move the gsm->receive initialization from gsm_activate_mux() to
> gsmld_attach_gsm().
> b) Avoid calling gsm->receive in gsmld_receive_buf() if not initialized.
>
> The current code might assume that gsm->receive is only called after MUX
> activation. Therefore, variant a) may break the code in other places.
> I see no need to initialize gsm->receive in gsm_activate_mux() and
> gsmld_attach_gsm().
Thanks for the feedback! I will send a v2 patch that implements a check
shortly.