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Fri, 12 Jun 2026 07:19:23 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-serial@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Fri, 12 Jun 2026 22:19:18 +0800 Message-Id: Cc: "Jiri Slaby" , "Daniel Starke" , , , "Xiang Mei" Subject: Re: [PATCH] tty: n_gsm: fix NULL deref of gsm->dlci[0] in control message handlers From: "Weiming Shi" To: "Greg Kroah-Hartman" , "Weiming Shi" X-Mailer: aerc 0.21.0 References: <20260611183217.2488508-2-bestswngs@gmail.com> <2026061101-hanky-uninstall-da53@gregkh> In-Reply-To: <2026061101-hanky-uninstall-da53@gregkh> On Fri Jun 12, 2026 at 2:50 AM CST, Greg Kroah-Hartman wrote: > On Thu, Jun 11, 2026 at 11:32:18AM -0700, Weiming Shi wrote: >> gsm_control_command() and gsm_control_reply() load gsm->dlci[0] and >> immediately dereference dlci->ftype without checking it for NULL. >>=20 >> On the receive path, gsm_queue() validates that gsm->dlci[0] is non-NULL >> and DLCI_OPEN before invoking the control handler, but the value is not >> held across that check: the receive worker runs from flush_to_ldisc() >> without taking gsm->mutex, while a concurrent GSMIOC_SETCONF ioctl can >> enter gsm_cleanup_mux(), which takes gsm->mutex, releases gsm->dlci[0] >> and sets it to NULL. If the mux is torn down between gsm_queue()'s check >> and the re-load inside gsm_control_command()/gsm_control_reply(), the >> handler dereferences a NULL dlci. >>=20 >> A peer that drives DLCI 0 control frames (e.g. CMD_TEST) while the mux >> owner reconfigures the line discipline can therefore crash the kernel >> (line numbers from decode_stacktrace.sh against the crashing build): >>=20 >> Oops: general protection fault, probably for non-canonical address >> KASAN: null-ptr-deref in range [0x0000000000000208-0x000000000000020f] >> RIP: 0010:gsm_control_reply (drivers/tty/n_gsm.c:1497) >> Call Trace: >> gsm_dlci_command (drivers/tty/n_gsm.c:2482) >> gsm_queue.part.0 (drivers/tty/n_gsm.c:2852) >> gsm0_receive (drivers/tty/n_gsm.c:2972) >> gsmld_receive_buf (drivers/tty/n_gsm.c:3629) >> tty_ldisc_receive_buf (drivers/tty/tty_buffer.c:391) >> tty_port_default_receive_buf (drivers/tty/tty_port.c:39) >> flush_to_ldisc (drivers/tty/tty_buffer.c:495) >> process_one_work >> worker_thread >> kthread >>=20 >> The other callers of these helpers (the keep-alive and negotiation timer >> paths) already guard the gsm->dlci[0] access; only the receive path is >> unguarded. The CMD_CLD handler in the same switch already checks the >> loaded dlci for NULL for the very same reason. Bail out early when >> gsm->dlci[0] has been cleared instead of dereferencing it. >>=20 >> Triggering this requires CAP_NET_ADMIN to attach the n_gsm line >> discipline (gsmld_open() uses capable(), not ns_capable()), so it is a >> local denial of service for a privileged mux owner racing its own >> control channel; harden the handlers regardless. >>=20 >> Fixes: 5767712668b8 ("tty: n_gsm: cleanup gsm_control_command and gsm_co= ntrol_reply") >> Reported-by: Xiang Mei >> Assisted-by: Claude:claude-opus-4-8 >> Signed-off-by: Weiming Shi >> --- >> drivers/tty/n_gsm.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >>=20 >> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c >> index 214abeb89aaa..860cfb91d510 100644 >> --- a/drivers/tty/n_gsm.c >> +++ b/drivers/tty/n_gsm.c >> @@ -1457,6 +1457,9 @@ static int gsm_control_command(struct gsm_mux *gsm= , int cmd, const u8 *data, >> struct gsm_msg *msg; >> struct gsm_dlci *dlci =3D gsm->dlci[0]; >> =20 >> + if (!dlci) >> + return -EINVAL; > > What precents dlci from being NULL right after you check this? > > thanks, > > greg k-h Hi greg, I'm sorry for taking so long to respond. After a closer look I think your review is correct. The real problem is that the receive path touches gsm->dlci[] with no lock. The teardown side holds gsm->mutex while it releases and frees the dlci, but the receive worker does not: gsm_queue() loads dlci =3D gsm->dlci[address] while it is still valid and passes it down through dlci->data() to gsm_control_command()/gsm_control_reply(), which also re-read gsm->dlci[0] and dereference dlci->ftype. Meanwhile GSMIOC_SETCONF -> gsm_cleanup_mux() takes gsm->mutex, closes DLCI0 and drops its reference via gsm_dlci_release(); the final tty_port_put() runs the gsm_dlci_free() destructor, which clears the slot and frees the object: ``` dlci->gsm->dlci[dlci->addr] =3D NULL; kfree(dlci); ``` If that happens while the worker is still in the dispatch above, it ends up dereferencing the freed dlci. I can reproduce this as a use-after-free: ``` [ 997.227486][ T46] BUG: KASAN: slab-use-after-free in gsm_control_reply= .isra.0 (drivers/tty/n_gsm.c:1162 drivers/tty/n_gsm.c:1494) [ 997.229052][ T46] Read of size 8 at addr ffff888029ae9000 by task kwor= ker/u16:2/46 [ 997.230517][ T46] [ 997.230952][ T46] CPU: 1 UID: 0 PID: 46 Comm: kworker/u16:2 Not tainte= d 7.1.0-rc7 #1 PREEMPT(full) [ 997.230958][ T46] Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PII= X, arch_caps fix, 1996), BIOS 1.16.3-debian-4 [ 997.230961][ T46] Workqueue: events_unbound flush_to_ldisc [ 997.230969][ T46] Call Trace: [ 997.230972][ T46] [ 997.230974][ T46] dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.= c:120) [ 997.230990][ T46] print_report (mm/kasan/report.c:378 mm/kasan/report= .c:482) [ 997.231008][ T46] kasan_report (mm/kasan/report.c:595) [ 997.231016][ T46] gsm_control_reply.isra.0 (drivers/tty/n_gsm.c:1162 = drivers/tty/n_gsm.c:1494) [ 997.231020][ T46] gsm_dlci_command (drivers/tty/n_gsm.c:1873 drivers/= tty/n_gsm.c:2477) [ 997.231036][ T46] gsmld_receive_buf (drivers/tty/n_gsm.c:3616) [ 997.231044][ T46] tty_ldisc_receive_buf (drivers/tty/tty_buffer.c:398= ) [ 997.231052][ T46] tty_port_default_receive_buf (drivers/tty/tty_port.= c:37) [ 997.231056][ T46] flush_to_ldisc (drivers/tty/tty_buffer.c:452 driver= s/tty/tty_buffer.c:502) [ 997.231066][ T46] process_one_work (kernel/workqueue.c:3314) [ 997.231082][ T46] worker_thread (kernel/workqueue.c:3397 kernel/workq= ueue.c:3478) [ 997.231091][ T46] kthread (kernel/kthread.c:436) [ 997.231103][ T46] ret_from_fork (arch/x86/kernel/process.c:158) [ 997.231120][ T46] ret_from_fork_asm (arch/x86/entry/entry_64.S:245) [ 997.231128][ T46] [ 997.231130][ T46] [ 997.267905][ T46] Allocated by task 5110: [ 997.268716][ T46] kasan_save_stack (mm/kasan/common.c:57) [ 997.269595][ T46] kasan_save_track (mm/kasan/common.c:78) [ 997.270483][ T46] __kasan_kmalloc (mm/kasan/common.c:398 mm/kasan/com= mon.c:415) [ 997.271353][ T46] gsm_dlci_alloc (./include/linux/slab.h:950 ./includ= e/linux/slab.h:1188 drivers/tty/n_gsm.c:2648) [ 997.272203][ T46] gsm_activate_mux (drivers/tty/n_gsm.c:3189) [ 997.273109][ T46] gsmld_ioctl (drivers/tty/n_gsm.c:3443 drivers/tty/n= _gsm.c:3846) [ 997.273981][ T46] tty_ioctl (drivers/tty/tty_io.c:2801) [ 997.274789][ T46] __x64_sys_ioctl (fs/ioctl.c:51 fs/ioctl.c:597 fs/io= ctl.c:583 fs/ioctl.c:583) [ 997.275682][ T46] do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/= x86/entry/syscall_64.c:94) [ 997.276544][ T46] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entr= y_64.S:121) [ 997.277658][ T46] [ 997.278108][ T46] Freed by task 5110: [ 997.278865][ T46] kasan_save_stack (mm/kasan/common.c:57) [ 997.279740][ T46] kasan_save_track (mm/kasan/common.c:78) [ 997.280615][ T46] kasan_save_free_info (mm/kasan/generic.c:584) [ 997.281554][ T46] __kasan_slab_free (mm/kasan/common.c:253 mm/kasan/c= ommon.c:285) [ 997.282435][ T46] kfree (./include/linux/kasan.h:235 mm/slub.c:2689 m= m/slub.c:6251 mm/slub.c:6566) [ 997.283159][ T46] gsm_cleanup_mux (drivers/tty/n_gsm.c:2711 drivers/t= ty/n_gsm.c:2744 drivers/tty/n_gsm.c:3161) [ 997.284050][ T46] gsmld_ioctl (drivers/tty/n_gsm.c:3415 drivers/tty/n= _gsm.c:3846) [ 997.284928][ T46] tty_ioctl (drivers/tty/tty_io.c:2801) [ 997.285746][ T46] __x64_sys_ioctl (fs/ioctl.c:51 fs/ioctl.c:597 fs/io= ctl.c:583 fs/ioctl.c:583) [ 997.286653][ T46] do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/= x86/entry/syscall_64.c:94) [ 997.287526][ T46] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entr= y_64.S:121) [ 997.288639][ T46] ``` The NULL deref I reported is the same unguarded access on the same path, just hitting the window after the slot is already cleared. Either way a NULL check in the handlers can't fix it, since in the UAF case dlci isn't NULL. I think the fix should serialize the receive side against gsm_cleanup_mux() instead of checking in the handlers. Two ways I can see: 1. take gsm->mutex around the dlci lookup and dispatch in gsm_queue(), or 2. pin the dlci across the dispatch using its existing tty_port ref (dlci_get/dlci_put), so gsm_dlci_free() can't run while it's in use. Do you have a preference, or is there a pattern in n_gsm you'd rather I use? I'll respin v2 once I know which way to go. And I'll send the reproducer and the config to trigger it in a separate mai= l. Thanks, Weiming