[syzbot] [serial?] memory leak in gsm_activate_mux

[syzbot] [serial?] memory leak in gsm_activate_mux

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    0bb80ecc33a8 Linux 6.6-rc1
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10a8aad8680000
kernel config:  https://syzkaller.appspot.com/x/.config?x=99ce3535087fc27
dashboard link: https://syzkaller.appspot.com/bug?extid=b5d1f455d385b2c7da3c
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=163a9e30680000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14fcd4a0680000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ce21ae500663/disk-0bb80ecc.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1d2f34908198/vmlinux-0bb80ecc.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9c6f5eecda20/bzImage-0bb80ecc.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b5d1f455d385b2c7da3c@syzkaller.appspotmail.com

BUG: memory leak
unreferenced object 0xffff888101230c00 (size 1024):
  comm "syz-executor201", pid 5036, jiffies 4294942174 (age 8.190s)
  hex dump (first 32 bytes):
    00 78 0f 11 81 88 ff ff 00 00 00 00 00 00 00 00  .x..............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff81573b75>] kmalloc_trace+0x25/0x90 mm/slab_common.c:1114
    [<ffffffff827ec407>] kmalloc include/linux/slab.h:599 [inline]
    [<ffffffff827ec407>] kzalloc include/linux/slab.h:720 [inline]
    [<ffffffff827ec407>] gsm_dlci_alloc+0x27/0x1f0 drivers/tty/n_gsm.c:2640
    [<ffffffff827ec5ec>] gsm_activate_mux+0x1c/0x1c0 drivers/tty/n_gsm.c:3129
    [<ffffffff827f310f>] gsm_config_ext drivers/tty/n_gsm.c:3434 [inline]
    [<ffffffff827f310f>] gsmld_ioctl+0x6cf/0x9f0 drivers/tty/n_gsm.c:3798
    [<ffffffff827d7bbb>] tty_ioctl+0x3eb/0xc70 drivers/tty/tty_io.c:2785
    [<ffffffff816b25f2>] vfs_ioctl fs/ioctl.c:51 [inline]
    [<ffffffff816b25f2>] __do_sys_ioctl fs/ioctl.c:871 [inline]
    [<ffffffff816b25f2>] __se_sys_ioctl fs/ioctl.c:857 [inline]
    [<ffffffff816b25f2>] __x64_sys_ioctl+0xf2/0x140 fs/ioctl.c:857
    [<ffffffff84b30008>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    [<ffffffff84b30008>] do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
    [<ffffffff84c0008b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

BUG: memory leak
unreferenced object 0xffff888104f38000 (size 4096):
  comm "syz-executor201", pid 5036, jiffies 4294942174 (age 8.190s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff815742ab>] __do_kmalloc_node mm/slab_common.c:1022 [inline]
    [<ffffffff815742ab>] __kmalloc+0x4b/0x150 mm/slab_common.c:1036
    [<ffffffff824f9f89>] kmalloc_array include/linux/slab.h:636 [inline]
    [<ffffffff824f9f89>] __kfifo_alloc+0x89/0xe0 lib/kfifo.c:43
    [<ffffffff827ec451>] gsm_dlci_alloc+0x71/0x1f0 drivers/tty/n_gsm.c:2645
    [<ffffffff827ec5ec>] gsm_activate_mux+0x1c/0x1c0 drivers/tty/n_gsm.c:3129
    [<ffffffff827f310f>] gsm_config_ext drivers/tty/n_gsm.c:3434 [inline]
    [<ffffffff827f310f>] gsmld_ioctl+0x6cf/0x9f0 drivers/tty/n_gsm.c:3798
    [<ffffffff827d7bbb>] tty_ioctl+0x3eb/0xc70 drivers/tty/tty_io.c:2785
    [<ffffffff816b25f2>] vfs_ioctl fs/ioctl.c:51 [inline]
    [<ffffffff816b25f2>] __do_sys_ioctl fs/ioctl.c:871 [inline]
    [<ffffffff816b25f2>] __se_sys_ioctl fs/ioctl.c:857 [inline]
    [<ffffffff816b25f2>] __x64_sys_ioctl+0xf2/0x140 fs/ioctl.c:857
    [<ffffffff84b30008>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    [<ffffffff84b30008>] do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
    [<ffffffff84c0008b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd



---
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[PATCH] tty: n_gsm: fix memory leak in gsm_activate_mux

[Minu Jin]

I found a memory leak in gsm_activate_mux() from the syzbot dashboard.
I have analyzed the root cause and created a fix.
Please test the following patch.

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git tty-next

---
diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index c13e050de83b..de3d30eac86e 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2645,7 +2645,12 @@ static int gsm_dlci_config(struct gsm_dlci *dlci, struct gsm_dlci_config *dc, in

 static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr)
 {
-       struct gsm_dlci *dlci = kzalloc_obj(struct gsm_dlci, GFP_ATOMIC);
+       struct gsm_dlci *dlci;
+
+       if (gsm->dlci[addr])
+               return gsm->dlci[addr];
+
+       dlci = kzalloc_obj(struct gsm_dlci, GFP_ATOMIC);
        if (dlci == NULL)
                return NULL;
        spin_lock_init(&dlci->lock);
@@ -3196,8 +3201,10 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
                gsm->receive = gsm1_receive;

        ret = gsm_register_devices(gsm_tty_driver, gsm->num);
-       if (ret)
+       if (ret) {
+               gsm_dlci_free(&dlci->port);
                return ret;
+       }

        gsm->has_devices = true;
        gsm->dead = false;              /* Tty opens are now permissible */

Re: [syzbot] [serial?] memory leak in gsm_activate_mux

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+b5d1f455d385b2c7da3c@syzkaller.appspotmail.com
Tested-by: syzbot+b5d1f455d385b2c7da3c@syzkaller.appspotmail.com

Tested on:

commit:         6596a02b Merge tag 'drm-next-2026-04-22' of https://gi..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git tty-next
console output: https://syzkaller.appspot.com/x/log.txt?x=17b574ce580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=943968b43250a7da
dashboard link: https://syzkaller.appspot.com/bug?extid=b5d1f455d385b2c7da3c
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1117ac36580000

Note: testing is done by a robot and is best-effort only.