Re: [PATCH] tty: n_gsm: fix NULL pointer dereference in gsmld_receive_buf()

[Jiri Slaby]

CC Daniel.

It looks like what he proposed at:
https://lore.kernel.org/all/DB9PR10MB58813E33D3516BAB526156B1E0659@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

On 11. 08. 22, 6:28, ChenXiaoSong wrote:
> Syzkaller reported NULL pointer dereference bug as follows:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> PGD 0 P4D 0
> Oops: 0010 [#1] PREEMPT SMP KASAN PTI
> CPU: 3 PID: 1360 Comm: a.out Not tainted 5.19.0-next-20220810 #9
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>   BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> RIP: 0010:0x0
> Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
> RSP: 0018:ffffc9000570fcf8 EFLAGS: 00010202
> RAX: 0000000000000001 RBX: ffff8880189c2000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8880189c2000
> RBP: ffffc9000570fda1 R08: 0000000000000000 R09: ffffffff8f59ab27
> R10: ffffc9000570fd58 R11: ffff8880196f8000 R12: 0000000000000001
> R13: 0000000000000000 R14: ffffc9000570fd90 R15: dffffc0000000000
> FS:  00007f382232c540(0000) GS:ffff888107d80000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffffffffffffffd6 CR3: 0000000101452005 CR4: 0000000000770ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> PKRU: 55555554
> Call Trace:
>   <TASK>
>   gsmld_receive_buf+0x1c7/0x320
>   tty_ioctl+0x50e/0x1520
>   __x64_sys_ioctl+0x19c/0x210
>   do_syscall_64+0x38/0x90
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f382225e9b9
> Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7
>   48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
>   ff 73 01 c3 48 8b 0d a7 54 0c 00 f7 d8 64 89 01 48
> RSP: 002b:00007ffd1c0fdd98 EFLAGS: 00000217 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f382225e9b9
> RDX: 00007ffd1c0fdda4 RSI: 0000000000005412 RDI: 0000000000000004
> RBP: 00007ffd1c0fddb0 R08: 000000000000000e R09: 000000001c0fdea0
> R10: 0000000000000000 R11: 0000000000000217 R12: 0000560d89007070
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>   </TASK>
> Modules linked in:
> CR2: 0000000000000000
> ---[ end trace 0000000000000000 ]---
>
> Reproducer(c program):
> 1. int fd = open("/dev/char/4:20", O_RDWR, 0);
> 2. int arg = 0x15; ioctl(fd, 0x5423, &arg);
> 3. int arg2 = 0; ioctl(fd, 0x5412, &arg2);
>
> gsm->receive is NULL when gsm_mux is not activated by gsm_activate_mux():
>
>    ioctl(fd, 0x5423, &arg)
>      vfs_ioctl
>        tty_ioctl
>          tiocsetd
>            tty_set_ldisc
>              tty_ldisc_open
>                gsmld_open
>                  gsm_alloc_mux
>                    gsm = kzalloc(sizeof(struct gsm_mux), GFP_KERNEL)
>                    gsm->receive == NULL
>    ioctl(fd, 0x5412, &arg2)
>      vfs_ioctl
>        tty_ioctl
>          tiocsti // fake input character
>            gsmld_receive_buf
>              gsm->receive == NULL
>
> Fix this by adding sanity check on struct gsm_mux->receive.
>
> Signed-off-by: ChenXiaoSong <chenxiaosong2@xxxxxxxxxx>
> ---
>   drivers/tty/n_gsm.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index caa5c14ed57f..f1621667b69f 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -2850,6 +2850,9 @@ static void gsmld_receive_buf(struct tty_struct *tty, const unsigned char *cp,
>   	struct gsm_mux *gsm = tty->disc_data;
>   	char flags = TTY_NORMAL;
>   
> +	if (!gsm->receive)
> +		return;
> +
>   	if (debug & 4)
>   		gsm_hex_dump_bytes(__func__, cp, count);
>   

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