BUG: unable to handle kernel NULL pointer dereference in write_port

BUG: unable to handle kernel NULL pointer dereference in write_port

[Kyungtae Kim]

We report a bug in v4.19-rc8 (4.20-rc1 as well):

kernel config: https://kt0755.github.io/etc/config-4.19-rc2.kmsan
repro: https://kt0755.github.io/etc/repro.e3752.c

This happens during data transition from user-supplied buffer to port
(using outb) pointed by ppos. (driver/mem/char.c:640)
Although there is a strict bound 65536 (driver/mem/char.c:632), user
buffer copy still causes crashes within the strict bound.
(In the experiment, the crash arose when loop count is beyond 0x7f )
To stop it, it probably needs a little tight bound check.

I think this is a little bit related to the crash I reported before
(https://lkml.org/lkml/2018/5/12/91)

Crash log
=========================================
BUG: unable to handle kernel NULL pointer dereference at 00000000000000af
PGD 8000000092c1a067 P4D 8000000092c1a067 PUD 93a74067 PMD 0
Oops: 0002 [#1] SMP PTI
CPU: 0 PID: 6651 Comm: syz-executor0 Not tainted 4.19.0-rc8+ #18
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
RIP: 0010:outb arch/x86/include/asm/io.h:333 [inline]
RIP: 0010:write_port+0x25b/0x420 drivers/char/mem.c:640
Code: 4c 8b 75 c8 4c 89 e7 e8 93 9f 5c fe 85 db 75 7d 48 8b 18 44 8b
22 e8 a4 de f8 fd 84 db 75 5a 48 8b 5d b0 85 db 75 5c 44 89 f0 <44> 89
66 90 49 83 c7 01 4d 89 ee 49 83 c6 01 31 c0 31 ff 45 31 e4
RSP: 0018:ffff88009312fbf0 EFLAGS: 00010246
RAX: 00000000200001e4 RBX: 0000000000000000 RCX: 0000000000010000
RDX: ffffc90003f89000 RSI: 000000000000011f RDI: 0000000000000120
RBP: ffff88009312fc98 R08: ffffffff7fffffff R09: ffff88009277fc60
R10: 0000000000000000 R11: ffffffff83772a30 R12: 0000000000000000
R13: 000000002000017f R14: 00000000200001e4 R15: 000000000000007f
FS:  00007f5c84a4d700(0000) GS:ffff88013fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000714fe4 CR3: 000000009498a000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __vfs_write+0x1f4/0xa50 fs/read_write.c:485
 vfs_write+0x495/0x8e0 fs/read_write.c:549
 ksys_write fs/read_write.c:598 [inline]
 __do_sys_write fs/read_write.c:610 [inline]
 __se_sys_write+0x17a/0x370 fs/read_write.c:607
 __x64_sys_write+0x4a/0x70 fs/read_write.c:607
 do_syscall_64+0xb8/0x100 arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
RIP: 0033:0x4497b9
Code: e8 8c 9f 02 00 48 83 c4 18 c3 0f 1f 80 00 00 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 0f 83 9b 6b fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f5c84a4cc68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f5c84a4d6cc RCX: 00000000004497b9
RDX: 00000000000000cc RSI: 0000000020000100 RDI: 0000000000000014
RBP: 000000000071bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 000000000000b250 R14: 00000000006f32f0 R15: 00007f5c84a4d700
Modules linked in:
Dumping ftrace buffer:
   (ftrace buffer empty)
CR2: 00000000000000af
---[ end trace 5b28bfcf889ce399 ]---
=========================================

Crash log (reported before)
=========================================
BUG: unable to handle kernel paging request at ffffc90001eb5f90
PGD 1150c2067 P4D 1150c2067 PUD 1150c3067 PMD 10cd1e067 PTE 0
Oops: 0002 [#1] SMP KASAN PTI
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 2925 Comm: syz-executor7 Not tainted 4.17.0-rc3 #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
RIP: 0010:outb arch/x86/include/asm/io.h:333 [inline]
RIP: 0010:write_port+0xda/0x190 drivers/char/mem.c:640
RSP: 0018:ffff88011095fc40 EFLAGS: 00010286
RAX: 000000001095feaf RBX: 000000002000163f RCX: ffffffff8286ca77
RDX: 0000000000010000 RSI: ffffc90001eb6000 RDI: ffff880114403f58
RBP: ffff88011095fc78 R08: ffffffffa9c6c9a0 R09: 0000000000000000
R10: ffff880115622813 R11: ffffed0022ac4503 R12: 000000000000007f
R13: 000000000000000c R14: 0000000000000000 R15: ffff88011095feaf
FS:  00007f40dbe46700(0000) GS:ffff880115600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffc90001eb5f90 CR3: 00000001117ca000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __vfs_write+0x10d/0x610 fs/read_write.c:485
 vfs_write+0x187/0x500 fs/read_write.c:549
 ksys_write+0xd4/0x1a0 fs/read_write.c:598
 __do_sys_write fs/read_write.c:610 [inline]
 __se_sys_write fs/read_write.c:607 [inline]
 __x64_sys_write+0x73/0xb0 fs/read_write.c:607
 do_syscall_64+0xa4/0x460 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4497b9
RSP: 002b:00007f40dbe45c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f40dbe466cc RCX: 00000000004497b9
RDX: 000000000000008c RSI: 00000000200015c0 RDI: 0000000000000013
RBP: 000000000071bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000009ee8 R14: 00000000006f0f88 R15: 00007f40dbe46700
Code: 49 81 fc ff ff 00 00 4c 8d 6b ff 0f 87 af 00 00 00 48 85 db 0f
84 a6 00 00 00 48 8b 5d d0 45 31 f6 eb 2c e8 59 00 b1 fe 44 89 f8 <44>
89 66 90 49 83 c4 01 48 83 c3 01 4d 85 ed 0f 95 c2 49 81 fc
RIP: outb arch/x86/include/asm/io.h:333 [inline] RSP: ffff88011095fc40
RIP: write_port+0xda/0x190 drivers/char/mem.c:640 RSP: ffff88011095fc40
CR2: ffffc90001eb5f90
---[ end trace 6917feb3b143574b ]---
=========================================

Thanks,
Kyungtae Kim

Re: BUG: unable to handle kernel NULL pointer dereference in write_port

[Arnd Bergmann]

On Tue, Nov 13, 2018 at 9:24 AM Kyungtae Kim <kt0755@gmail.com> wrote:
>
> We report a bug in v4.19-rc8 (4.20-rc1 as well):
>
> kernel config: https://kt0755.github.io/etc/config-4.19-rc2.kmsan
> repro: https://kt0755.github.io/etc/repro.e3752.c
>
> This happens during data transition from user-supplied buffer to port
> (using outb) pointed by ppos. (driver/mem/char.c:640)
> Although there is a strict bound 65536 (driver/mem/char.c:632), user
> buffer copy still causes crashes within the strict bound.
> (In the experiment, the crash arose when loop count is beyond 0x7f )
> To stop it, it probably needs a little tight bound check.
>
> I think this is a little bit related to the crash I reported before
> (https://lkml.org/lkml/2018/5/12/91)
>
> Crash log
> =========================================
> BUG: unable to handle kernel NULL pointer dereference at 00000000000000af

The first thing that comes to mind is that this would be qemu specific.
Note that writing arbitrary data into arbitrary I/O ports is likely to crash
any x86 PC, but it's possible that qemu reports a different set of
exceptions.

Looking in /proc/ioports for a real PC, I find

    0080-008f : dma page reg
    00a0-00a1 : pic2
    00c0-00df : dma2
    00f0-00ff : fpu

so it appears that you have just written into the interrupt
controller here.

       Arnd

Re: BUG: unable to handle kernel NULL pointer dereference in write_port

[Kyungtae Kim]

Thank you for your reply.
But I think this kind of crash can occur in real PC as well, and I'm
just thinking of some way to stop it in the first place (if possible).
because malicious users can use this, so as to make the whole system
(kernel) work incorrectly.

Thanks,
Kyungtae

Re: BUG: unable to handle kernel NULL pointer dereference in write_port

[Dmitry Vyukov]

On Wed, Nov 21, 2018 at 11:11 AM, Kyungtae Kim <kt0755@gmail.com> wrote:
> Thank you for your reply.
> But I think this kind of crash can occur in real PC as well, and I'm
> just thinking of some way to stop it in the first place (if possible).
> because malicious users can use this, so as to make the whole system
> (kernel) work incorrectly.

If I am not mistaken this program writes to /dev/port. /dev/port is
strictly privileged.

p.s. if you are extending syzkaller to do new things, it can make
sense to send pull requests upstream. Then we can review the changes
and figure out what should be tested, and what does not make sense to
fuzz. E.g. also don't give /dev/mem to syzkaller ;)

Re: BUG: unable to handle kernel NULL pointer dereference in write_port

[Kyungtae Kim]

Ah, I got that. Thanks.

Kyungtae