Re: UBSAN: Undefined behaviour in drivers/pps/pps.c

[Rodolfo Giometti <giometti@enneenne.com>]

On 08/01/2019 21:24, Kyungtae Kim wrote:
> We report a bug in linux-4.20: "UBSAN: Undefined behaviour in drivers/pps/pps.c"
> 
> kernel config: https://kt0755.github.io/etc/config_v4.20_stable
> repro: https://kt0755.github.io/etc/repro.a6372.c
> 
> pps_cdev_pps_fetch() lacks the bounds checking for computing
> fdata->timeout.sec * HZ, that causes such integer overflow when the result
> is larger than the boundary.
> The  patch below checks the possibility of overflow right before the
> multiplication.
> 
> =========================================
> UBSAN: Undefined behaviour in drivers/pps/pps.c:82:30
> signed integer overflow:
> -7557201428062104791 * 100 cannot be represented in type 'long long int'
> CPU: 0 PID: 10159 Comm: syz-executor6 Not tainted 4.20.0 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0xb1/0x118 lib/dump_stack.c:113
>   ubsan_epilogue+0x12/0x94 lib/ubsan.c:159
>   handle_overflow+0x1cf/0x21a lib/ubsan.c:190
>   __ubsan_handle_mul_overflow+0x2a/0x35 lib/ubsan.c:214
>   pps_cdev_pps_fetch+0x575/0x5b0 drivers/pps/pps.c:82
>   pps_cdev_ioctl+0x567/0x910 drivers/pps/pps.c:191
>   vfs_ioctl fs/ioctl.c:46 [inline]
>   do_vfs_ioctl+0x1aa/0x1160 fs/ioctl.c:698
>   ksys_ioctl+0x9e/0xb0 fs/ioctl.c:713
>   __do_sys_ioctl fs/ioctl.c:720 [inline]
>   __se_sys_ioctl fs/ioctl.c:718 [inline]
>   __x64_sys_ioctl+0x7e/0xc0 fs/ioctl.c:718
>   do_syscall_64+0xbe/0x4f0 arch/x86/entry/common.c:290
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 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:00007f8cf875bc68 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00007f8cf875c6cc RCX: 00000000004497b9
> RDX: 0000000020000240 RSI: 00000000c00870a4 RDI: 0000000000000014
> RBP: 000000000071bea0 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
> R13: 0000000000005c10 R14: 00000000006eecb0 R15: 00007f8cf875c700
> =========================================
> 
> ---
>   drivers/pps/pps.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> index 8febacb..66002e1 100644
> --- a/drivers/pps/pps.c
> +++ b/drivers/pps/pps.c
> @@ -79,6 +79,8 @@ static int pps_cdev_pps_fetch(struct pps_device
> *pps, struct pps_fdata *fdata)
>                  dev_dbg(pps->dev, "timeout %lld.%09d\n",
>                                  (long long) fdata->timeout.sec,
>                                  fdata->timeout.nsec);
> +               if (fdata->timeout.sec > S64_MAX / HZ)
> +                               return -EINVAL;
>                  ticks = fdata->timeout.sec * HZ;
>                  ticks += fdata->timeout.nsec / (NSEC_PER_SEC / HZ);

It looks good to me. Do you think is better adding a check for timeout.nsec also?

Now you have to produce a patch according to 
linux/Documentation/process/submitting-patches.rst and then submitting it! :-)

Ciao,

Rodolfo

-- 
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