oss-security - Re: Linux Kernel: out of bounds array access in dm-ioctl.c

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Date: Mon, 19 Apr 2021 15:19:58 +0800
From: - Nop <nopitydays@...il.com>
To: oss-security@...ts.openwall.com, John Haxby <john.haxby@...cle.com>
Subject: Re: Linux Kernel: out of bounds array access in dm-ioctl.c

Hi,

sorry for the late reply.

> DM_LIST_DEVICES_CMD, and in fact, any function called from ctl_ioctl is limited to users with CAP_SYS_ADMIN.  Without that root-equivalent privilege I don't see any way to exploit this bug. Did you find a way to exploit it as an unprivileged user?

Yes, this IOCTL does need CAP_SYS_ADMIN capability which is very close
to the real root user.

The only possible exploitable scenario that I can imagine is, a user
with CAP_SYS_ADMIN cap in a container attacks the shared kernel to
break through the seccomp limitation.

However, it is quite rare.


Thanks,

Bodong


On Sun, Mar 28, 2021 at 11:47 AM - Nop <nopitydays@...il.com> wrote:

> Hi,
>
> We found an out of bounds array accessing bug in drivers/md/dm-ioctl.c,
> and reproduced it in the latest kernel (v5.11.10).
>
> The root cause of this BUG is :
>
> The field "data_size" in function ctl_ioctl is fully controlled by users
> and this argument controls the size of kvmalloc in function copy_params.
>
> When the data_size is in a range of [0x131,0x138], the allocated memory
> which is pointed by the variable "param" used in ioctl
> "DM_LIST_DEVICES_CMD" is too small, causing an oob bug at line "nl->dev =
> 0; /* Flags no data */" (
> https://github.com/torvalds/linux/blob/0d02ec6b3136c73c09e7859f0d0e4e2c4c07b49b/drivers/md/dm-ioctl.c#L538
> )
>
> Attachments are the poc, kernel config and Kernel report.
>
> The patch:
>
> https://github.com/torvalds/linux/commit/4edbe1d7bcffcd6269f3b5eb63f710393ff2ec7a
>      * Grab our output buffer.
>      */
>      nl = orig_nl = get_result_buffer(param, param_size, &len);
> -    if (len < needed) {
> +    if (len < needed || len < sizeof(nl->dev)) {
>          param->flags |= DM_BUFFER_FULL_FLAG;
>          goto out;
>      }
>
> Regards,
> Bodong Zhao of NISL lab, Tsinghua University
>
>

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