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From: Paul Moore <paul@paul-moore.com>
To: Kees Cook <keescook@chromium.org>, Casey Schaufler <casey@schaufler-ca.com>
CC: Jann Horn <jannh@google.com>, James Morris <jmorris@namei.org>, "Serge E. Hallyn" <serge@hallyn.com>, John Johansen <john.johansen@canonical.com>, <linux-security-module@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Date: Thu, 15 Feb 2024 08:02:18 -0500
Message-ID: <18dacdbced8.28a4.85c95baa4474aabc7814e68940a78392@paul-moore.com>
In-Reply-To: <202402141644.808307E71@keescook>
References: <20240214160538.1086089-1-jannh@google.com>
 <a4a77e0e-8d5e-4c9d-aff4-9fe0d8b89cf0@schaufler-ca.com>
 <202402141644.808307E71@keescook>
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Subject: Re: [PATCH] security: fix integer overflow in lsm_set_self_attr() syscall
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On February 14, 2024 7:45:43 PM Kees Cook <keescook@chromium.org> wrote:
> On Wed, Feb 14, 2024 at 08:53:52AM -0800, Casey Schaufler wrote:
>> On 2/14/2024 8:05 AM, Jann Horn wrote:
>>> security_setselfattr() has an integer overflow bug that leads to
>>> out-of-bounds access when userspace provides bogus input:
>>> `lctx->ctx_len + sizeof(*lctx)` is checked against `lctx->len` (and,
>>> redundantly, also against `size`), but there are no checks on
>>> `lctx->ctx_len`.
>>> Therefore, userspace can provide an `lsm_ctx` with `->ctx_len` set to a
>>> value between `-sizeof(struct lsm_ctx)` and -1, and this bogus `->ctx_len`
>>> will then be passed to an LSM module as a buffer length, causing LSM
>>> modules to perform out-of-bounds accesses.
>>>
>>> The following reproducer will demonstrate this under ASAN (if AppArmor is
>>> loaded as an LSM):
>>> ```
>>> #define _GNU_SOURCE
>>> #include <unistd.h>
>>> #include <stdint.h>
>>> #include <stdlib.h>
>>> #include <sys/syscall.h>
>>>
>>> struct lsm_ctx {
>>> uint64_t id;
>>> uint64_t flags;
>>> uint64_t len;
>>> uint64_t ctx_len;
>
> Do we want to take the opportunity to reduce this to u32 for len and u32
> for ctx_len? All FS operations are limited to INT_MAX anyway...

Not at this point, no.

--
paul-moore.com