linux-kernel - Re: [PATCH] security: fix integer overflow in lsm_set_self

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Date: Wed, 14 Feb 2024 16:45:42 -0800
From: Kees Cook <keescook@...omium.org>
To: Casey Schaufler <casey@...aufler-ca.com>
Cc: Jann Horn <jannh@...gle.com>, Paul Moore <paul@...l-moore.com>,
	James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	John Johansen <john.johansen@...onical.com>,
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] security: fix integer overflow in lsm_set_self_attr()
 syscall

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...

-Kees

-- 
Kees Cook