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Date: Tue, 12 Mar 2024 14:50:07 -0700
From: Kees Cook <keescook@chromium.org>
To: "Dmitry V. Levin" <ldv@strace.io>
Cc: Casey Schaufler <casey@schaufler-ca.com>,
	Paul Moore <paul@paul-moore.com>,
	linux-security-module@vger.kernel.org, jmorris@namei.org,
	serge@hallyn.com, john.johansen@canonical.com,
	penguin-kernel@i-love.sakura.ne.jp, stephen.smalley.work@gmail.com,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	mic@digikod.net
Subject: Re: [PATCH v15 05/11] LSM: Create lsm_list_modules system call
Message-ID: <202403121449.17AB66665@keescook>
References: <20230912205658.3432-1-casey@schaufler-ca.com>
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 <20240312101630.GA903@altlinux.org>
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On Tue, Mar 12, 2024 at 08:28:20PM +0200, Dmitry V. Levin wrote:
> On Tue, Mar 12, 2024 at 10:44:38AM -0700, Casey Schaufler wrote:
> > On 3/12/2024 10:06 AM, Paul Moore wrote:
> > > On Tue, Mar 12, 2024 at 11:27 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
> > >> On 3/12/2024 6:25 AM, Paul Moore wrote:
> > >>> On Tue, Mar 12, 2024 at 6:16 AM Dmitry V. Levin <ldv@strace.io> wrote:
> > >>>> On Tue, Sep 12, 2023 at 01:56:50PM -0700, Casey Schaufler wrote:
> > >>>> [...]
> > >>>>> --- a/security/lsm_syscalls.c
> > >>>>> +++ b/security/lsm_syscalls.c
> > >>>>> @@ -55,3 +55,42 @@ SYSCALL_DEFINE4(lsm_get_self_attr, unsigned int, attr, struct lsm_ctx __user *,
> > >>>>>  {
> > >>>>>       return security_getselfattr(attr, ctx, size, flags);
> > >>>>>  }
> > >>>>> +
> > >>>>> +/**
> > >>>>> + * sys_lsm_list_modules - Return a list of the active security modules
> > >>>>> + * @ids: the LSM module ids
> > >>>>> + * @size: pointer to size of @ids, updated on return
> > >>>>> + * @flags: reserved for future use, must be zero
> > >>>>> + *
> > >>>>> + * Returns a list of the active LSM ids. On success this function
> > >>>>> + * returns the number of @ids array elements. This value may be zero
> > >>>>> + * if there are no LSMs active. If @size is insufficient to contain
> > >>>>> + * the return data -E2BIG is returned and @size is set to the minimum
> > >>>>> + * required size. In all other cases a negative value indicating the
> > >>>>> + * error is returned.
> > >>>>> + */
> > >>>>> +SYSCALL_DEFINE3(lsm_list_modules, u64 __user *, ids, size_t __user *, size,
> > >>>>> +             u32, flags)
> > >>>> I'm sorry but the size of userspace size_t is different from the kernel one
> > >>>> on 32-bit compat architectures.
> > >>> D'oh, yes, thanks for pointing that out.  It would have been nice to
> > >>> have caught that before v6.8 was released, but I guess it's better
> > >>> than later.
> > >>>
> > >>>> Looks like there has to be a COMPAT_SYSCALL_DEFINE3(lsm_list_modules, ..)
> > >>>> now.  Other two added lsm syscalls also have this issue.
> > >>> Considering that Linux v6.8, and by extension these syscalls, are only
> > >>> a few days old, I think I'd rather see us just modify the syscalls and
> > >>> avoid the compat baggage.  I'm going to be shocked if anyone has
> > >>> shifted to using the new syscalls yet, and even if they have (!!),
> > >>> moving from a "size_t" type to a "u64" should be mostly transparent
> > >>> for the majority of native 64-bit systems.  Those running the absolute
> > >>> latest kernels on 32-bit systems with custom or bleeding edge
> > >>> userspace *may* see a slight hiccup, but I think that user count is in
> > >>> the single digits, if not zero.
> > >>>
> > >>> Let's fix this quickly with /size_t/u64/ in v6.8.1 and avoid the
> > >>> compat shim if we can.
> > >>>
> > >>> Casey, do you have time to put together a patch for this (you should
> > >>> fix the call chains below the syscalls too)?  If not, please let me
> > >>> know and I'll get a patch out ASAP.
> > >> Grumble. Yes, I'll get right on it.
> > > Great, thanks Casey.
> > 
> > Look like lsm_get_self_attr() needs the same change. lsm_set_self_attr()
> > doesn't, need it, but I'm tempted to change it as well for consistency.
> > Thoughts?
> 
> As lsm_get_self_attr() has the same issue, it needs the same treatment.
> 
> lsm_set_self_attr() could be left unchanged.  In fact, changing the type
> of syscall arguments from size_t to an explicit 64-bit type would be
> problematic because 32-bit syscalls cannot have 64-bit arguments.

Using u32 should be totally fine for both. Nearly ever kernel internal
limits sizes to INT_MAX anyway. :)

-- 
Kees Cook