From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.hallyn.com (mail.hallyn.com [178.63.66.53]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9BE473E468 for ; Fri, 15 Mar 2024 17:00:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=178.63.66.53 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710522058; cv=none; b=U4Bjd8QO34NlqDWqIWMlfkK8UokFecQF33+fKjPRBgDFZY3pgS7RuGY6tQnM6EKm+b2S+psFIJyzKT9p7ihFLjuR8RpcvjL5xW4wJBJz7q0BmbFuHpz6NAB11b+GQyCNAm+6ZAIOZqrocZHTkdXBzmv4g6GjoxVKyqr+PFP8ia4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710522058; c=relaxed/simple; bh=31Tl3Yg+bKjuBUah7xAGOblXnxSzu++EAcBQm6ADnag=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=EaGXLbXKUnmvvqXat2r6v7baqhCXTa4svEias1FDk/p0mTV2m2zvij7dCFrRGsfaE1uCDxMVOo5GSBvJDI80xp/aOQasYfFy3Ip9OFotXPvufB5EcKyxSU/NTgRyJyHyqVL62GjkOr3orCtQ/PZUHp2OD64VIvnFfw5n3HZMovY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=hallyn.com; spf=pass smtp.mailfrom=mail.hallyn.com; arc=none smtp.client-ip=178.63.66.53 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=hallyn.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=mail.hallyn.com Received: by mail.hallyn.com (Postfix, from userid 1001) id 84EE9625; Fri, 15 Mar 2024 12:00:52 -0500 (CDT) Date: Fri, 15 Mar 2024 12:00:52 -0500 From: "Serge E. Hallyn" To: Paul Moore Cc: "Serge E. Hallyn" , Casey Schaufler , linux-security-module@vger.kernel.org Subject: Re: [PATCH] lsm: handle the NULL buffer case in lsm_fill_user_ctx() Message-ID: <20240315170052.GA310015@mail.hallyn.com> References: <20240314022202.599471-2-paul@paul-moore.com> <20240315150208.GA307433@mail.hallyn.com> <7956284d-5687-465d-bbcc-d45435dac42e@schaufler-ca.com> <20240315161345.GA309097@mail.hallyn.com> <20240315162831.GA309358@mail.hallyn.com> Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Mar 15, 2024 at 12:42:27PM -0400, Paul Moore wrote: > On Fri, Mar 15, 2024 at 12:28 PM Serge E. Hallyn wrote: > > On Fri, Mar 15, 2024 at 12:19:05PM -0400, Paul Moore wrote: > > > On Fri, Mar 15, 2024 at 12:13 PM Serge E. Hallyn wrote: > > > > On Fri, Mar 15, 2024 at 09:08:47AM -0700, Casey Schaufler wrote: > > > > > On 3/15/2024 8:02 AM, Serge E. Hallyn wrote: > > > > > > On Wed, Mar 13, 2024 at 10:22:03PM -0400, Paul Moore wrote: > > > > > >> Passing a NULL buffer into the lsm_get_self_attr() syscall is a valid > > > > > >> way to quickly determine the minimum size of the buffer needed to for > > > > > >> the syscall to return all of the LSM attributes to the caller. > > > > > >> Unfortunately we/I broke that behavior in commit d7cf3412a9f6 > > > > > >> ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()") > > > > > >> such that it returned an error to the caller; this patch restores the > > > > > >> original desired behavior of using the NULL buffer as a quick way to > > > > > >> correctly size the attribute buffer. > > > > > >> > > > > > >> Cc: stable@vger.kernel.org > > > > > >> Fixes: d7cf3412a9f6 ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()") > > > > > >> Signed-off-by: Paul Moore > > > > > >> --- > > > > > >> security/security.c | 8 +++++++- > > > > > >> 1 file changed, 7 insertions(+), 1 deletion(-) > > > > > >> > > > > > >> diff --git a/security/security.c b/security/security.c > > > > > >> index 5b2e0a15377d..7e118858b545 100644 > > > > > >> --- a/security/security.c > > > > > >> +++ b/security/security.c > > > > > >> @@ -780,7 +780,9 @@ static int lsm_superblock_alloc(struct super_block *sb) > > > > > >> * @id: LSM id > > > > > >> * @flags: LSM defined flags > > > > > >> * > > > > > >> - * Fill all of the fields in a userspace lsm_ctx structure. > > > > > >> + * Fill all of the fields in a userspace lsm_ctx structure. If @uctx is NULL > > > > > >> + * simply calculate the required size to output via @utc_len and return > > > > > >> + * success. > > > > > >> * > > > > > >> * Returns 0 on success, -E2BIG if userspace buffer is not large enough, > > > > > >> * -EFAULT on a copyout error, -ENOMEM if memory can't be allocated. > > > > > >> @@ -799,6 +801,10 @@ int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len, > > > > > >> goto out; > > > > > >> } > > > > > >> > > > > > >> + /* no buffer - return success/0 and set @uctx_len to the req size */ > > > > > >> + if (!uctx) > > > > > >> + goto out; > > > > > > If the user just passes in *uctx_len=0, then they will get -E2BIG > > > > > > but still will get the length in *uctx_len. > > > > > > > > > > Yes. > > > > > > > > > > > To use it this new way, they have to first set *uctx_len to a > > > > > > value larger than nctx_len could possibly be, else they'll... > > > > > > still get -E2BIG. > > > > > > > > > > Not sure I understand the problem. A return of 0 or E2BIG gets the > > > > > caller the size. > > > > > > > > The problem is that there are two ways of doing the same thing, with > > > > different behavior. People are bound to get it wrong at some point, > > > > and it's more corner cases to try and maintain (once we start). > > > > > > I have a different perspective on this, you can supply either a NULL > > > buffer and/or a buffer that is too small, including a size of zero, > > > and you'll get back an -E2BIG and a minimum buffer size. What's the > > > old wisdom, be conservative in what you send and liberal in what you > > > accept? > > > > But if you pass a NULL uctx, then surely you should send *uctx_len=0. > > And that is already handled. > > Why should we assume that userspace is always going to behave a > certain way? Userspace is going to do crazy stuff, that's a given, I > just want to make sure that we protect ourselves against the really > crazy stuff, and if we can do something useful with the moderately > crazy stuff I think we should. > > > What you are adding is that the user can pass NULL uctx, but a large > > uctx_len value. > > > > Perhaps should change my objection, and say that I would prefer the > > comment fix to suggest passing in uctx_len=0 and uctx=NULL, and expect > > a -E2BIG. The NULL check can stay (though I still think it should > > return -E2BIG). > > > > Because with the current comment update, the user may pass in > > uctx=NULL, but the actual rv will change between 0 and -E2BIG > > depending on the uctx_len they sent in. Which is whack, since > > the incoming value means nothing. > > I think that's a desirable behavior, if you pass in a NULL buffer > we'll provide you with the required size and return -E2BIG if the size > you gave us was too small, and zero/success if the size you provided > was adequate. > > Maybe I'm being stupid and this really is "whack", but you've got to > help me understand what harm can come from the behavior above. Returning success if uctx==NULL and uctx_len is big enough sends the message that this -sending a non-zero length and uctx=NULL is a recommended use case. It should not be a recommended use. If the user wants to find out the size of the buffer, they can already do so more reliably in the pre-existing way, passing in a *length=0 (and a NULL or random uctx). I say more reliable, because they can predict the function return value in this case: -E2BIG. So the only way uctx==NULL and uctx_len!=0 should happen is by accident, and in that case sending back 'success' is misleading. > ... What's the > old wisdom, be conservative in what you send and liberal in what you > accept? If you've received garbage, you should let the sender know, rather than return 'success' in the hopes that it wasn't important. But anyway I'll stop here - it doesn't break anything directly, I just think it's a bad API with the potential for future harder to spot bugs. -serge