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 F33C71773D for ; Fri, 15 Mar 2024 16:28:32 +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=1710520116; cv=none; b=gTBuRhWN9zWNsVgaOBzH+692JrgEtUFPmNmNw1xxWFK+jgAjczrwjYUlTTJNfXYNux/7WqwEC97JQQDswU4QBYeIoRDofyyn8pOytw95rrxKuZPeJggQ+Xt0e4GwNt7r6bkHIMOS2Vp3x3ciimbC7Gc+yKMT+88aszAoY7/1tj8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710520116; c=relaxed/simple; bh=BmTACyeRWYIt+Zg/gwh69HRRM7Amr+agnN1VNya/qcM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=aEDK45NY3rPPDvUKakNW+cWhcDOeoXDtd2tTURJTWfnPskUG6CrAgTCkpc6rELbuTrXkTJAAbvlMcNOfvQ1CBLgorkMj3TnqINWUyYmY46o6E3SILb54XofNx/LYfe6m2kmFR4sLVWFkk6msvwyP8Dyq1yl/cMYsWz+0bccdRQ4= 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 245AAC12; Fri, 15 Mar 2024 11:28:31 -0500 (CDT) Date: Fri, 15 Mar 2024 11:28:31 -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: <20240315162831.GA309358@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> 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 12:19:05PM -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. 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. -serge