linux-kernel - Re: [PATCH v2] fs: don't audit the capability check in simple_xattr

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Message-ID: <20221103153030.ep5rqq2uetpclm3z@wittgenstein>
Date:   Thu, 3 Nov 2022 16:30:30 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Ondrej Mosnacek <omosnace@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, Martin Pitt <mpitt@...hat.com>
Subject: Re: [PATCH v2] fs: don't audit the capability check in
 simple_xattr_list()

On Thu, Nov 03, 2022 at 04:12:05PM +0100, Ondrej Mosnacek wrote:
> The check being unconditional may lead to unwanted denials reported by
> LSMs when a process has the capability granted by DAC, but denied by an
> LSM. In the case of SELinux such denials are a problem, since they can't
> be effectively filtered out via the policy and when not silenced, they
> produce noise that may hide a true problem or an attack.
> 
> Checking for the capability only if any trusted xattr is actually
> present wouldn't really address the issue, since calling listxattr(2) on
> such node on its own doesn't indicate an explicit attempt to see the
> trusted xattrs. Additionally, it could potentially leak the presence of
> trusted xattrs to an unprivileged user if they can check for the denials
> (e.g. through dmesg).
> 
> Therefore, it's best (and simplest) to keep the check unconditional and
> instead use ns_capable_noaudit() that will silence any associated LSM
> denials.
> 
> Fixes: 38f38657444d ("xattr: extract simple_xattr code from tmpfs")
> Reported-by: Martin Pitt <mpitt@...hat.com>
> Suggested-by: Christian Brauner (Microsoft) <brauner@...nel.org>
> Signed-off-by: Ondrej Mosnacek <omosnace@...hat.com>
> ---

Looks good,
Reviewed-by: Christian Brauner (Microsoft) <brauner@...nel.org>