From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=amazon.com header.i=@amazon.com header.b="tmtPZpbg" Received: from smtp-fw-9105.amazon.com (smtp-fw-9105.amazon.com [207.171.188.204]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E421012A; Fri, 1 Dec 2023 12:59:51 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1701464392; x=1733000392; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=nCap8NwzKwphCukdUu1OlpulweBp55bEAVvRfRiRsUE=; b=tmtPZpbga/5zaP3+9H5UXuW+ZTP9KA1zU96y38I/GzFHM2fSVXrAEr0v p+Ma59GZclHXwgWsIfVbNczLHzytH3aejuQBFsgx8MzuR1X1Dd14r9dVE S2hVMubfV6KuIaTkUQs0X8bxVuUdyDe9d2rzIhDBvMh6myJ6J+J4+XmK3 Q=; X-IronPort-AV: E=Sophos;i="6.04,242,1695686400"; d="scan'208";a="688425735" Received: from pdx4-co-svc-p1-lb2-vlan2.amazon.com (HELO email-inbound-relay-pdx-2a-m6i4x-af372327.us-west-2.amazon.com) ([10.25.36.210]) by smtp-border-fw-9105.sea19.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Dec 2023 20:59:51 +0000 Received: from smtpout.prod.us-west-2.prod.farcaster.email.amazon.dev (pdx2-ws-svc-p26-lb5-vlan2.pdx.amazon.com [10.39.38.66]) by email-inbound-relay-pdx-2a-m6i4x-af372327.us-west-2.amazon.com (Postfix) with ESMTPS id 1601560A61; Fri, 1 Dec 2023 20:59:51 +0000 (UTC) Received: from EX19MTAUWA001.ant.amazon.com [10.0.7.35:56175] by smtpin.naws.us-west-2.prod.farcaster.email.amazon.dev [10.0.38.133:2525] with esmtp (Farcaster) id 98e2ec98-9b24-410c-804e-0c935446c316; Fri, 1 Dec 2023 20:59:50 +0000 (UTC) X-Farcaster-Flow-ID: 98e2ec98-9b24-410c-804e-0c935446c316 Received: from EX19D010UWA004.ant.amazon.com (10.13.138.204) by EX19MTAUWA001.ant.amazon.com (10.250.64.218) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1118.39; Fri, 1 Dec 2023 20:59:50 +0000 Received: from dev-dsk-kamatam-2b-b66a5860.us-west-2.amazon.com (10.169.6.191) by EX19D010UWA004.ant.amazon.com (10.13.138.204) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1118.39; Fri, 1 Dec 2023 20:59:50 +0000 From: Munehisa Kamata To: , CC: , , , Subject: Re: Fw: [PATCH] proc: Update inode upon changing task security attribute Date: Fri, 1 Dec 2023 20:59:40 +0000 Message-ID: <20231201205940.23095-1-kamatam@amazon.com> X-Mailer: git-send-email 2.40.1 In-Reply-To: <5f8b18b0-0744-4cf5-9ec5-b0bb0451dd18@p183> References: <5f8b18b0-0744-4cf5-9ec5-b0bb0451dd18@p183> Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: EX19D046UWB004.ant.amazon.com (10.13.139.164) To EX19D010UWA004.ant.amazon.com (10.13.138.204) Hi Alexey, On Fri, 2023-12-01 09:30:00 +0000, Alexey Dobriyan wrote: > > On Wed, Nov 29, 2023 at 05:11:22PM -0800, Andrew Morton wrote: > > > > fyi... > > > > (yuk!) > > > > > > > > Begin forwarded message: > > > > Date: Thu, 30 Nov 2023 00:37:04 +0000 > > From: Munehisa Kamata > > To: , > > Cc: , , "Munehisa Kamata" > > Subject: [PATCH] proc: Update inode upon changing task security attribute > > > > > > I'm not clear whether VFS is a better (or worse) place[1] to fix the > > problem described below and would like to hear opinion. > > > > If the /proc/[pid] directory is bind-mounted on a system with Smack > > enabled, and if the task updates its current security attribute, the task > > may lose access to files in its own /proc/[pid] through the mountpoint. > > > > $ sudo capsh --drop=cap_mac_override -- > > # mkdir -p dir > > # mount --bind /proc/$$ dir > > # echo AAA > /proc/$$/task/current # assuming built-in echo > > # cat /proc/$$/task/current # revalidate > > AAA > > # echo BBB > dir/attr/current > > # cat dir/attr/current > > cat: dir/attr/current: Permission denied > > # ls dir/ > > ls: cannot access dir/: Permission denied > > # cat /proc/$$/attr/current # revalidate > > BBB > > # cat dir/attr/current > > BBB > > # echo CCC > /proc/$$/attr/current > > # cat dir/attr/current > > cat: dir/attr/current: Permission denied > > > > This happens because path lookup doesn't revalidate the dentry of the > > /proc/[pid] when traversing the filesystem boundary, so the inode security > > blob of the /proc/[pid] doesn't get updated with the new task security > > attribute. Then, this may lead security modules to deny an access to the > > directory. Looking at the code[2] and the /proc/pid/attr/current entry in > > proc man page, seems like the same could happen with SELinux. Though, I > > didn't find relevant reports. > > > > The steps above are quite artificial. I actually encountered such an > > unexpected denial of access with an in-house application sandbox > > framework; each app has its own dedicated filesystem tree where the > > process's /proc/[pid] is bind-mounted to and the app enters into via > > chroot. > > > > With this patch, writing to /proc/[pid]/attr/current (and its per-security > > module variant) updates the inode security blob of /proc/[pid] or > > /proc/[pid]/task/[tid] (when pid != tid) with the new attribute. > > > > [1] https://lkml.kernel.org/linux-fsdevel/4A2D15AF.8090000@sun.com/ > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/security/selinux/hooks.c#n4220 > > > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > > Signed-off-by: Munehisa Kamata > > --- > > fs/proc/base.c | 23 ++++++++++++++++++++--- > > 1 file changed, 20 insertions(+), 3 deletions(-) > > > > diff --git a/fs/proc/base.c b/fs/proc/base.c > > index dd31e3b6bf77..bdb7bea53475 100644 > > --- a/fs/proc/base.c > > +++ b/fs/proc/base.c > > @@ -2741,6 +2741,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf, > > { > > struct inode * inode = file_inode(file); > > struct task_struct *task; > > + const char *name = file->f_path.dentry->d_name.name; > > void *page; > > int rv; > > > > @@ -2784,10 +2785,26 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf, > > if (rv < 0) > > goto out_free; > > > > - rv = security_setprocattr(PROC_I(inode)->op.lsm, > > - file->f_path.dentry->d_name.name, page, > > - count); > > + rv = security_setprocattr(PROC_I(inode)->op.lsm, name, page, count); > > mutex_unlock(¤t->signal->cred_guard_mutex); > > + > > + /* > > + * Update the inode security blob in advance if the task's security > > + * attribute was updated > > + */ > > + if (rv > 0 && !strcmp(name, "current")) { > > + struct pid *pid; > > + struct proc_inode *cur, *ei; > > + > > + rcu_read_lock(); > > + pid = get_task_pid(current, PIDTYPE_PID); > > + hlist_for_each_entry(cur, &pid->inodes, sibling_inodes) > > + ei = cur; > > Should this "break;"? Why is only the last inode in the list updated? > Should it be the first? All of them? If it picks up the first node, it may end up updating /proc/[pid]/task/[tid] rather than /proc/[pid] (when pid == tid) and the task may be denied access to its own /proc/[pid] afterward. I think updating all of them won't hurt. But, as long as /proc/[pid] is accessible, the rest of the inodes should be updated upon path lookup via revalidation as usual. When pid != tid, it only updates /proc/[pid]/task/[tid] and the thread may lose an access to /proc/[pid], but I think it's okay as it's a matter of security policy enforced by security modules. Casey, do you have any comments here? Regards, Munehisa > > + put_pid(pid); > > + pid_update_inode(current, &ei->vfs_inode); > > + rcu_read_unlock(); > > + } >