From mboxrd@z Thu Jan 1 00:00:00 1970 From: Topi Miettinen Date: Wed, 16 Dec 2020 13:05:05 +0000 Subject: Re: Antw: [EXT] Re: [systemd-devel] Creating executable device nodes in /dev? Message-Id: <815056f6-cb4b-0d48-ea2c-1cde97af45d4@gmail.com> List-Id: References: <0f17eade-5e99-be29-fd09-2d0a1949ac7f@gmail.com> <9DF5C88B-5156-455A-BA3F-EB19CAA0411B@amacapital.net> <20201209001521.GA64007@kernel.org> <20201211104635.GD12091@kernel.org> <27796c04-249e-6cf0-c3e1-0fd657a82f9c@gmail.com> <5FD7137E020000A10003D81B@gwsmtp.uni-regensburg.de> <20201215041903.GA21875@kernel.org> <5FD9DB8D020000A10003D8DC@gwsmtp.uni-regensburg.de> In-Reply-To: <5FD9DB8D020000A10003D8DC@gwsmtp.uni-regensburg.de> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: Ulrich Windl , jarkko@kernel.org Cc: luto@amacapital.net, Ben Hutchings , jethro@fortanix.com, bruce.schlobohm@intel.com, kai.svahn@intel.com, luto@kernel.org, haitao.huang@linux.intel.com, jarkko.sakkinen@linux.intel.com, "systemd-devel@lists.freedesktop.org" , casey@schaufler-ca.com, sds@tycho.nsa.gov, linux-hotplug@vger.kernel.org, linux-sgx@vger.kernel.org On 16.12.2020 12.03, Ulrich Windl wrote: >>>> Jarkko Sakkinen schrieb am 15.12.2020 um 05:19 in > Nachricht > <20201215041903.GA21875@kernel.org>: >> On Mon, Dec 14, 2020 at 08:25:50AM +0100, Ulrich Windl wrote: >>>>>> Topi Miettinen schrieb am 11.12.2020 um 12:46 in >>> Nachricht >>> <27796c04-249e-6cf0-c3e1-0fd657a82f9c@gmail.com>: >>>> On 11.12.2020 12.46, Jarkko Sakkinen wrote: >>>>> On Wed, Dec 09, 2020 at 10:35:21AM +0200, Topi Miettinen wrote: >>>>>> On 9.12.2020 2.15, Jarkko Sakkinen wrote: >>>>>>> On Wed, Dec 09, 2020 at 01:15:27AM +0200, Topi Miettinen wrote: >>>>>>>>>>> As a further argument, I just did this on a Fedora system: >>>>>>>>>>> $ find /dev ‑perm /ugo+x ‑a \! ‑type d ‑a \! ‑type l >>>>>>>>>>> No results. So making /dev noexec doesn't seem to have any > benefit. >>>>>>>>>> >>>>>>>>>> It's no surprise that there aren't any executables in /dev since >>>>>>>>>> removing MAKEDEV ages ago. That's not the issue, which is that >>>>>>>>>> /dev is a writable directory (for UID=0 but no capabilities are >>>>>>>>>> needed) and thus a potential location for constructing unapproved >>>>>>>>>> executables if it is also mounted exec (W^X). >>>>>>>>> >>>>>>>>> UID 0 can just change mount options, though, unless SELinux or > similar >>> is >>>> used. And SELinux can protect /dev just fine without noexec. >>>>>>>> >>>>>>>> Well, mounting would need CAP_SYS_ADMIN in addition to UID 0. Also >>> SELinux >>>>>>>> is not universal and the policies might not contain all users or >>> services. >>>>>>>> >>>>>>>> ‑Topi >>>>>>> >>>>>>> What's the data that supports having noexec /dev anyway? With root >>>>>>> access I can then just use something else like /dev/shm mount. >>>>>>> >>>>>>> Has there been out in the wild real world cases that noexec mount >>>>>>> of would have prevented? >>>>>>> >>>>>>> For me this sounds a lot just something that "feels more secure" >>>>>>> without any measurable benefit. Can you prove me wrong? >>>>>> >>>>>> I don't think security works that way. An attacker has various methods > to >>>>>> choose from, some are more interesting than others. The case where >>> rw,exec >>>>>> /dev would be interesting would imply that easier or more common > avenues >>>>>> would be blocked, for example rw,exec /dev/shm, /tmp, /var/tmp, or >>>>>> /run/user/$UID/ for user. Also fileless malware with pure ROP/JOP >>> approach >>>>>> with no need for any file system access is getting more common. It > does >>> not >>>>>> mean that it would not be prudent to block the relatively easy > approaches >>>>>> too, including /dev. >>>>> >>>>> What if we add a new mount option "chrexec", which allows exec >>>>> for character devices (S_IFCHR). >>>> >>>> I think devices are a bad match for SGX because devices haven't been >>>> executable and SGX is actually an operation for memory. So something >>>> like memfd_create(, MFD_SGX) or mmap(,, PROT_READ|PROT_EXEC|PROT_SGX) >>>> would be much more natural. Even better would be something that >>>> conceptully also works for AMD version (either with the same flags or >>>> MFD_SGX / MFD_whatever_the_AMD_version_is). >>> >>> +1 >> >> SGX reserved memory from kernel's point of view is IO memory. >> >> Mapping SGX to memfd would not be a great idea, as it does not map >> into concept of anonymous file backed by regular memory. >> >> A device file is very natural match actually. We have ioctl API for >> uploading enclave pages during the build procedure to the enclave and >> custom #PF handler. Conceptually it's a lot like video memory or such >> special device specific memory area. >> >> There's no AMD equivalent of this technology. > > Hi! > > Back to "noexec": AFAIR the execute bit does not make sense for device files, > and the purpose probably was to avoid execution of non-device files (e.g. > regular executables) from inside /dev (where they should not be). So in this > view "noexec" makes sense. > There were similar arguments for not allowing device files in user > directories. PR#17940 (https://github.com/systemd/systemd/pull/17940) was merged, so /dev will now on be mounted with "exec" by systemd. I made issue #17942 (https://github.com/systemd/systemd/issues/17942) to discuss related hardening options. I'm leaning towards NoExecPaths=/ExecPaths= as it would enable nice hardening by allow-listing of all executable content for system services with simple directives like: [Service] NoExecPaths=/ ExecPaths=/usr/sbin/daemon /lib64/ld-linux-x86-64.so.2 /usr/lib Then a service infected with malware would not be able to execute a shell present in the system or downloaded later, if that was not explicitly allowed. /dev would also not have "exec" flag by default, but SGX could be allowed with "ExecPaths=/dev/sgx" when needed. -Topi