On Wed, Dec 16, 2020 at 03:05:05PM +0200, Topi Miettinen wrote: > On 16.12.2020 12.03, Ulrich Windl wrote: > > > > > Jarkko Sakkinen <jarkko@xxxxxxxxxx> schrieb am 15.12.2020 um 05:19 in > > Nachricht > > <20201215041903.GA21875@xxxxxxxxxx>: > > > On Mon, Dec 14, 2020 at 08:25:50AM +0100, Ulrich Windl wrote: > > > > > > > Topi Miettinen <toiwoton@xxxxxxxxx> schrieb am 11.12.2020 um 12:46 in > > > > Nachricht > > > > <27796c04-249e-6cf0-c3e1-0fd657a82f9c@xxxxxxxxx>: > > > > > 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. OK, this sounds relieving, thank you (late resp, I was vacation last week). > -Topi /Jarkko
