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). > -Topi /Jarkko
