>>> 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
