Kernel security: beyond bug fixing [LWN.net]

Kernel security: beyond bug fixing

Posted Nov 5, 2015 10:45 UTC (Thu) by renox (guest, #23785)
In reply to: Kernel security: beyond bug fixing by PaXTeam
Parent article: Kernel security: beyond bug fixing

Yes, it's able to write anything anywhere the process can, but it doesn't necessarily makes its possible to overwrite the return address, which makes it harder to exploit the flaw (especially if you have w^x and randomisation).

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Kernel security: beyond bug fixing

Posted Nov 5, 2015 11:25 UTC (Thu) by PaXTeam (guest, #24616) [Link] (8 responses)

the return address is writable by the process and therefore by the arbitrary write primitive too. whether there're defenses that make it harder or not is orthogonal to your original statement 'you cannot use a buffer overflow to override a return address'.

Kernel security: beyond bug fixing

Posted Nov 5, 2015 12:25 UTC (Thu) by renox (guest, #23785) [Link] (1 responses)

> the return address is writable by the process and therefore by the arbitrary write primitive too. whether there're defenses that make it harder or not is orthogonal to your original statement 'you cannot use a buffer overflow to override a return address'.

The 'arbitrary write' can overwrite the return address only if the address of the return address is known, which can be quite difficult if there is randomisation.

Also for the Mill CPU(unfortunately paperware only currently) I think that the separated address stack is managed directly by the CPU, so an 'arbitrary write' cannot overwrite a return address.

Kernel security: beyond bug fixing

Posted Nov 5, 2015 12:46 UTC (Thu) by PaXTeam (guest, #24616) [Link]

it seems to me that you're mixing up defense techniques with exploit techniques, which one are you arguing about now? ;) your original post made a blanket statement about something which is demonstrably false, that's all i wanted to point out. how you can make exploit techniques fail with deterministic or probabilistic methods is irrelevant for that discussion. case in point, my simple example may very well be non-exploitable if the compiler places some of those variables into registers which by definition are not subject to memory corruption, but that of course was not the point of the example.

Kernel security: beyond bug fixing

Posted Nov 5, 2015 12:28 UTC (Thu) by hummassa (guest, #307) [Link] (5 responses)

The only defense against THAT would be "only the CALL instruction can write on the return stack".

Kernel security: beyond bug fixing

Posted Nov 5, 2015 12:52 UTC (Thu) by PaXTeam (guest, #24616) [Link] (4 responses)

there're other defenses against that.

Kernel security: beyond bug fixing

Posted Nov 10, 2015 16:39 UTC (Tue) by hummassa (guest, #307) [Link] (3 responses)

Care to elaborate? If any instruction can write to the return stack, it's always possible to point the return address to an address of my choosing, and that way calling anything the current function's security context has the right to call. Which is mostly everything, because proper security contexts are a pain in the arse.

Kernel security: beyond bug fixing

Posted Nov 10, 2015 17:29 UTC (Tue) by PaXTeam (guest, #24616) [Link] (2 responses)

there's a whole bunch of public research on control flow integrity, some of which also addresses protecting the return address despite its being writable by the attacker. i also recently presented my ideas on this at H2HC, see the slides for more details. in short, all these defenses boil down to restricting the 'address of my choosing' to the point that at least privilege escalation is no longer possible.

Kernel security: beyond bug fixing

Posted Nov 24, 2015 13:43 UTC (Tue) by hummassa (guest, #307) [Link] (1 responses)

I couldn't find the slides. Link, please? :D

Kernel security: beyond bug fixing

Posted Nov 24, 2015 16:25 UTC (Tue) by PaXTeam (guest, #24616) [Link]

https://pax.grsecurity.net/docs/PaXTeam-H2HC15-RAP-RIP-RO... (there's some small errata in there that i'll fix eventually so better check the doc page for the updated version)