Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
On Wed, Jun 16, 2021 at 11:32 AM Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
Prevent security holes by recording when all of the registers are
available so generic code changes do not result in security holes
on alpha.
Please no, not this way. ldl/stc is extremely expensive on some alpha cpus.
I really think thatTIF_ALLREGS_SAVED bit isn't worth it, except
perhaps for debugging.
And even for debugging, I think it would be both easier and cheaper to
just add a magic word to the entry stack instead.
I think I can do something like that.
Looking at arch/alpha/asm/cache.h it looks like alpha had either 32byte
or 64bit cachelines. Which makes struct switch_stack a full 10 or 5
cachelines in size. So pushing something extra might hit an extra
cacheline.
However it looks like struct pt_regs is 16 bytes short of a full cache
line so struct switch_stack isn't going to be cacheline aligned. Adding
an extra 8 bytes of magic number will hopefully be in the noise.
If I can I would like to find something that is cheap enough that I can
always leave on. Mostly because there is little enough testing that a
bug that allows anyone to stomp the kernel stack has existed for 17 years
without being noticed.
If you want it to be a debug option only I can certainly make that
happen. I am still going "Eek! Arbitrary stack smash!" in my head.
Eric
