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
