On Tue, Oct 30, 2018 at 02:02:12PM -0700, Andy Lutomirski wrote: > But I dislike allowing regular writes in the protected region. We > really only need four write primitives: > > 1. Just write one value. Call at any time (except NMI). > > 2. Just copy some bytes. Same as (1) but any number of bytes. Given the !preempt/!IRQ contraints I'd certainly put an upper limit on the number of bytes there. > 3,4: Same as 1 and 2 but must be called inside a special rare write > region. This is purely an optimization. > > Actually getting a modifiable pointer should be disallowed for two > reasons: > > 1. Some architectures may want to use a special > write-different-address-space operation. You're thinking of s390 ? :-) > Heck, x86 could, too: make > the actual offset be a secret and shove the offset into FSBASE or > similar. Then %fs-prefixed writes would do the rare writes. > 2. Alternatively, x86 could set the U bit. Then the actual writes > would use the uaccess helpers, giving extra protection via SMAP. Cute, and yes, something like that would be nice. > We don’t really want a situation where an unchecked pointer in the > rare write region completely defeats the mechanism. Agreed.
