On Wed, Aug 19, 2020 at 10:02 AM Josh Poimboeuf <[email protected]> wrote: > > On Wed, Aug 19, 2020 at 09:39:10AM -0700, Andy Lutomirski wrote: > > On Wed, Aug 19, 2020 at 7:50 AM Josh Poimboeuf <[email protected]> wrote: > > > +/* > > > + * Sanitize a uaccess pointer such that it becomes NULL if it's not a > > > valid > > > + * user pointer. This blocks speculative dereferences of user-controlled > > > + * pointers. > > > + */ > > > +#define uaccess_mask_ptr(ptr) \ > > > + (__typeof__(ptr)) array_index_nospec((__force unsigned long)ptr, > > > user_addr_max()) > > > + > > > > If I dug through all the macros correctly, this is generating a fairly > > complex pile of math to account for the fact that user_addr_max() is > > variable and that it's a nasty number. > > The math is actually pretty simple. It's identical to what getuser.S is > doing: > > cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX > sbb %_ASM_DX, %_ASM_DX > and %_ASM_DX, %_ASM_AX > > > But I don't think there's any particular need to use the real maximum > > user address here. Allowing a mis-speculated user access to a > > non-canonical address or to the top guard page of the lower canonical > > region is harmless. With current kernels, a sequence like: > > > > if (likely((long)addr > 0) { > > masked_addr = addr & 0x7FFFFFFFFFFFFFFFUL; > > } else { > > if (kernel fs) { > > masked_addr = addr; > > } else { > > EFAULT; > > } > > } > > The masking has to be done without conditional branches, otherwise it > defeats the point.
Ugh. Fair enough. > Yeah, we could do that. Though in the meantime, the simple merge > conflict resolution with Christoph's patches would be > s/user_addr_max/TASK_SIZE_MAX/ in my uaccess_mask_ptr() macro. Okay. I will try to help remember :) --Andy
