From: Linus Torvalds > Sent: 04 September 2020 00:26 > > On Thu, Sep 3, 2020 at 2:30 PM David Laight <David.Laight@xxxxxxxxxx> wrote: > > > > A non-canonical (is that the right term) address between the highest > > valid user address and the lowest valid kernel address (7ffe to fffe?) > > will fault anyway. > > Yes. > > But we actually warn against that fault, because it's been a good way > to catch places that didn't use the proper "access_ok()" pattern. > > See ex_handler_uaccess() and the > > WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in > user access. Non-canonical address?"); > > warning. It's been good for randomized testing - a missing range check > on a user address will often hit this. > > Of course, you should never see it in real life (and hopefully not in > testing either any more). But belt-and-suspenders.. That could still be effective, just pick an address limit that is appropriate for the one access_ok() is using. Even if access_ok() uses 1<<63 there are plenty of addresses above it that fault. But the upper limit for 5-level page tables could be used all the time. One option is to test '(address | length) < (3<<62)' in access_ok(). That is also moderately suitable for masking invalid addresses to 0. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
