On 3/10/2022 10:49 PM, David Laight wrote: > From: Dave Hansen <dave.hansen@xxxxxxxxx> >> Sent: 10 March 2022 16:46 >> >> On 3/10/22 06:32, David Laight wrote: >>>> UAI allows software to store a tag in the upper 7 bits of a logical >>>> address [63:57]. When enabled, the processor will suppress the >>>> traditional canonical address checks on the addresses. More information >>>> about UAI can be found in section 5.10 of 'AMD64 Architecture >>>> Programmer's Manual, Vol 2: System Programming' which is available from >>>> >>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fattachment.cgi%3Fid%3D300549&data=04%7C01%7Cbharata%40amd.com%7Ca1de24223931481b3fcb08da02ba2e6f%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637825295938946622%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=HijEAUq172r8YwkcCuhvl99Vk5BwE6iSROXcSQXmJHk%3D&reserved=0 >>> Is that really allowing bit 63 to be used? >>> That is normally the user-kernel bit. >>> I can't help feeling that will just badly break things. >> >> Yeah, this does seem worrisome. The LAM approach[1] retains >> canonicality checking for bit 63. > > Actually it is rather worse than 'worrisome'. > Allowing the user all address upto the base of the valid > kernel addresses (probably tags to 3e, but not 3f) > means that you can't use a fast address check in access_ok(). > You are forced to use the strict test that 32bit kernels use.
