Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 24 Apr 2015 13:50:05 -0700 | Subject | Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue |
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On Fri, Apr 24, 2015 at 1:46 PM, Denys Vlasenko <vda.linux@googlemail.com> wrote: > On Fri, Apr 24, 2015 at 10:21 PM, Andy Lutomirski <luto@amacapital.net> wrote: >> On Thu, Apr 23, 2015 at 7:15 PM, Andy Lutomirski <luto@kernel.org> wrote: >>> AMD CPUs don't reinitialize the SS descriptor on SYSRET, so SYSRET >>> with SS == 0 results in an invalid usermode state in which SS is >>> apparently equal to __USER_DS but causes #SS if used. >>> >>> Work around the issue by replacing NULL SS values with __KERNEL_DS >>> in __switch_to, thus ensuring that SYSRET never happens with SS set >>> to NULL. >>> >>> This was exposed by a recent vDSO cleanup. >>> >>> Fixes: e7d6eefaaa44 x86/vdso32/syscall.S: Do not load __USER32_DS to %ss >>> Signed-off-by: Andy Lutomirski <luto@kernel.org> >>> --- >>> >>> Tested only on Intel, which isn't very interesting. I'll tidy up >>> and send a test case, too, once Borislav confirms that it works. >>> >>> Please don't actually apply this until we're sure we understand the >>> scope of the issue. If this doesn't affect SYSRETQ, then we might >>> to fix it on before SYSRETL to avoid impacting 64-bit processes >>> at all. >>> >> >> After sleeping on it, I think I want to offer a different, more >> complicated approach. AFAIK there are really only two ways that this >> issue can be visible: >> >> 1. SYSRETL. We can fix that up in the AMD SYSRETL path. I think >> there's a decent argument that that path is less performance-critical >> than context switches. >> >> 2. SYSRETQ. The only way that I know of to see the problem is SYSRETQ >> followed by a far jump or return. This is presumably *extremely* >> rare. >> >> What if we fixed #2 up in do_stack_segment. We should double-check >> the docs, but I think that this will only ever manifest as #SS(0) with >> regs->ss == __USER_DS and !user_mode_64bit(regs). We need to avoid >> infinite retry looks, but this might be okay. I think that #SS(0) >> from userspace under those conditions can *only* happen as a result of >> this issue. Even if not, we could come up with a way to only retry >> once per syscall (e.g. set some ti->status flag in the 64-bit syscall >> path on AMD and clear it in do_stack_segment). >> >> This might be way more trouble than it's worth. > > Exactly my feeling. What are you trying to save? About four CPU > cycles of checking %ss != __KERNEL_DS on each switch_to? > That's not worth bothering about. Your last patch seems to be perfect.
We'll have to do the write to ss almost every time an AMD CPU sleeps in a syscall. Maybe that's still not a big deal.
--Andy
-- Andy Lutomirski AMA Capital Management, LLC
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