Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 23 Apr 2015 19:18:47 -0700 | Subject | Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue |
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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.
Even if the issue affects SYSRETQ, it could be that we don't care. If the extent of the info leak is whether we context switched during a 64-bit syscall to a non-syscall context, then this is basically uninteresting. In that case, we could either ignore the 64-bit issue entirely or fix it the other way: force SS to NULL on context switch (much faster, I presume) and fix it up before SYSRETL as Denys suggested.
We clearly don't have a spate of crashes in programs that do SYSCALL from a 64-bit CS and then far jump/return to a 32-bit CS without first reloading SS, since this bug has been here forever. I agree that the issue is ugly (if it exists in the first place), but maybe we don't need to fix it.
Thoughts?
--Andy
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