Messages in this thread | | | Date | Thu, 23 Apr 2015 14:37:27 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary |
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On 04/23/2015 02:10 PM, Borislav Petkov wrote: > On Thu, Apr 23, 2015 at 10:01:16PM +0200, Denys Vlasenko wrote: >> Naturally, CS can't be NULL, and up until today >> I thought SS also can't. But the bit is probably implemented >> for all eight cached descriptors. > > There's this section about NULL selector in APM v2. It says that NULL > selectors are used to invalidate segment registers and software can load > a NULL selector in SS in CPL0. > > So, if an interrupt happens and as you quoted earlier that SS gets set > to NULL as a result of an interrupt, there's that SS leak causing the SS > exception. >
Yes, the NULL SS is a special thing in 64-bit mode. I agree that context-switching it is probably the way to go; it should be cheap enough. We might even be able to conditionalize it on an X86_BUG_ flag.
-hpa
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