Messages in this thread Patch in this message | | | From | Andy Lutomirski <> | Date | Thu, 24 Oct 2019 09:11:53 -0700 | Subject | Re: [PATCH] x86/stackframe/32: repair 32-bit Xen PV |
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On Mon, Oct 7, 2019 at 3:41 AM Jan Beulich <jbeulich@suse.com> wrote: > > Once again RPL checks have been introduced which don't account for a > 32-bit kernel living in ring 1 when running in a PV Xen domain. The > case in FIXUP_FRAME has been preventing boot; adjust BUG_IF_WRONG_CR3 > as well just in case.
I'm okay with the generated code, but IMO the macro is too indirect for something that's trivial.
> > Fixes: 3c88c692c287 ("x86/stackframe/32: Provide consistent pt_regs") > Signed-off-by: Jan Beulich <jbeulich@suse.com> > > --- a/arch/x86/entry/entry_32.S > +++ b/arch/x86/entry/entry_32.S > @@ -48,6 +48,17 @@ > > #include "calling.h" > > +#ifndef CONFIG_XEN_PV > +# define USER_SEGMENT_RPL_MASK SEGMENT_RPL_MASK > +#else > +/* > + * When running paravirtualized on Xen the kernel runs in ring 1, and hence > + * simple mask based tests (i.e. ones not comparing against USER_RPL) have to > + * ignore bit 0. See also the C-level get_kernel_rpl(). > + */
How about:
/* * When running on Xen PV, the actual %cs register in the kernel is 1, not 0. * If we need to distinguish between a %cs from kernel mode and a %cs from * user mode, we can do test $2 instead of test $3. */ #define USER_SEGMENT_RPL_MASK 2
but...
> +# define USER_SEGMENT_RPL_MASK (SEGMENT_RPL_MASK & ~1) > +#endif > + > .section .entry.text, "ax" > > /* > @@ -172,7 +183,7 @@ > ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI > .if \no_user_check == 0 > /* coming from usermode? */ > - testl $SEGMENT_RPL_MASK, PT_CS(%esp) > + testl $USER_SEGMENT_RPL_MASK, PT_CS(%esp)
Shouldn't PT_CS(%esp) be 0 if we came from the kernel? I'm guessing the actual bug is in whatever code put 1 in here in the first place.
In other words, I'm having trouble understanding why there is any context in which some value would be 3 for user mode and 1 for kernel mode. Obviously if we're manually IRETing to kernel mode, we need to set CS to 1, but if we're filling in our own PT_CS, we should just write 0.
The supposedly offending commit (""x86/stackframe/32: Provide consistent pt_regs") looks correct to me, so I suspect that the problem is elsewhere. Or is it intentional that Xen PV's asm (arch/x86/xen/whatever) sticks 1 into the CS field on the stack?
Also, why are we supporting 32-bit Linux PV guests at all? Can we just delete this code instead?
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