Re: [PATCH v3] x86/livepatch: Fix livepatch application when CET is active

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

> From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Sent: Tuesday, April 18, 2023 12:10 PM
> To: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Jan Beulich 
> <jbeulich@xxxxxxxx>; Jan Beulich <JBeulich@xxxxxxxx>; Roger Pau Monne 
> <roger.pau@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Konrad Rzeszutek Wilk 
> <konrad.wilk@xxxxxxxxxx>; Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> Subject: [PATCH v3] x86/livepatch: Fix livepatch application when CET is 
> active 
>  
> Right now, trying to apply a livepatch on any system with CET shstk (AMD Zen3
> or later, Intel Tiger Lake or Sapphire Rapids and later) fails as follows:
> 
>   (XEN) livepatch: lp: Verifying enabled expectations for all functions
>   (XEN) common/livepatch.c:1591: livepatch: lp: timeout is 30000000ns
>   (XEN) common/livepatch.c:1703: livepatch: lp: CPU28 - IPIing the other 127 
> CPUs
>   (XEN) livepatch: lp: Applying 1 functions
>   (XEN) hi_func: Hi! (called 1 times)
>   (XEN) Hook executing.
>   (XEN) Assertion 'local_irq_is_enabled() || cpumask_subset(mask, 
> cpumask_of(cpu))' failed at arch/x86/smp.c:265
>   (XEN) *** DOUBLE FAULT ***
>   <many double faults>
> 
> The assertion failure is from a global (system wide) TLB flush initiated by
> modify_xen_mappings().  I'm not entirely sure when this broke, and I'm not
> sure exactly what causes the #DF's, but it doesn't really matter either
> because they highlight a latent bug that I'd overlooked with the CET-SS vs
> patching work the first place.
> 
> While we're careful to arrange for the patching CPU to avoid encountering
> non-shstk memory with transient shstk perms, other CPUs can pick these
> mappings up too if they need to re-walk for uarch reasons.
> 
> Another bug is that for livepatching, we only disable CET if shadow stacks are
> in use.  Running on Intel CET systems when Xen is only using CET-IBT will
> crash in arch_livepatch_quiesce() when trying to clear CR0.WP with CR4.CET
> still active.
> 
> Also, we never went and cleared the dirty bits on .rodata.  This would
> matter (for the same reason it matters on .text - it becomes a valid target
> for WRSS), but we never actually patch .rodata anyway.
> 
> Therefore rework how we do patching for both alternatives and livepatches.
> 
> Introduce modify_xen_mappings_lite() with a purpose similar to
> modify_xen_mappings(), but stripped down to the bare minimum as it's used in
> weird contexts.  Leave all complexity to the caller to handle.
> 
> Instead of patching by clearing CR0.WP (and having to jump through some
> fragile hoops to disable CET in order to do this), just transiently relax the
> permissions on .text via l2_identmap[].
> 
> Note that neither alternatives nor livepatching edit .rodata, so we don't need
> to relax those permissions at this juncture.
> 
> The perms are relaxed globally, but is safe enough.  Alternatives run before
> we boot APs, and Livepatching runs in a quiesced state where the other CPUs
> are not doing anything interesting.
> 
> This approach is far more robust.
> 
> Fixes: 48cdc15a424f ("x86/alternatives: Clear CR4.CET when clearing CR0.WP")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx> (live patching bits)