Re: [Xen-devel] [PATCH] x86/vvmx: Fix deadlock with MSR bitmap merging

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Mar 11, 2020 at 06:34:55PM +0000, Andrew Cooper wrote:
> c/s c47984aabead "nvmx: implement support for MSR bitmaps" introduced a use of
> map_domain_page() which may get used in the middle of context switch.
> 
> This is not safe, and causes Xen to deadlock on the mapcache lock:
> 
>   (XEN) Xen call trace:
>   (XEN)    [<ffff82d08022d6ae>] R _spin_lock+0x34/0x5e
>   (XEN)    [<ffff82d0803219d7>] F map_domain_page+0x250/0x527
>   (XEN)    [<ffff82d080356332>] F do_page_fault+0x420/0x780
>   (XEN)    [<ffff82d08038da3d>] F 
> x86_64/entry.S#handle_exception_saved+0x68/0x94
>   (XEN)    [<ffff82d08031729f>] F __find_next_zero_bit+0x28/0x69
>   (XEN)    [<ffff82d080321a4d>] F map_domain_page+0x2c6/0x527
>   (XEN)    [<ffff82d08029eeb2>] F nvmx_update_exec_control+0x1d7/0x323
>   (XEN)    [<ffff82d080299f5a>] F vmx_update_cpu_exec_control+0x23/0x40
>   (XEN)    [<ffff82d08029a3f7>] F 
> arch/x86/hvm/vmx/vmx.c#vmx_ctxt_switch_from+0xb7/0x121
>   (XEN)    [<ffff82d08031d796>] F 
> arch/x86/domain.c#__context_switch+0x124/0x4a9
>   (XEN)    [<ffff82d080320925>] F context_switch+0x154/0x62c
>   (XEN)    [<ffff82d080252f3e>] F 
> common/sched/core.c#sched_context_switch+0x16a/0x175
>   (XEN)    [<ffff82d080253877>] F common/sched/core.c#schedule+0x2ad/0x2bc
>   (XEN)    [<ffff82d08022cc97>] F common/softirq.c#__do_softirq+0xb7/0xc8
>   (XEN)    [<ffff82d08022cd38>] F do_softirq+0x18/0x1a
>   (XEN)    [<ffff82d0802a2fbb>] F vmx_asm_do_vmentry+0x2b/0x30
> 
> Convert the domheap page into being a xenheap page.

Fixes: c47984aabead5391 ('nvmx: implement support for MSR bitmaps')

> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> 
> I suspect this is the not-quite-consistent-enough-to-bisect issue which
> OSSTest is hitting and interfering with pushes to master.
> ---
>  xen/arch/x86/hvm/vmx/vvmx.c        | 19 ++++---------------
>  xen/include/asm-x86/hvm/vmx/vvmx.h |  2 +-
>  2 files changed, 5 insertions(+), 16 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
> index 926a11c15f..f049920196 100644
> --- a/xen/arch/x86/hvm/vmx/vvmx.c
> +++ b/xen/arch/x86/hvm/vmx/vvmx.c
> @@ -130,12 +130,9 @@ int nvmx_vcpu_initialise(struct vcpu *v)
>  
>      if ( cpu_has_vmx_msr_bitmap )
>      {
> -        nvmx->msr_merged = alloc_domheap_page(d, MEMF_no_owner);
> +        nvmx->msr_merged = alloc_xenheap_page();

Could we also use __map_domain_page_global here (keeping the domheap
allocation) in order to map the page on init and keep it mapped until
the domain is destroyed?

>          if ( !nvmx->msr_merged )
> -        {
> -            gdprintk(XENLOG_ERR, "nest: allocation for MSR bitmap failed\n");

I know the message is not specially helpful, but that seems to be
inline with how error handling is done in nvmx_vcpu_initialise.

Thanks, Roger.

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