Re: [PATCH v2] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv_inhibit()

[Sean Christopherson <seanjc@google.com>]

On Wed, Nov 02, 2022, Ben Gardon wrote:
> kvm_zap_gfn_range() must be called in an SRCU read-critical section, but
> there is no SRCU annotation in __kvm_set_or_clear_apicv_inhibit(). This
> can lead to the following warning via
> kvm_arch_vcpu_ioctl_set_guest_debug() if a Shadow MMU is in use (TDP
> MMU disabled or nesting):
> 
> [ 1416.659809] =============================
> [ 1416.659810] WARNING: suspicious RCU usage
> [ 1416.659839] 6.1.0-dbg-DEV #1 Tainted: G S        I
> [ 1416.659853] -----------------------------
> [ 1416.659854] include/linux/kvm_host.h:954 suspicious rcu_dereference_check() usage!
> [ 1416.659856]
> ...
> [ 1416.659904]  dump_stack_lvl+0x84/0xaa
> [ 1416.659910]  dump_stack+0x10/0x15
> [ 1416.659913]  lockdep_rcu_suspicious+0x11e/0x130
> [ 1416.659919]  kvm_zap_gfn_range+0x226/0x5e0
> [ 1416.659926]  ? kvm_make_all_cpus_request_except+0x18b/0x1e0
> [ 1416.659935]  __kvm_set_or_clear_apicv_inhibit+0xcc/0x100
> [ 1416.659940]  kvm_arch_vcpu_ioctl_set_guest_debug+0x350/0x390
> [ 1416.659946]  kvm_vcpu_ioctl+0x2fc/0x620
> [ 1416.659955]  __se_sys_ioctl+0x77/0xc0
> [ 1416.659962]  __x64_sys_ioctl+0x1d/0x20
> [ 1416.659965]  do_syscall_64+0x3d/0x80
> [ 1416.659969]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> Always take the KVM SRCU read lock in __kvm_set_or_clear_apicv_inhibit()
> to protect the GFN to memslot translation. The SRCU read lock is not
> technically required when no Shadow MMUs are in use, since the TDP MMU
> walks the paging structures from the roots and does not need to look up
> GFN translations in the memslots, but make the SRCU locking
> unconditional for simplicty.
> 
> In most cases, the SRCU locking is taken care of in the vCPU run loop,
> but when called through the KVM_SET_GUEST_DEBUG IOCTL, the SRCU read
> lock is missing.

Nit, it not just KVM_SET_GUEST_DEBUG.  If it were just KVM_SET_GUEST_DEBUG, I
might have advocated putting the fix KVM_SET_GUEST_DEBUG.

> Tested: ran tools/testing/selftests/kvm/x86_64/debug_regs on a DBG
> 	build. This patch causes the suspicious RCU warning to disappear.
> 	Note that the warning is hit in __kvm_zap_rmaps(), so
> 	kvm_memslots_have_rmaps() must return true in order for this to
> 	repro (i.e. the TDP MMU must be off or nesting in use.)
> 
> Reported-by: Greg Thelen <gthelen@google.com>
> Fixes: 36222b117e36 ("KVM: x86: don't disable APICv memslot when inhibited")
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>