linux-kernel - Re: [PATCH v2 7/7] KVM: selftests: Avoid infinite loop if ucall

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Date:   Mon, 12 Dec 2022 11:38:47 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Oliver Upton <oliver.upton@...ux.dev>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Jones <andrew.jones@...ux.dev>,
        Peter Gonda <pgonda@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        Ricardo Koller <ricarkol@...gle.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/7] KVM: selftests: Avoid infinite loop if
 ucall_alloc() fails

On 12/9/22 22:03, Sean Christopherson wrote:
> From: Sean Christopherson<seanjc@...gle.com>
> Date: Fri, 9 Dec 2022 12:55:44 -0800
> Subject: [PATCH] KVM: selftests: Use magic value to signal ucall_alloc()
>   failure
> 
> Use a magic value to signal a ucall_alloc() failure instead of simply
> doing GUEST_ASSERT().  GUEST_ASSERT() relies on ucall_alloc() and so a
> failure puts the guest into an infinite loop.
> 
> Use -1 as the magic value, as a real ucall struct should never wrap.
> 
> Reported-by: Oliver Upton<oliver.upton@...ux.dev>
> Signed-off-by: Sean Christopherson<seanjc@...gle.com>
> ---
>   tools/testing/selftests/kvm/lib/ucall_common.c | 16 ++++++++++++++--
>   1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
> index 0cc0971ce60e..2f0e2ea941cc 100644
> --- a/tools/testing/selftests/kvm/lib/ucall_common.c
> +++ b/tools/testing/selftests/kvm/lib/ucall_common.c
> @@ -4,6 +4,8 @@
>   #include "linux/bitmap.h"
>   #include "linux/atomic.h"
>   
> +#define GUEST_UCALL_FAILED -1
> +
>   struct ucall_header {
>   	DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
>   	struct ucall ucalls[KVM_MAX_VCPUS];
> @@ -41,7 +43,8 @@ static struct ucall *ucall_alloc(void)
>   	struct ucall *uc;
>   	int i;
>   
> -	GUEST_ASSERT(ucall_pool);
> +	if (!ucall_pool)
> +		goto ucall_failed;
>   
>   	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>   		if (!test_and_set_bit(i, ucall_pool->in_use)) {
> @@ -51,7 +54,13 @@ static struct ucall *ucall_alloc(void)
>   		}
>   	}
>   
> -	GUEST_ASSERT(0);
> +ucall_failed:
> +	/*
> +	 * If the vCPU cannot grab a ucall structure, make a bare ucall with a
> +	 * magic value to signal to get_ucall() that things went sideways.
> +	 * GUEST_ASSERT() depends on ucall_alloc() and so cannot be used here.
> +	 */
> +	ucall_arch_do_ucall(GUEST_UCALL_FAILED);
>   	return NULL;
>   }
>   
> @@ -93,6 +102,9 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
>   
>   	addr = ucall_arch_get_ucall(vcpu);
>   	if (addr) {
> +		TEST_ASSERT(addr != (void *)GUEST_UCALL_FAILED,
> +			    "Guest failed to allocate ucall struct");
> +
>   		memcpy(uc, addr, sizeof(*uc));
>   		vcpu_run_complete_io(vcpu);
>   	} else {
> 
> base-commit: dc2efbe4813e0dc4368779bc36c5f0e636cb8eb2
> -- 

Queued, thanks.

Paolo