linux-kernel - Re: [PATCH] KVM/x86: remove WARN_ON() for when vm

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Message-ID: <20180201153310.GD31080@flask>
Date:   Thu, 1 Feb 2018 16:33:10 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] KVM/x86: remove WARN_ON() for when vm_munmap() fails

2018-01-31 17:30-0800, Eric Biggers:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> On x86, special KVM memslots such as the TSS region have anonymous
> memory mappings created on behalf of userspace, and these mappings are
> removed when the VM is destroyed.
> 
> It is however possible for removing these mappings via vm_munmap() to
> fail.  This can most easily happen if the thread receives SIGKILL while
> it's waiting to acquire ->mmap_sem.   This triggers the 'WARN_ON(r < 0)'
> in __x86_set_memory_region().  syzkaller was able to hit this, using
> 'exit()' to send the SIGKILL.  Note that while the vm_munmap() failure
> results in the mapping not being removed immediately, it is not leaked
> forever but rather will be freed when the process exits.
> 
> It's not really possible to handle this failure properly, so almost

We could check "r < 0 && r != -EINTR" to get rid of the easily
triggerable warning.

> every other caller of vm_munmap() doesn't check the return value.  It's
> a limitation of having the kernel manage these mappings rather than
> userspace.
> 
> So just remove the WARN_ON() so that users can't spam the kernel log
> with this warning.
> 
> Fixes: f0d648bdf0a5 ("KVM: x86: map/unmap private slots in __x86_set_memory_region")
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---

Removing it altogether doesn't sound that bad, though ...
Queued, thanks.

>  arch/x86/kvm/x86.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c53298dfbf50..53b57f18baec 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8272,10 +8272,8 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
>  			return r;
>  	}
>  
> -	if (!size) {
> -		r = vm_munmap(old.userspace_addr, old.npages * PAGE_SIZE);
> -		WARN_ON(r < 0);
> -	}
> +	if (!size)
> +		vm_munmap(old.userspace_addr, old.npages * PAGE_SIZE);
>  
>  	return 0;
>  }
> -- 
> 2.16.0.rc1.238.g530d649a79-goog
>