2018-01-31 17:30-0800, Eric Biggers:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
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
>
