Patch "KVM/x86: remove WARN_ON() for when vm_munmap() fails" has been added to the 4.14-stable tree

Patch "KVM/x86: remove WARN_ON() for when vm_munmap() fails" has been added to the 4.14-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    KVM/x86: remove WARN_ON() for when vm_munmap() fails

to the 4.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-x86-remove-warn_on-for-when-vm_munmap-fails.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 103c763c72dd2df3e8c91f2d7ec88f98ed391111 Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Wed, 31 Jan 2018 17:30:21 -0800
Subject: KVM/x86: remove WARN_ON() for when vm_munmap() fails
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Eric Biggers <ebiggers@google.com>

commit 103c763c72dd2df3e8c91f2d7ec88f98ed391111 upstream.

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
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@googlegroups.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/x86/kvm/x86.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8251,10 +8251,8 @@ int __x86_set_memory_region(struct kvm *
 			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;
 }


Patches currently in stable-queue which might be from ebiggers@google.com are

queue-4.14/kvm-x86-remove-warn_on-for-when-vm_munmap-fails.patch