Re: [PATCH] KVM/x86: remove WARN_ON() for when vm_munmap() fails

[Eric Biggers <ebiggers3@gmail.com>]

On Thu, Feb 01, 2018 at 12:12:00PM -0500, Paolo Bonzini wrote:
> On 01/02/2018 10:33, Radim KrA?mA!A? wrote:
> > 2018-01-31 17:30-0800, Eric Biggers:
> >> From: Eric Biggers <ebiggers@google.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.
> 
> Considering that vm_munmap uses down_write_killable, that would be
> preferrable I think.
> 

Don't be so sure that vm_munmap() can't fail for other reasons as well :-)
Remember, userspace can mess around with its address space.

And indeed, looking closer, I see there was a previous report of this same WARN
on an older kernel which in vm_munmap() still had down_write() instead of
down_write_killable().  The reproducer in that case concurrently called
personality(ADDR_LIMIT_3GB) to reduce its address limit after the mapping was
already created above 3 GiB.  Then the vm_munmap() returned EINVAL since
'start > TASK_SIZE'.

So I don't think we should check for specific error codes.  We could make it a
pr_warn_ratelimited() though, if we still want some notification that there was
a problem without implying it is a kernel bug as WARN_ON() does.

- Eric

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