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    SubjectRe: [PATCH 0/2] Don’t leave executable TL B entries to freed pages
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    > On Nov 27, 2018, at 5:06 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
    >
    >> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:
    >>
    >> Sometimes when memory is freed via the module subsystem, an executable
    >> permissioned TLB entry can remain to a freed page. If the page is re-used to
    >> back an address that will receive data from userspace, it can result in user
    >> data being mapped as executable in the kernel. The root of this behavior is
    >> vfree lazily flushing the TLB, but not lazily freeing the underlying pages.
    >>
    >> There are sort of three categories of this which show up across modules, bpf,
    >> kprobes and ftrace:
    >>
    >> 1. When executable memory is touched and then immediatly freed
    >>
    >> This shows up in a couple error conditions in the module loader and BPF JIT
    >> compiler.
    >
    > Interesting!
    >
    > Note that this may cause conflict with "x86: avoid W^X being broken during
    > modules loading”, which I recently submitted.

    I actually have not looked on the vmalloc() code too much recent, but it
    seems … strange:

    void vm_unmap_aliases(void)
    {

    ...
    mutex_lock(&vmap_purge_lock);
    purge_fragmented_blocks_allcpus();
    if (!__purge_vmap_area_lazy(start, end) && flush)
    flush_tlb_kernel_range(start, end);
    mutex_unlock(&vmap_purge_lock);
    }

    Since __purge_vmap_area_lazy() releases the memory, it seems there is a time
    window between the release of the region and the TLB flush, in which the
    area can be allocated for another purpose. This can result in a
    (theoretical) correctness issue. No?

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