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Date: Tue, 27 Nov 2018 16:07:52 -0800
From: Rick Edgecombe <rick.p.edgecombe@...el.com>
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Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: [PATCH 0/2] Don’t leave executable TLB entries to freed pages
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.
2. When executable memory is set to RW right before being freed
In this case (on x86 and probably others) there will be a TLB flush when its
set to RW and so since the pages are not touched between setting the
flush and the free, it should not be in the TLB in most cases. So this
category is not as big of a concern. However, techinically there is still a
race where an attacker could try to keep it alive for a short window with a
well timed out-of-bound read or speculative read, so ideally this could be
blocked as well.
3. When executable memory is freed in an interrupt
At least one example of this is the freeing of init sections in the module
loader. Since vmalloc reuses the allocation for the work queue linked list
node for the deferred frees, the memory actually gets touched as part of the
vfree operation and so returns to the TLB even after the flush from resetting
the permissions.
I have only actually tested category 1, and identified 2 and 3 just from reading
the code.
To catch all of these, module_alloc for x86 is changed to use a new flag that
instructs the unmap operation to flush the TLB before freeing the pages.
If this solution seems good I can plug the flag in for other architectures that
define PAGE_KERNEL_EXEC.
Rick Edgecombe (2):
vmalloc: New flag for flush before releasing pages
x86/modules: Make x86 allocs to flush when free
arch/x86/kernel/module.c | 4 ++--
include/linux/vmalloc.h | 1 +
mm/vmalloc.c | 13 +++++++++++--
3 files changed, 14 insertions(+), 4 deletions(-)
--
2.17.1
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