x86/mm/pti: Clear Global bit more aggressively
commit eac7073aa69aa1cac819aa712146284f53f642b1 upstream.
The kernel image starts out with the Global bit set across the entire
kernel image. The bit is cleared with set_memory_nonglobal() in the
configurations with PCIDs where the performance benefits of the Global bit
are not needed.
However, this is fragile. It means that we are stuck opting *out* of the
less-secure (Global bit set) configuration, which seems backwards. Let's
start more secure (Global bit clear) and then let things opt back in if
they want performance, or are truly mapping common data between kernel and
userspace.
This fixes a bug. Before this patch, there are areas that are unmapped
from the user page tables (like like everything above 0xffffffff82600000 in
the example below). These have the hallmark of being a wrong Global area:
they are not identical in the 'current_kernel' and 'current_user' page
table dumps. They are also read-write, which means they're much more
likely to contain secrets.
Before this patch:
current_kernel:---[ High Kernel Mapping ]---
current_kernel-0xffffffff80000000-0xffffffff81000000 16M pmd
current_kernel-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_kernel-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_kernel-0xffffffff81e11000-0xffffffff82000000 1980K RW GLB NX pte
current_kernel-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_kernel-0xffffffff82600000-0xffffffff82c00000 6M RW PSE GLB NX pmd
current_kernel-0xffffffff82c00000-0xffffffff82e00000 2M RW GLB NX pte
current_kernel-0xffffffff82e00000-0xffffffff83200000 4M RW PSE GLB NX pmd
current_kernel-0xffffffff83200000-0xffffffffa0000000 462M pmd
current_user:---[ High Kernel Mapping ]---
current_user-0xffffffff80000000-0xffffffff81000000 16M pmd
current_user-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_user-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_user-0xffffffff81e11000-0xffffffff82000000 1980K RW GLB NX pte
current_user-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_user-0xffffffff82600000-0xffffffffa0000000 474M pmd
After this patch:
current_kernel:---[ High Kernel Mapping ]---
current_kernel-0xffffffff80000000-0xffffffff81000000 16M pmd
current_kernel-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_kernel-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_kernel-0xffffffff81e11000-0xffffffff82000000 1980K RW NX pte
current_kernel-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_kernel-0xffffffff82600000-0xffffffff82c00000 6M RW PSE NX pmd
current_kernel-0xffffffff82c00000-0xffffffff82e00000 2M RW NX pte
current_kernel-0xffffffff82e00000-0xffffffff83200000 4M RW PSE NX pmd
current_kernel-0xffffffff83200000-0xffffffffa0000000 462M pmd
current_user:---[ High Kernel Mapping ]---
current_user-0xffffffff80000000-0xffffffff81000000 16M pmd
current_user-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_user-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_user-0xffffffff81e11000-0xffffffff82000000 1980K RW NX pte
current_user-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_user-0xffffffff82600000-0xffffffffa0000000 474M pmd
Fixes: 0f561fce4d69 ("x86/pti: Enable global pages for shared areas")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: keescook@google.com
Cc: aarcange@redhat.com
Cc: jgross@suse.com
Cc: jpoimboe@redhat.com
Cc: gregkh@linuxfoundation.org
Cc: peterz@infradead.org
Cc: torvalds@linux-foundation.org
Cc: bp@alien8.de
Cc: luto@kernel.org
Cc: ak@linux.intel.com
Cc: Kees Cook <keescook@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Link: https://lkml.kernel.org/r/20180802225825.A100C071@viggo.jf.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2 files changed
