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Subject[PATCH v3] [x86] detect and report lack of NX protections
It is possible for x86_64 systems to lack the NX bit (see check_efer())
either due to the hardware lacking support or the BIOS having turned
off the CPU capability, so NX status should be reported. Additionally,
anyone booting NX-capable CPUs in 32bit mode without PAE will lack NX
functionality, so this change provides feedback for that case as well.

v2: use "Alert:" instead of "Warning:" to avoid confusion with WARN_ON()
v3: use "Notice:" instead of "Alert:" to avoid confusion with KERN_ALERT,
and switch to KERN_NOTICE, in keeping with its use for "normal but
significant condition" messages.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
---
arch/x86/mm/init.c | 10 ++++++++++
arch/x86/mm/setup_nx.c | 2 ++
2 files changed, 12 insertions(+), 0 deletions(-)
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 73ffd55..8472293 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -149,6 +149,16 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
set_nx();
if (nx_enabled)
printk(KERN_INFO "NX (Execute Disable) protection: active\n");
+ else if (cpu_has_pae)
+#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
+ /* PAE kernel, PAE CPU, without NX */
+ printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
+ "missing in CPU or disabled in BIOS!\n");
+#else
+ /* 32bit non-PAE kernel, PAE CPU */
+ printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
+ "cannot be enabled: non-PAE kernel!\n");
+#endif

/* Enable PSE if available */
if (cpu_has_pse)
diff --git a/arch/x86/mm/setup_nx.c b/arch/x86/mm/setup_nx.c
index 513d8ed..b039a4c 100644
--- a/arch/x86/mm/setup_nx.c
+++ b/arch/x86/mm/setup_nx.c
@@ -53,6 +53,8 @@ void __init set_nx(void)
#else
void set_nx(void)
{
+ /* notice if _PAGE_NX was removed during check_efer() */
+ nx_enabled = ((__supported_pte_mask & _PAGE_NX) == _PAGE_NX);
}
#endif

--
1.6.3.3

--
Kees Cook
Ubuntu Security Team


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