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Subject[tip:x86/apic] x86/apic: Initialize TPR to block interrupts 16-31
Commit-ID:  229b969b3d38bc28bcd55841ee7ca9a9afb922f3
Gitweb: https://git.kernel.org/tip/229b969b3d38bc28bcd55841ee7ca9a9afb922f3
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Sun, 14 Jul 2019 08:23:14 -0700
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Mon, 22 Jul 2019 10:12:32 +0200

x86/apic: Initialize TPR to block interrupts 16-31

The APIC, per spec, is fundamentally confused and thinks that interrupt
vectors 16-31 are valid. This makes no sense -- the CPU reserves vectors
0-31 for exceptions (faults, traps, etc). Obviously, no device should
actually produce an interrupt with vector 16-31, but robustness can be
improved by setting the APIC TPR class to 1, which will prevent delivery of
an interrupt with a vector below 32.

Note: This is *not* intended as a security measure against attackers who
control malicious hardware. Any PCI or similar hardware that can be
controlled by an attacker MUST be behind a functional IOMMU that remaps
interrupts. The purpose of this change is to reduce the chance that a
certain class of device malfunctions crashes the kernel in hard-to-debug
ways.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/dc04a9f8b234d7b0956a8d2560b8945bcd9c4bf7.1563117760.git.luto@kernel.org

---
arch/x86/kernel/apic/apic.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index f5291362da1a..84032bf81476 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1561,11 +1561,14 @@ static void setup_local_APIC(void)
#endif

/*
- * Set Task Priority to 'accept all'. We never change this
- * later on.
+ * Set Task Priority to 'accept all except vectors 0-31'. An APIC
+ * vector in the 16-31 range could be delivered if TPR == 0, but we
+ * would think it's an exception and terrible things will happen. We
+ * never change this later on.
*/
value = apic_read(APIC_TASKPRI);
value &= ~APIC_TPRI_MASK;
+ value |= 0x10;
apic_write(APIC_TASKPRI, value);

apic_pending_intr_clear();
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