linux-kernel - [tip: x86/urgent] x86/debug: Prevent data breakpoints on cpu

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Date:   Fri, 05 Feb 2021 19:15:11 -0000
From:   "tip-bot2 for Lai Jiangshan" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/debug: Prevent data breakpoints on cpu_dr7

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     3943abf2dbfae9ea4d2da05c1db569a0603f76da
Gitweb:        https://git.kernel.org/tip/3943abf2dbfae9ea4d2da05c1db569a0603f76da
Author:        Lai Jiangshan <laijs@...ux.alibaba.com>
AuthorDate:    Thu, 04 Feb 2021 23:27:07 +08:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Fri, 05 Feb 2021 20:13:12 +01:00

x86/debug: Prevent data breakpoints on cpu_dr7

local_db_save() is called at the start of exc_debug_kernel(), reads DR7 and
disables breakpoints to prevent recursion.

When running in a guest (X86_FEATURE_HYPERVISOR), local_db_save() reads the
per-cpu variable cpu_dr7 to check whether a breakpoint is active or not
before it accesses DR7.

A data breakpoint on cpu_dr7 therefore results in infinite #DB recursion.

Disallow data breakpoints on cpu_dr7 to prevent that.

Fixes: 84b6a3491567a("x86/entry: Optimize local_db_save() for virt")
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/r/20210204152708.21308-2-jiangshanlai@gmail.com

---
 arch/x86/kernel/hw_breakpoint.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 012ed82..668a4a6 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -307,6 +307,14 @@ static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
 				(unsigned long)&per_cpu(cpu_tlbstate, cpu),
 				sizeof(struct tlb_state)))
 			return true;
+
+		/*
+		 * When in guest (X86_FEATURE_HYPERVISOR), local_db_save()
+		 * will read per-cpu cpu_dr7 before clear dr7 register.
+		 */
+		if (within_area(addr, end, (unsigned long)&per_cpu(cpu_dr7, cpu),
+				sizeof(cpu_dr7)))
+			return true;
 	}
 
 	return false;