linux-kernel - [PATCH 3/3] x86/perf/hw_breakpoint: Fix check for kernelspace breakpoints

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Message-Id: <136be387950e78f18cea60e9d1bef74465d0ee8f.1438312874.git.luto@kernel.org>
Date:	Thu, 30 Jul 2015 20:32:42 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Cc:	Brian Gerst <brgerst@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	X86 ML <x86@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 3/3] x86/perf/hw_breakpoint: Fix check for kernelspace breakpoints

The check looked wrong, although I think it was actually safe.  TASK_SIZE
is unnecessarily small for compat tasks, and it wasn't possible to make
a range breakpoint so large it started in user space and ended in kernel
space.

Nonetheless, let's fix up the check for the benefit of future
readers.  A breakpoint is in the kernel if either end is in the
kernel.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/hw_breakpoint.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 6f345d302cf6..50a3fad5b89f 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -180,7 +180,11 @@ int arch_check_bp_in_kernelspace(struct perf_event *bp)
 	va = info->address;
 	len = bp->attr.bp_len;
 
-	return (va >= TASK_SIZE) && ((va + len - 1) >= TASK_SIZE);
+	/*
+	 * We don't need to worry about va + len - 1 overflowing:
+	 * we already require that va is aligned to a multiple of len.
+	 */
+	return (va >= TASK_SIZE_MAX) || ((va + len - 1) >= TASK_SIZE_MAX);
 }
 
 int arch_bp_generic_fields(int x86_len, int x86_type,
-- 
2.4.3

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