linux-kernel - [PATCH AUTOSEL 4.9 3/4] x86: Clear .brk area at early boot

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Message-Id: <20220714042707.282675-3-sashal@kernel.org>
Date:   Thu, 14 Jul 2022 00:27:06 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Juergen Gross <jgross@...e.com>, Borislav Petkov <bp@...e.de>,
        Sasha Levin <sashal@...nel.org>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, brijesh.singh@....com, michael.roth@....com,
        ak@...ux.intel.com, thomas.lendacky@....com
Subject: [PATCH AUTOSEL 4.9 3/4] x86: Clear .brk area at early boot

From: Juergen Gross <jgross@...e.com>

[ Upstream commit 38fa5479b41376dc9d7f57e71c83514285a25ca0 ]

The .brk section has the same properties as .bss: it is an alloc-only
section and should be cleared before being used.

Not doing so is especially a problem for Xen PV guests, as the
hypervisor will validate page tables (check for writable page tables
and hypervisor private bits) before accepting them to be used.

Make sure .brk is initially zero by letting clear_bss() clear the brk
area, too.

Signed-off-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lore.kernel.org/r/20220630071441.28576-3-jgross@suse.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/kernel/head64.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index b5785c197e53..2b2060d842d1 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -106,6 +106,8 @@ static void __init clear_bss(void)
 {
 	memset(__bss_start, 0,
 	       (unsigned long) __bss_stop - (unsigned long) __bss_start);
+	memset(__brk_base, 0,
+	       (unsigned long) __brk_limit - (unsigned long) __brk_base);
 }
 
 static unsigned long get_cmd_line_ptr(void)
-- 
2.35.1