linux-kernel - [PATCH 2/2] x86: fix setup of brk area

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Date:   Wed, 22 Jun 2022 18:10:48 +0200
From:   Juergen Gross <jgross@...e.com>
To:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 2/2] x86: fix setup of brk area

Commit e32683c6f7d2 ("x86/mm: Fix RESERVE_BRK() for older binutils")
put the brk area into the .bss segment, causing it not to be cleared
initially. As the brk area is used to allocate early page tables, these
might contain garbage in not explicitly written entries.

This 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. There have been reports
of early crashes of PV guests due to illegal page table contents.

Fix that by letting clear_bss() clear the brk area, too.

Fixes: e32683c6f7d2 ("x86/mm: Fix RESERVE_BRK() for older binutils")
Signed-off-by: Juergen Gross <jgross@...e.com>
---
 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 e7e233209a8c..6a3cfaf6b72a 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -430,6 +430,8 @@ 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)
-- 
2.35.3