[tip: x86/urgent] x86: Clear .brk area at early boot

[tip-bot2 for Juergen Gross]

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

Commit-ID:     38fa5479b41376dc9d7f57e71c83514285a25ca0
Gitweb:        https://git.kernel.org/tip/38fa5479b41376dc9d7f57e71c83514285a25ca0
Author:        Juergen Gross <jgross@xxxxxxxx>
AuthorDate:    Thu, 30 Jun 2022 09:14:40 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Fri, 01 Jul 2022 11:11:34 +02:00

x86: Clear .brk area at early boot

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@xxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lore.kernel.org/r/20220630071441.28576-3-jgross@xxxxxxxx
---
 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 e7e2332..6a3cfaf 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)