x86: Clear .brk area at early boot

stable inclusion
from stable-4.19.253
commit 36e2f161fb01795722f2ff1a24d95f08100333dd
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I5JTYM


CVE: CVE-2022-36123

--------------------------------

[ 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@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220630071441.28576-3-jgross@suse.com


Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>

arch/x86/kernel/head64.c

@@ -383,6 +383,8 @@ 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)