x86: Clear .brk area at early boot

[ 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.

RELEASE_NOTE=Fixed CVE-2022-36123 in the Linux kernel.

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>

cos-patch: security-moderate
Change-Id: I19bd20c3fc96f18078c5edeb798307c6ec0e1420
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/36068
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 206a4b6..9502860 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -383,6 +383,8 @@
 	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)