x86: Clear .brk area at early boot

[ Upstream commit 9f89c64452a62e9845e3741058ce24605fed9bfc ]

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>

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 206a4b6144c2e1d9a0780a82424c4817bf390d7d..950286016f63c616dfbe8a6a71a732a3f1d6e8ce 100644 (file)
--- a/arch/x86/kernel/head64.c
+++ b/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)