[PATCH 1/2] x86: Increase brk randomness entropy on x86_64

[Kees Cook <keescook@chromium.org>]

In commit c1d171a00294 ("x86: randomize brk"), arch_randomize_brk() was
defined to use a 32MB range (13 bits of entropy), but was never increased
when moving to 64-bit. The default arch_randomize_brk() uses 32MB for
32-bit tasks, and 1GB (18 bits of entropy) for 64-bit tasks. Update
x86_64 to match the entropy used by arm64 and other 64-bit architectures.

Reported-by: y0un9n132@gmail.com
Closes: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=dzOUG5E4gV7ayCbu0MPJTZzWkw@mail.gmail.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: x86@kernel.org
---
 arch/x86/kernel/process.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index ab49ade31b0d..45a9d496fe2a 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -1030,7 +1030,10 @@ unsigned long arch_align_stack(unsigned long sp)
 
 unsigned long arch_randomize_brk(struct mm_struct *mm)
 {
-	return randomize_page(mm->brk, 0x02000000);
+	if (mmap_is_ia32())
+		return randomize_page(mm->brk, SZ_32M);
+
+	return randomize_page(mm->brk, SZ_1G);
 }
 
 /*
-- 
2.34.1