linux-hardening - [PATCH 1/2] x86: Increase brk randomness entropy on x86

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Date: Fri, 16 Feb 2024 22:25:43 -0800
From: Kees Cook <keescook@...omium.org>
To: Jiri Kosina <jikos@...nel.org>
Cc: Kees Cook <keescook@...omium.org>,
	y0un9n132@...il.com,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Qi Zheng <zhengqi.arch@...edance.com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	x86@...nel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>,
	Eric Biederman <ebiederm@...ssion.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Brian Gerst <brgerst@...il.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Tony Battersby <tonyb@...ernetics.com>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH 1/2] x86: Increase brk randomness entropy on x86_64

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@...il.com
Closes: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=dzOUG5E4gV7ayCbu0MPJTZzWkw@mail.gmail.com/
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Cc: Jiri Kosina <jikos@...nel.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Qi Zheng <zhengqi.arch@...edance.com>
Cc: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: x86@...nel.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