x86: Increase brk randomness entropy for 64-bit systems

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
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Jiri Kosina <jkosina@suse.com>
Closes: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=dzOUG5E4gV7ayCbu0MPJTZzWkw@mail.gmail.com/
Link: https://lore.kernel.org/r/20240217062545.1631668-1-keescook@chromium.org

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index ab49ade31b0dc46616916d405c694ad6307fdd27..45a9d496fe2a0f04cbd2f753619c739b08f83fd4 100644 (file)
--- 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);
 }
 
 /*