linux-kernel - [PATCH 29/29] x86: Use INT3 instead of NOP for linker fill bytes

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Message-Id: <20190926175602.33098-30-keescook@chromium.org>
Date:   Thu, 26 Sep 2019 10:56:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Kees Cook <keescook@...omium.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-c6x-dev@...ux-c6x.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Michal Simek <monstr@...str.eu>, linux-parisc@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 29/29] x86: Use INT3 instead of NOP for linker fill bytes

Instead of using 0x90 (NOP) to fill bytes between functions, which makes
it easier to sloppily target functions in function pointer overwrite
attacks, fill with 0xCC (INT3) to force a trap. Also drops the space
between "=" and the value to better match the binutils documentation
https://sourceware.org/binutils/docs/ld/Output-Section-Fill.html#Output-Section-Fill

Example "objdump -d" before:

...
ffffffff810001e0 <start_cpu0>:
ffffffff810001e0:       48 8b 25 e1 b1 51 01    mov 0x151b1e1(%rip),%rsp        # ffffffff8251b3c8 <initial_stack>
ffffffff810001e7:       e9 d5 fe ff ff          jmpq   ffffffff810000c1 <secondary_startup_64+0x91>
ffffffff810001ec:       90                      nop
ffffffff810001ed:       90                      nop
ffffffff810001ee:       90                      nop
ffffffff810001ef:       90                      nop

ffffffff810001f0 <__startup_64>:
...

After:

...
ffffffff810001e0 <start_cpu0>:
ffffffff810001e0:       48 8b 25 41 79 53 01    mov 0x1537941(%rip),%rsp        # ffffffff82537b28 <initial_stack>
ffffffff810001e7:       e9 d5 fe ff ff          jmpq   ffffffff810000c1 <secondary_startup_64+0x91>
ffffffff810001ec:       cc                      int3
ffffffff810001ed:       cc                      int3
ffffffff810001ee:       cc                      int3
ffffffff810001ef:       cc                      int3

ffffffff810001f0 <__startup_64>:
...

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 arch/x86/kernel/vmlinux.lds.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index a5c8571e4967..a37817fafb22 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -144,7 +144,7 @@ SECTIONS
 		*(.text.__x86.indirect_thunk)
 		__indirect_thunk_end = .;
 #endif
-	} :text = 0x9090
+	} :text =0xcccc
 
 	/* End of text section, which should occupy whole number of pages */
 	_etext = .;
-- 
2.17.1