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From: Joao Moreira <joao@overdrivepizza.com>
To: Peter Collingbourne <pcc@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Kees Cook <keescook@chromium.org>,
	x86@kernel.org, hjl.tools@gmail.com, jpoimboe@redhat.com,
	andrew.cooper3@citrix.com, linux-kernel@vger.kernel.org,
	ndesaulniers@google.com, samitolvanen@google.com,
	llvm@lists.linux.dev
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups
Date: Tue, 01 Mar 2022 19:32:47 -0800	[thread overview]
Message-ID: <f07f50b634e160f543c2a77ec39a2b4f@overdrivepizza.com> (raw)
In-Reply-To: <Yh7fLRYl8KgMcOe5@google.com>

On 2022-03-01 19:06, Peter Collingbourne wrote:
> Hi Peter,
> 
> One issue with this call sequence is that:
> 
> On Fri, Feb 11, 2022 at 02:38:03PM +0100, Peter Zijlstra wrote:
>> caller:
>> 	cmpl	$0xdeadbeef, -0x4(%rax)		# 7 bytes
> 
> Because this instruction ends in the constant 0xdeadbeef, it may
> be used as a "gadget" that would effectively allow branching to an
> arbitrary address in %rax if the attacker can arrange to set ZF=1.
> 
>> 	je	1f				# 2 bytes
>> 	ud2					# 2 bytes
>> 1:	call	__x86_indirect_thunk_rax	# 5 bytes
>> 
>> 
>> 	.align 16
>> 	.byte 0xef, 0xbe, 0xad, 0xde		# 4 bytes
>> func:
>> 	endbr					# 4 bytes
>> 	...
>> 	ret
> 
> I think we can avoid this problem with a slight tweak to your
> instruction sequence, at the cost of 2 bytes per function prologue.
> First, change the call sequence like so:
> 
>  	cmpl	$0xdeadbeef, -0x6(%rax)		# 6 bytes
> 	je	1f				# 2 bytes
> 	ud2					# 2 bytes
> 1:	call	__x86_indirect_thunk_rax	# 5 bytes
> 
> The key difference is that we've changed 0x4 to 0x6.
> 
> Then change the function prologue to this:
> 
> 	.align 16
> 	.byte 0xef, 0xbe, 0xad, 0xde		# 4 bytes
> 	.zero 2					# 2 bytes
> func:
> 
> The end result of the above is that the constant embedded in the cmpl
> instruction may only be used to reach the following ud2 instruction,
> which will "harmlessly" terminate execution in the same way as if
> the prologue signature did not match.

FWIIW, this makes sense IMHO. These additional pre-prologue bytes are 
also what might be needed to enable the smaller version of FineIBT that 
I suggested in this thread some time ago.

  reply	other threads:[~2022-03-02  3:32 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-22 17:03 [RFC][PATCH 0/6] x86: Kernel IBT beginnings Peter Zijlstra
2021-11-22 17:03 ` [RFC][PATCH 1/6] x86: Annotate _THIS_IP_ Peter Zijlstra
2021-11-23 13:53   ` Mark Rutland
2021-11-23 14:14     ` Peter Zijlstra
2021-11-24 18:18       ` Josh Poimboeuf
2021-11-22 17:03 ` [RFC][PATCH 2/6] x86: Base IBT bits Peter Zijlstra
2022-02-08 23:32   ` Kees Cook
2021-11-22 17:03 ` [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self Peter Zijlstra
2021-11-22 18:09   ` Peter Zijlstra
2022-02-08 23:33     ` Kees Cook
2021-11-22 17:03 ` [RFC][PATCH 4/6] objtool: Read the _THIS_IP_ hints Peter Zijlstra
2021-11-22 17:03 ` [RFC][PATCH 5/6] x86: Sprinkle ENDBR dust Peter Zijlstra
2021-11-23 14:00   ` Mark Rutland
2021-11-23 14:21     ` Peter Zijlstra
2022-02-08 23:38     ` Kees Cook
2021-11-22 17:03 ` [RFC][PATCH 6/6] objtool: Add IBT validation / fixups Peter Zijlstra
2021-11-24 19:30   ` Josh Poimboeuf
2022-02-08 23:43     ` Kees Cook
2022-02-09  5:09       ` Josh Poimboeuf
2022-02-09 11:41       ` Peter Zijlstra
2022-02-09 11:45         ` Peter Zijlstra
2021-12-24  2:05   ` joao
2022-02-08 23:42     ` Kees Cook
2022-02-09  2:21       ` Joao Moreira
2022-02-09  4:05         ` Kees Cook
2022-02-09  5:18           ` Joao Moreira
2022-02-11 13:38             ` Peter Zijlstra
2022-02-14 21:38               ` Sami Tolvanen
2022-02-14 22:25                 ` Peter Zijlstra
2022-02-15 16:56                   ` Sami Tolvanen
2022-02-15 20:03                     ` Kees Cook
2022-02-15 21:05                       ` Peter Zijlstra
2022-02-15 23:05                         ` Kees Cook
2022-02-15 23:38                           ` Joao Moreira
2022-02-16 12:24                         ` Peter Zijlstra
2022-02-15 20:53                     ` Peter Zijlstra
2022-02-15 22:45               ` Joao Moreira
2022-02-16  0:57               ` Andrew Cooper
2022-03-02  3:06               ` Peter Collingbourne
2022-03-02  3:32                 ` Joao Moreira [this message]
2022-06-08 17:53                 ` Fāng-ruì Sòng
2022-06-09  0:05                   ` Sami Tolvanen
2021-11-23  7:58 ` [RFC][PATCH 0/6] x86: Kernel IBT beginnings Christoph Hellwig
2021-11-23  9:02   ` Peter Zijlstra
2022-02-08 23:48 ` Kees Cook
2022-02-09  0:09 ` Nick Desaulniers

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