Re: [PATCH v2 2/2] x86/kprobes: Prohibit probing on compiler generated CFI checking code

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 11 Jul 2023 10:50:58 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Prohibit probing on the compiler generated CFI typeid checking code
> because it is used for decoding typeid when CFI error happens.
> 
> The compiler generates the following instruction sequence for indirect
> call checks on x86;
> 
>    movl    -<id>, %r10d       ; 6 bytes
>    addl    -4(%reg), %r10d    ; 4 bytes
>    je      .Ltmp1             ; 2 bytes
>    ud2                        ; <- regs->ip
> 
> And handle_cfi_failure() decodes these instructions (movl and addl)
> for the typeid and the target address. Thus if we put a kprobe on
> those instructions, the decode will fail and report a wrong typeid
> and target address.
> 
> 

Hi Peter,

Can I pick this to probes/fixes branch ?

Thank you,

> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/kernel/kprobes/core.c |   34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index f7f6042eb7e6..fa8c2b41cbaf 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -54,6 +54,7 @@
>  #include <asm/insn.h>
>  #include <asm/debugreg.h>
>  #include <asm/ibt.h>
> +#include <asm/cfi.h>
>  
>  #include "common.h"
>  
> @@ -293,7 +294,40 @@ static int can_probe(unsigned long paddr)
>  #endif
>  		addr += insn.length;
>  	}
> +	if (IS_ENABLED(CONFIG_CFI_CLANG)) {
> +		/*
> +		 * The compiler generates the following instruction sequence
> +		 * for indirect call checks and cfi.c decodes this;
> +		 *
> +		 *   movl    -<id>, %r10d       ; 6 bytes
> +		 *   addl    -4(%reg), %r10d    ; 4 bytes
> +		 *   je      .Ltmp1             ; 2 bytes
> +		 *   ud2                        ; <- regs->ip
> +		 *   .Ltmp1:
> +		 *
> +		 * Also, these movl and addl are used for showing expected
> +		 * type. So those must not be touched.
> +		 */
> +		__addr = recover_probed_instruction(buf, addr);
> +		if (!__addr)
> +			return 0;
> +
> +		if (insn_decode_kernel(&insn, (void *)__addr) < 0)
> +			return 0;
> +
> +		if (insn.opcode.value == 0xBA)
> +			offset = 12;
> +		else if (insn.opcode.value == 0x3)
> +			offset = 6;
> +		else
> +			goto out;
> +
> +		/* This movl/addl is used for decoding CFI. */
> +		if (is_cfi_trap(addr + offset))
> +			return 0;
> +	}
>  
> +out:
>  	return (addr == paddr);
>  }
>  
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>