RE: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

[David Laight <David.Laight@ACULAB.COM>]

From: Sami Tolvanen
> Sent: 13 May 2022 21:22
> 
> With CONFIG_CFI_CLANG, the compiler injects a type preamble
> immediately before each function and a check to validate the target
> function type before indirect calls:
> 
>   ; type preamble
>   __cfi_function:
>     int3
>     int3
>     mov <id>, %eax

Interesting - since this code can't be executed there is no
point adding an instruction 'prefix' to the 32bit constant.
 
>     int3
>     int3
>   function:
>     ...
>   ; indirect call check
>     cmpl    <id>, -6(%r11)
>     je      .Ltmp1
>     ud2
>   .Ltmp1:
>     call    __x86_indirect_thunk_r11
> 
> Define the __CFI_TYPE helper macro for manual type annotations in
> assembly code, add error handling for the CFI ud2 traps, and allow
> CONFIG_CFI_CLANG to be selected on x86_64.
> 
...
> +
> +	/*
> +	 * The compiler generates the following instruction sequence
> +	 * for indirect call checks:
> +	 *
> +	 *   cmpl    <id>, -6(%reg)	; 7 bytes

If the <id> is between -128 and 127 then an 8bit constant
(sign extended) might be used.
Possibly the compiler forces the assembler to generate the
long form.

There could also be a REX prefix.
That will break any code that tries to use %reg.

> +	 *   je      .Ltmp1		; 2 bytes
> +	 *   ud2			; <- addr
> +	 *   .Ltmp1:
> +	 *
> +	 * Both the type and the target address can be decoded from the
> +	 * cmpl instruction.
> +	 */
> +	if (copy_from_kernel_nofault(buffer, (void *)regs->ip - 9, MAX_INSN_SIZE))
> +		return;
> +	if (insn_decode_kernel(&insn, buffer))
> +		return;
> +	if (insn.opcode.value != 0x81 || X86_MODRM_REG(insn.modrm.value) != 7)
> +		return;

Since you are looking for a very specific opcode why bother
calling insn_decode_kernel() - just check for the required (masked)
byte values.

> +
> +	*type = insn.immediate.value;
> +
> +	offset = insn_get_modrm_rm_off(&insn, regs);

Given the expected instruction, isn't that -6 ??
> +	if (offset < 0)
> +		return;
> +
> +	*target = *(unsigned long *)((void *)regs + offset);

WTF is that calculating??

> +}
...

	David

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