Messages in this thread | | | From | David Laight <> | Subject | RE: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG | Date | Mon, 16 May 2022 08:32:18 +0000 |
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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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