linux-kernel - re: x86/kprobes: Use int3 instead of debug trap for single-step

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Date:   Wed, 24 Mar 2021 15:48:34 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: re: x86/kprobes: Use int3 instead of debug trap for single-step

Hi,

Static analysis on linux-next using Coverity has detected an issue in
the following commit:

commit 6256e668b7af9d81472e03c6a171630c08f8858a
Author: Masami Hiramatsu <mhiramat@...nel.org>
Date:   Wed Mar 3 00:25:46 2021 +0900

    x86/kprobes: Use int3 instead of debug trap for single-step

The analysis is as follows:

160        switch (opcode & 0xf0) {
161        case 0x60:
162                /* can't boost "bound" */
163                return (opcode != 0x62);
164        case 0x70:
165                return 0; /* can't boost conditional jump */
166        case 0x90:
167                return opcode != 0x9a;  /* can't boost call far */
168        case 0xc0:
169                /* can't boost software-interruptions */
170                return (0xc1 < opcode && opcode < 0xcc) || opcode ==
0xcf;
171        case 0xd0:
172                /* can boost AA* and XLAT */
173                return (opcode == 0xd4 || opcode == 0xd5 || opcode ==
0xd7);
174        case 0xe0:
175                /* can boost in/out and absolute jmps */
176                return ((opcode & 0x04) || opcode == 0xea);
177        case 0xf0:
178                /* clear and set flags are boostable */
179                return (opcode == 0xf5 || (0xf7 < opcode && opcode <
0xfe));

   dead_error_condition: The switch governing value opcode & 0xf0 cannot
be 255.
  undefined (#1 of 1): Logically dead code (DEADCODE)
  dead_error_begin: Execution cannot reach this statement: case 255:

180        case 0xff:
181                /* indirect jmp is boostable */
182               return X86_MODRM_REG(insn->modrm.bytes[0]) == 4;

the case 0xff statement can never be reached because the switch
statement is acting on opcode & 0xf0.

Colin