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Message-Id: <20230726122317.9058a6bed272835f0c1a3b29@kernel.org>
Date: Wed, 26 Jul 2023 12:23:17 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Petr Pavlu <petr.pavlu@...e.com>, tglx@...utronix.de,
mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
hpa@...or.com, samitolvanen@...gle.com, x86@...nel.org,
linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] x86/kprobes: Prohibit probing on compiler
generated CFI checking code
On Tue, 11 Jul 2023 10:50:58 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> From: Masami Hiramatsu (Google) <mhiramat@...nel.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@...nel.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.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@...nel.org>
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