Re: [PATCH v2 01/11] linux-user: handle /proc/self/exe for execve

[Laurent Vivier]

Le 31/05/2021 à 07:50, YAMAMOTO Takashi a écrit :
> It seems somehow common to execve /proc/self/exe in docker
> or golang community these days.
> At least, moby "reexec" and runc "libcontainer" do that.
> 
> Signed-off-by: YAMAMOTO Takashi <yamamoto@midokura.com>
> ---
>  linux-user/syscall.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index c9f812091c..a2b03ecb8b 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -8470,6 +8470,7 @@ static abi_long do_syscall1(void *cpu_env, int num, 
> abi_long arg1,
>  #endif
>      case TARGET_NR_execve:
>          {
> +            const char *path;
>              char **argp, **envp;
>              int argc, envc;
>              abi_ulong gp;
> @@ -8537,7 +8538,11 @@ static abi_long do_syscall1(void *cpu_env, int num, 
> abi_long arg1,
>               * before the execve completes and makes it the other
>               * program's problem.
>               */
> -            ret = get_errno(safe_execve(p, argp, envp));
> +            path = p;
> +            if (is_proc_myself(path, "exe")) {
> +                path = exec_path;
> +            }
> +            ret = get_errno(safe_execve(path, argp, envp));
>              unlock_user(p, arg1, 0);
>  
>              goto execve_end;
> 

The problem here is QEMU can fail to execute the file directly.

The binary can be launched with binfmt_misc and the 'O' flag:

     ``O`` - open-binary
            Legacy behavior of binfmt_misc is to pass the full path
            of the binary to the interpreter as an argument. When this flag is
            included, binfmt_misc will open the file for reading and pass its
            descriptor as an argument, instead of the full path, thus allowing
            the interpreter to execute non-readable binaries. This feature
            should be used with care - the interpreter has to be trusted not to
            emit the contents of the non-readable binary.

You should use do_openat() (that resolves the /proc/self/exe path) and 
fexecve().

Thanks,
Laurent