Re: [Qemu-devel] [PATCH v2 1/2] linux-user: limit number of arguments to execve

[Peter Maydell]

On 6 March 2017 at 07:17, P J P <address@hidden> wrote:
> From: Prasad J Pandit <address@hidden>
>
> Limit the number of arguments passed to execve(2) call from
> a user program, as large number of them could lead to a bad
> guest address error.
>
> Reported-by: Jann Horn <address@hidden>
> Signed-off-by: Prasad J Pandit <address@hidden>
> ---
>  linux-user/syscall.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> Update per: use gemu_log() to report error
>   -> https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg00750.html
>
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 9be8e95..86a4a9c 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -7766,6 +7766,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> arg1,
>  #endif
>      case TARGET_NR_execve:
>          {
> +#define ARG_MAX 65535
>              char **argp, **envp;
>              int argc, envc;
>              abi_ulong gp;
> @@ -7794,6 +7795,11 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> arg1,
>                  envc++;
>              }
>
> +            if (argc > ARG_MAX || envc > ARG_MAX) {
> +                gemu_log("argc(%d), envc(%d) exceed %d\n", argc, envc, 
> ARG_MAX);
> +                ret = -TARGET_E2BIG;
> +                break;
> +            }
>              argp = alloca((argc + 1) * sizeof(void *));
>              envp = alloca((envc + 1) * sizeof(void *));


We need to fix this by not using alloca(), not by imposing
an arbitrary limit that's still rather over-large for an
alloca allocation, as Eric suggested.

thanks
-- PMM