Re: [Qemu-devel] [PATCH v2 2/2] linux-user: allocate heap memory for execve arguments

[Eric Blake <eblake@redhat.com>]

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On 03/06/2017 01:17 AM, P J P wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
> 
> Arguments passed to execve(2) call from user program could
> be large, allocating stack memory for them via alloca(3) call
> would lead to bad behaviour. Use 'g_malloc0' to allocate memory
> for such arguments.
> 
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  linux-user/syscall.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)

Is this patch alone (without 1/2) sufficient to solve the problem?  If
so, then drop 1/2.

> 
> Update per: replace alloca() with g_malloc0()
>   -> 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 86a4a9c..404fb0b 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -7800,8 +7800,8 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,

What version of qemu are you patching?  Line 7800 of current master is
nowhere near 'case TARGET_NR_execve:' (line 7899)

>                  ret = -TARGET_E2BIG;
>                  break;
>              }

and current master has 'goto efault' rather than directly setting ret at
this point.

> -            argp = alloca((argc + 1) * sizeof(void *));
> -            envp = alloca((envc + 1) * sizeof(void *));
> +            argp = g_malloc0((argc + 1) * sizeof(void *));
> +            envp = g_malloc0((envc + 1) * sizeof(void *));

Subject to a potential multiplication overflow.  I'd prefer:

g_new0(void *, argc + 1);

as that is guaranteed to not overflow.

>  
>              for (gp = guest_argp, q = argp; gp;
>                    gp += sizeof(abi_ulong), q++) {
> @@ -7862,6 +7862,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
>                      break;
>                  unlock_user(*q, addr, 0);
>              }
> +
> +            g_free(argp);
> +            g_free(envp);
>          }
>          break;
>      case TARGET_NR_chdir:
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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