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

[Eric Blake]

On 03/06/2017 09:42 AM, Peter Maydell wrote:
> 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(+)
>>

>>          {
>> +#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.

And patch 2/2 does that. Does that patch in isolation fix the problem?
In which case, we don't want this patch.

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

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