Re: [Qemu-trivial] [PATCH] target-ppc: kvm: Fix memory overflow issue about strncat()

[Michael Tokarev <mjt@tls.msk.ru>]

On 10/13/2014 06:47 PM, Alexander Graf wrote:
> On 13.10.14 16:36, Chen Gang wrote:
>> strncat() will append additional '\0' to destination buffer, so need
>> additional 1 byte for it, or may cause memory overflow, just like other
>> area within QEMU have done.
>>
>> Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
> 
> I agree with this patch. However, the code is pretty ugly - I'm sure it
> must've been me who wrote it :).
> 
> Could you please instead rewrite it to use g_strdup_printf() rather than
> strncat()s? That way we resolve all string pitfalls automatically - and
> this code is not the fast path, so doing an extra memory allocation is ok.

I'd just use snprintf() like this:

diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 9c23c6b..5eaa36c 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1794,8 +1794,7 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
         return -1;
     }

-    strncat(buf, "/", sizeof(buf) - strlen(buf));
-    strncat(buf, propname, sizeof(buf) - strlen(buf));
+    snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "/%s", propname);

     f = fopen(buf, "rb");
     if (!f) {

the buffer is of size PATH_MAX, and we're looking at /proc filesystem where
names should be rather short so we're extremly unlikely to hit this prob in
practice, there's no need to dynamically allocate a buffer for this stuff.

(Or alternatively there's asprintf(), but still I think it is overkill).

I can apply the above if everyone agrees.

Thanks,

/mjt

>>  target-ppc/kvm.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
>> index 9c23c6b..66e7ce5 100644
>> --- a/target-ppc/kvm.c
>> +++ b/target-ppc/kvm.c
>> @@ -1794,8 +1794,8 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
>>          return -1;
>>      }
>>  
>> -    strncat(buf, "/", sizeof(buf) - strlen(buf));
>> -    strncat(buf, propname, sizeof(buf) - strlen(buf));
>> +    strncat(buf, "/", sizeof(buf) - strlen(buf) - 1);
>> +    strncat(buf, propname, sizeof(buf) - strlen(buf) - 1);
>>  
>>      f = fopen(buf, "rb");
>>      if (!f) {
>>
>