Re: [Qemu-devel] [PATCH] Avoid crash in epoll_ctl with EPOLL_CTL_DEL

[Laurent Vivier]

Le 30/05/2019 à 18:00, Laurent Vivier a écrit :
> Le 30/05/2019 à 17:25, Giuseppe Musacchio a écrit :
>> The `event` parameter is ignored by the kernel if `op` is EPOLL_CTL_DEL,
>> do the same and avoid returning EFAULT if garbage is passed instead of a
>> valid pointer.
>>
>> Signed-off-by: Giuseppe Musacchio <address@hidden>
>> ---
>>  linux-user/syscall.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
>> index 5e29e675e9..32d463d58d 100644
>> --- a/linux-user/syscall.c
>> +++ b/linux-user/syscall.c
>> @@ -11329,7 +11329,7 @@ static abi_long do_syscall1(void *cpu_env, int
>> num, abi_long arg1,
>>      {
>>          struct epoll_event ep;
>>          struct epoll_event *epp = 0;
>> -        if (arg4) {
>> +        if (arg2 != EPOLL_CTL_DEL && arg4) {
>>              struct target_epoll_event *target_ep;
>>              if (!lock_user_struct(VERIFY_READ, target_ep, arg4, 1)) {
>>                  return -TARGET_EFAULT;
> 
> Reviewed-by: Laurent Vivier <address@hidden>
> 

in fact, the BUGS section of epoll_ctl(2) says:

"In kernel versions before 2.6.9, the EPOLL_CTL_DEL operation required a
 non-null pointer in event, even though this argument is ignored.  Since
 Linux 2.6.9, event can be specified as NULL when  using  EPOLL_CTL_DEL.
 Applications  that  need  to be portable to kernels before 2.6.9 should
 specify a non-null pointer in event."

So something like this would be more portable:

@@ -11329,6 +11329,7 @@ static abi_long do_syscall1(void *cpu_env, int
num, abi_long arg1,
         struct epoll_event ep;
         struct epoll_event *epp = 0;
         if (arg4) {
+            if (arg2 != EPOLL_CTL_DEL) {
                 struct target_epoll_event *target_ep;
                 if (!lock_user_struct(VERIFY_READ, target_ep, arg4, 1)) {
                     return -TARGET_EFAULT;
@@ -11340,6 +11341,11 @@ static abi_long do_syscall1(void *cpu_env, int
num, abi_long arg1,
                  */
                 ep.data.u64 = tswap64(target_ep->data.u64);
                 unlock_user_struct(target_ep, arg4, 0);
+            }
+            /*
+             * before kernel 2.6.9, EPOLL_CTL_DEL operation required a
+             * non-null pointer, even though this argument is ignored.
+             * */
             epp = &ep;
         }

Thanks,
Laurent