Re: [PATCH V2 1/4] selftests/sgx: Fix NULL-pointer-dereference upon early test failure

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Shuah and Dave,

On 2/2/2022 10:01 AM, Shuah Khan wrote:
> On 2/1/22 3:47 PM, Reinette Chatre wrote:
>> == Background ==
>>
>> The SGX selftests track parts of the enclave binaries in an array:
>> encl->segment_tbl[]. That array is dynamically allocated early
>> (but not first) in the test's lifetime. The array is referenced
>> at the end of the test in encl_delete().
>>
>> == Problem ==
>>
>> encl->segment_tbl[] can be NULL if the test fails before its
>> allocation. That leads to a NULL-pointer-dereference in encl_delete().
>> This is triggered during early failures of the selftest like if the
>> enclave binary ("test_encl.elf") is deleted.
>>
>> == Solution ==
>>
> 
> "==" usage looks a bit odd in the change log.

This is a new trend in the x86/ area and I was asked to modify the commit
message to follow suit in:
https://lore.kernel.org/linux-sgx/df2248d2-eb61-22d6-3a51-d8091f9eaad6@intel.com/


>> Ensure encl->segment_tbl[] is valid before attempting to access
>> its members. The offset with which it is accessed, encl->nr_segments,
>> is initialized before encl->segment_tbl[] and thus considered valid
>> to use after the encl->segment_tbl[] check succeeds.
>>
>> Fixes: 3200505d4de6 ("selftests/sgx: Create a heap for the test enclave")
>> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
>> ---
>> Changes since V1:
>> - Rewrite commit message (Dave).
>>
>>   tools/testing/selftests/sgx/load.c | 9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/testing/selftests/sgx/load.c b/tools/testing/selftests/sgx/load.c
>> index 9d4322c946e2..006b464c8fc9 100644
>> --- a/tools/testing/selftests/sgx/load.c
>> +++ b/tools/testing/selftests/sgx/load.c
>> @@ -21,7 +21,7 @@
>>     void encl_delete(struct encl *encl)
>>   {
>> -    struct encl_segment *heap_seg = &encl->segment_tbl[encl->nr_segments - 1];
>> +    struct encl_segment *heap_seg;
>>         if (encl->encl_base)
>>           munmap((void *)encl->encl_base, encl->encl_size);
>> @@ -32,10 +32,11 @@ void encl_delete(struct encl *encl)
>>       if (encl->fd)
>>           close(encl->fd);
>>   -    munmap(heap_seg->src, heap_seg->size);
>> -
>> -    if (encl->segment_tbl)
>> +    if (encl->segment_tbl) {
>> +        heap_seg = &encl->segment_tbl[encl->nr_segments - 1];
>> +        munmap(heap_seg->src, heap_seg->size);
>>           free(encl->segment_tbl);
>> +    }
>>         memset(encl, 0, sizeof(*encl));
>>   }
>>
> 
> The rest looks good to me. I can take this through kselftest tree, if not,
> 
> Acked-by: Shuah Khan <skhan@linuxfoundation.org>
> 

Thank you very much for reviewing the changes.

None of the patches in this series have external dependencies (all patches
in the "Fixes:" can be found in v5.17-rc1) but my understanding is that Dave
(for now) prefers to take them via the tip.git tree. This is because there
are more SGX features and tests for those features [1] in flight to the
SGX area and at least for now it would make things easier if the changes to
the SGX selftests are contained in the same tree.

Dave: please do correct me if I am wrong.

Reinette

[1] https://lore.kernel.org/linux-sgx/cover.1638381245.git.reinette.chatre@intel.com/