[PATCH] Fixing returning a userspace address for return value

[Casey Schaufler]

On 10/19/2021 11:41 AM, James Morris wrote:
> On Tue, 19 Oct 2021, T. Williams wrote:
>
>> Fixing user memory dereference bug.
>>
>> Signed-off-by: Thelford Williams <tdwilliamsiv at gmail.com>
> Casey, can you check the logic on this?
>
>> ---
>>   security/security.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/security/security.c b/security/security.c
>> index 9ffa9e9c5c55..7c41b5d732ab 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -1737,6 +1737,8 @@ int security_kernel_read_file(struct file *file, enum
>> kernel_read_file_id id,
>>          int ret;
>>
>>          ret = call_int_hook(kernel_read_file, 0, file, id, contents);
>> +       if (ret > 0)
>> +               return -EINVAL;

Does the failing configuration include a BPF program that might be
invoked for this hook? SELinux is the only traditional security module
that provides a hook, and it never returns a positive value. I don't
see a case where the proposed check makes any sense. If the problem is
in a BPF program it should be fixed there.

>>          if (ret)
>>                  return ret;
>>          return ima_read_file(file, id, contents);
>> -- 
>> 2.25.1
>>
>> This commit is to fix a userspace address dereference found by
>> syzkaller.
>> The crash is triggered by passing a file descriptor to an incorrectly
>> formed kernel module to finit_module.
>>
>> Kernel/module.c:4175 : Within the finit_module, info.len is set to the
>> return value from kernel_read_file_from_fd. This value is then
>> dereferenced by memcmp within module_sig_check from inside load_module.
>> The value is 0xb000000 so the kernel dereferences user memory and kernel
>> panics.
>>
>> To prevent this adding a check from within security_kernel_read_file to
>> make sure call_int_hook doesn't return an address which in the syzkaller
>> program is what causes the return value to be 0xb000000. Then the return
>> value of security_kernel_read_file is returned to kernel_read_file(also
>> in security/security.c) which returns the value to
>> kernel_read_file_from_fd (fs/kernel_read_file.c) and this returns the
>> value into err then being set to info.len causing the dereference when
>> info is passed into load_module.
>>