I was just taking a look at some ReactOS code this evening and it got me thinking about what our general strategy for handling invalid parameters is.
For example, in kernel32.dll there is a function DebugBreakProcess. This function takes one parameter, a process handle.
This parameter is passed through the following list of functions unchecked by any of them, until the final one will return a failure...
DebugBreakProcess DbgUiIssueRemoteBreakIn RtlCreateUserThread RtlpCreateUserStack ZwAllocateVirtualMemory ObReferenceObjectByHandle
Now I know this isn't a security vulnerability, which is what I was originally looking for, but it did make me think of the question of where should bounds checking be added?
In this example, the process handle must be a value greater than zero. Should this simple check be added to DebugBreakProcess, or all of the above? Is there some sort of standard that everyone should work to? e.g. should all functions check their own parameters. Sure it might make it a little slower due to multiple checks but it would make ReactOS very robust.
Any thoughts on this?
Martin
PS: Lack of activity recently had been due to uni. exams.
M Bealby wrote:
I was just taking a look at some ReactOS code this evening and it got me thinking about what our general strategy for handling invalid parameters is.
For example, in kernel32.dll there is a function DebugBreakProcess. This function takes one parameter, a process handle.
This parameter is passed through the following list of functions unchecked by any of them, until the final one will return a failure...
DebugBreakProcess
Adding the check here means that if someone calls the Native API directly the check is skipped, and this the one in the win32 api is useless.
DbgUiIssueRemoteBreakIn
Adding the check here cuts the problem short, but now it means every API that uses a process handle now needs to check the parameter, thus adding thousands of lines of duplicated code. The same argument goes for the ones below
RtlCreateUserThread RtlpCreateUserStack ZwAllocateVirtualMemory
Since process handles are the responsability of the object manager, it is the lowest place and the only place that should report this failure. Rtl code shouldn't make assumptions about what is a valid handle and what is not. If one day a new object manager is created which uses negative handles as correct handles, and the 0 handle as "Current process" handle, then thousands of lines of code woul dhave to be changed. By leaving the responsability of determing what is and what isn't a valid handle to the object manager itself, this keeps the kernel componentized and mostly independent.
ObReferenceObjectByHandle
Best regards, Alex Ionescu
Since process handles are the responsability of the object manager, it is the lowest place and the only place that should report this failure. Rtl code shouldn't make assumptions about what is a valid handle and what is not. If one day a new object manager is created which uses negative handles as correct handles, and the 0 handle as "Current process" handle, then thousands of lines of code woul dhave to be changed. By leaving the responsability of determing what is and what isn't a valid handle to the object manager itself, this keeps the kernel componentized and mostly independent.
So what you're saying is that the functions in the modules of code which actually do the work should perform the checking. What about this: If a parameter is passed straight through a function to another no checking should take place, but if any manipulation of the parameters occurs then they should be checked? I think this might be slightly different to your idea Alex. What does everyone think? I personally think this is the best approach as it requires little, if any duplicated checking code.
Comments welcome, Martin
M Bealby wrote:
So what you're saying is that the functions in the modules of code which actually do the work should perform the checking. What about this: If a parameter is passed straight through a function to another no checking should take place, but if any manipulation of the parameters occurs then they should be checked? I think this might be slightly different to your idea Alex. What does everyone think? I personally think this is the best approach as it requires little, if any duplicated checking code.
I agree with Alex because the only place the handle can be checked is in ob. It doesn't make any sense to add pseudo checks.
- Thomas
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Alex Ionescu -
M Bealby -
Thomas Weidenmueller