Re: Is ext4_dio_read_iter() broken?

[Matthew Bobrowski <mbobrowski@mbobrowski.org>]

On Thu, Mar 12, 2020 at 09:19:39AM +0000, David Howells wrote:
> Hi Matthew,
> 
> Is ext4_dio_read_iter() broken?  It calls:
> 
> 	file_accessed(iocb->ki_filp);
> 
> at the end of the function - but surely iocb should be expected to have been
> freed when iocb->ki_complete() was called?
> 
> In my cachefiles rewrite, I'm seeing the attached kasan dump.  The offending
> RIP, ext4_file_read_iter+0x12b is at the above line, where it is trying to
> read iocb->ki_filp.
>
> Here's an excerpt of the relevant bits from my code:
> 
> 	static void cachefiles_read_complete(struct kiocb *iocb, long ret, long ret2)
> 	{
> 		struct cachefiles_kiocb *ki =
> 			container_of(iocb, struct cachefiles_kiocb, iocb);
> 		struct fscache_io_request *req = ki->req;
> 	...
> 		fput(ki->iocb.ki_filp);
> 		kfree(ki);
> 		fscache_end_io_operation(req->cookie);
> 	...
> 	}

I'm not exactly sure what you're rewriting, although from this excerpt
the way that this is implemented would sure cause the UAF in
ext4_dio_read_iter().

I don't forsee any issues with calling file_accessed(iocb->ki_filp);
prior to calling into the iomap infrastructure, unless I'm totally
missing something obvious...

/M