On Mon, Jan 31, 2022 at 10:26:11AM -0600, Eric W. Biederman wrote: > Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > > > On Mon, Jan 31, 2022 at 10:03:31AM -0600, Eric W. Biederman wrote: > >> "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> writes: > >> > >> > I'm not sure if the VMA list can change under us, but dump_vma_snapshot() > >> > is very careful to take the mmap_lock in write mode. We only need to > >> > take it in read mode here as we do not care if the size of the stack > >> > VMA changes underneath us. > >> > > >> > If it can be changed underneath us, this is a potential use-after-free > >> > for a multithreaded process which is dumping core. > >> > >> The problem is not multi-threaded process so much as processes that > >> share their mm. > > > > I don't understand the difference. I appreciate that another process can > > get read access to an mm through, eg, /proc, but how can another process > > (that isn't a thread of this process) modify the VMAs? > > There are a couple of ways. > > A classic way is a multi-threads process can call vfork, and the > mm_struct is shared with the child until exec is called. While true, I thought the semantics of vfork() were that the parent was suspended. Given that, it can't core dump until the child execs ... right? > A process can do this more deliberately by forking a child using > clone(CLONE_VM) and not including CLONE_THREAD. Supporting this case > is a hold over from before CLONE_THREAD was supported in the kernel and > such processes were used to simulate threads. That is a multithreaded process then! Maybe not in the strict POSIX compliance sense, but the intent is to be a multithreaded process. ie multiple threads of execution, sharing an address space. > It also happens that there are subsystems in the kernel that do things > like kthread_use_mm that can also be modifying the mm during a coredump. Yikes. That's terrifying. It's really legitimate for a kthread to attach to a process and start tearing down VMAs? > > Uhh .. that seems like it needs a lot more understanding of binfmt_elf > > than I currently possess. I'd rather spend my time working on folios > > than learning much more about binfmt_elf. I was just trying to fix an > > assertion failure with the maple tree patches (we now assert that you're > > holding a lock when walking the list of VMAs). > > Fair enough. I will put it on my list of things to address. Thanks. Now that I've disclosed it's a UAF, I hope you're able to get to it soon. Otherwise we should put this band-aid in for now and you can address it properly in the fullness of time.
