On Wed, Apr 29, 2020 at 11:49:49PM +0200, Jann Horn wrote: > At the moment, we have that rather ugly mmget_still_valid() helper to > work around <https://crbug.com/project-zero/1790>: ELF core dumping > doesn't take the mmap_sem while traversing the task's VMAs, and if > anything (like userfaultfd) then remotely messes with the VMA tree, > fireworks ensue. So at the moment we use mmget_still_valid() to bail > out in any writers that might be operating on a remote mm's VMAs. > > With this series, I'm trying to get rid of the need for that as > cleanly as possible. > In particular, I want to avoid holding the mmap_sem across unbounded > sleeps. > > > Patches 1, 2 and 3 are relatively unrelated cleanups in the core > dumping code. > > Patches 4 and 5 implement the main change: Instead of repeatedly > accessing the VMA list with sleeps in between, we snapshot it at the > start with proper locking, and then later we just use our copy of > the VMA list. This ensures that the kernel won't crash, that VMA > metadata in the coredump is consistent even in the presence of > concurrent modifications, and that any virtual addresses that aren't > being concurrently modified have their contents show up in the core > dump properly. > > The disadvantage of this approach is that we need a bit more memory > during core dumping for storing metadata about all VMAs. > > After this series has landed, we should be able to rip out > mmget_still_valid(). > > > Testing done so far: > > - Creating a simple core dump on X86-64 still works. > - The created coredump on X86-64 opens in GDB, and both the stack and the > exectutable look vaguely plausible. > - 32-bit ARM compiles with FDPIC support, both with MMU and !MMU config. > > I'm CCing some folks from the architectures that use FDPIC in case > anyone wants to give this a spin. I've never had any reason to use FDPIC, and I don't have any binaries that would use it. Nicolas Pitre added ARM support, so I guess he would be the one to talk to about it. (Added Nicolas.) -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up
