On Wed, 29 Apr 2020, Russell King - ARM Linux admin wrote: > 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.) It's been a while since I worked with it. However Christophe Lyon (in CC) added support for ARM FDPIC to mainline gcc recently, so hopefully he might still be set up and able to help. Nicolas
