Re: [PATCH v2 0/5] Fix ELF / FDPIC ELF core dumping, and use mmap_sem properly in there

Russell King - ARM Linux admin
2020-04-29T21:56:20Z

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.)

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