| Message ID | 20200826151448.3404695-1-jannh@google.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Fix ELF / FDPIC ELF core dumping, and use mmap_lock properly in there | expand |
On Wed, Aug 26, 2020 at 8:15 AM Jann Horn <jannh@google.com> wrote: > > After this series has landed, we should be able to rip out > mmget_still_valid(). I think you should just add that to the series, since it's kind of the point of it all. But ack on this, it now looks saner than what we used to have regardless. I do wonder if we should possibly just limit the number of vma's we dump based on core size ulimit too, but that, I think, might be a "further improvement" rather than base cleanup. Linus
new in v4: - simplify patch 4/5 by replacing the heuristic for dumping the first pages of ELF mappings with what Linus suggested 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. ("cleanly" meaning "avoid holding the mmap_lock 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(). I have tested: - Creating a simple core dump on X86-64 still works. - The created coredump on X86-64 opens in GDB and looks plausible. - X86-64 core dumps contain the first page for executable mappings at offset 0, and don't contain the first page for non-executable file mappings or executable mappings at offset !=0. - NOMMU 32-bit ARM can still generate plausible-looking core dumps through the FDPIC implementation. (I can't test this with GDB because GDB is missing some structure definition for nommu ARM, but I've poked around in the hexdump and it looked decent.) Jann Horn (5): binfmt_elf_fdpic: Stop using dump_emit() on user pointers on !MMU coredump: Let dump_emit() bail out on short writes coredump: Refactor page range dumping into common helper binfmt_elf, binfmt_elf_fdpic: Use a VMA list snapshot mm/gup: Take mmap_lock in get_dump_page() fs/binfmt_elf.c | 147 ++++++++------------------------------- fs/binfmt_elf_fdpic.c | 106 +++++++++++----------------- fs/coredump.c | 125 ++++++++++++++++++++++++++++++--- include/linux/coredump.h | 11 +++ mm/gup.c | 61 ++++++++-------- 5 files changed, 227 insertions(+), 223 deletions(-) base-commit: 06a4ec1d9dc652e17ee3ac2ceb6c7cf6c2b75cdd