From 10424f5af7bae663f726944a471af2767ba1a569 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 24 Apr 2020 11:10:58 -0700 Subject: [PATCH] mm: check that mm is still valid in madvise() IORING_OP_MADVISE can end up basically doing mprotect() on the VM of another process, which means that it can race with our crazy core dump handling which accesses the VM state without holding the mmap_sem (because it incorrectly thinks that it is the final user). This is clearly a core dumping problem, but we've never fixed it the right way, and instead have the notion of "check that the mm is still ok" using mmget_still_valid() after getting the mmap_sem for writing in any situation where we're not the original VM thread. See commit ed8c070537a3 ("coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping") for more background on this whole mmget_still_valid() thing. You might want to have a barf bag handy when you do. We're discussing just fixing this properly in the only remaining core dumping routines. But even if we do that, let's make do_madvise() do the right thing, and then when we fix core dumping, we can remove all these mmget_still_valid() checks. Reported-and-tested-by: Jann Horn Fixes: bad805556e8b ("io_uring: add IORING_OP_MADVISE") Acked-by: Jens Axboe Signed-off-by: Linus Torvalds --- mm/madvise.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/mm/madvise.c b/mm/madvise.c index 4bb30ed6c8d21..8cbd8c1bfe159 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -27,6 +27,7 @@ #include #include #include +#include #include @@ -1090,6 +1091,23 @@ int do_madvise(unsigned long start, size_t len_in, int behavior) if (write) { if (down_write_killable(¤t->mm->mmap_sem)) return -EINTR; + + /* + * We may have stolen the mm from another process + * that is undergoing core dumping. + * + * Right now that's io_ring, in the future it may + * be remote process management and not "current" + * at all. + * + * We need to fix core dumping to not do this, + * but for now we have the mmget_still_valid() + * model. + */ + if (!mmget_still_valid(current->mm)) { + up_write(¤t->mm->mmap_sem); + return -EINTR; + } } else { down_read(¤t->mm->mmap_sem); } -- 2.39.5