From: Linus Torvalds torvalds@linux-foundation.org
[ Upstream commit bc0c4d1e176eeb614dc8734fc3ace34292771f11 ]
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 04f5866e41fb ("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 jannh@google.com Fixes: c1ca757bd6f4 ("io_uring: add IORING_OP_MADVISE") Acked-by: Jens Axboe axboe@kernel.dk Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 <linux/swapops.h> #include <linux/shmem_fs.h> #include <linux/mmu_notifier.h> +#include <linux/sched/mm.h>
#include <asm/tlb.h>
@@ -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); }