[PATCH bpf-next 7/9] fork: Let the new task point to its mm before uprobes can find it

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

From: Alexei Starovoitov <ast@kernel.org>

uprobe_perf_filter() compares event->hw.target->mm with mm. The target of
an inherited event is the new task. copy_mm() sets tsk->mm after dup_mm()
returned, but register_for_each_vma() can find the new mm as soon as
dup_mmap() releases dup_mmap_sem. No task points to this mm yet, so
the filter says that nobody wants it.

The walk that removes the breakpoint, because another consumer of the same
uprobe goes away, sees !filter_chain() and removes it from the child.
Nobody installs it again, and the inherited event doesn't see this child.

It's an old bug with a short window. The previous patch releases
dup_mmap_sem before dup_userfaultfd_complete(), so with UFFD_EVENT_FORK
the window is as long as the monitor likes.

Set tsk->mm at the end of dup_mmap() before it releases dup_mmap_sem.
Not earlier. The new task can be reached via its perf events and from bpf
progs. They shouldn't find mm that is half copied. Clear tsk->mm when
dup_mm() fails after dup_mmap(). Do both under task_lock() like exec_mmap()
and exit_mm().

The tasks with such event fork, while another consumer of the same uprobe
comes and goes. 5 cpu VM, 90 sec and 7000 children per run. The number of
children that the inherited event didn't see:

  without this series          11 to 24, 8 runs
  patches 1, 2, 5, 6 only      23, 12, 21
  this series                  0, 0, 0

With UFFD_EVENT_FORK monitor that reads the event after 5 sec the child
lost the breakpoint in 5 of 5 runs without this patch, and in 0 of 5 with.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 kernel/fork.c |  9 ++++++++-
 mm/internal.h |  3 ++-
 mm/mmap.c     | 14 +++++++++++++-
 mm/nommu.c    |  3 ++-
 4 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 8fd6be932afe..5a2f571b64b7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1539,7 +1539,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
 	if (!mm_init(mm, tsk))
 		goto fail_nomem;
 
-	err = dup_mmap(mm, oldmm);
+	err = dup_mmap(tsk, mm, oldmm);
 	if (err)
 		goto free_pt;
 
@@ -1552,6 +1552,13 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
 	return mm;
 
 free_pt:
+	/*
+	 * dup_mmap() has set it if it succeeded. get_task_mm() must not find
+	 * it after the mmput() below.
+	 */
+	task_lock(tsk);
+	tsk->mm = NULL;
+	task_unlock(tsk);
 	/* don't put binfmt in mmput, we haven't got module yet */
 	mm->binfmt = NULL;
 	mm_init_owner(mm, NULL);
diff --git a/mm/internal.h b/mm/internal.h
index 9e3be236f18c..70e40306d881 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1520,7 +1520,8 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start,
 			  pgd_t *pgd, void *private);
 
 void dup_mm_exe_file(struct mm_struct *mm, struct mm_struct *oldmm);
-int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm);
+int dup_mmap(struct task_struct *tsk, struct mm_struct *mm,
+	     struct mm_struct *oldmm);
 
 int remap_pfn_range_prepare(struct vm_area_desc *desc);
 int remap_pfn_range_complete(struct vm_area_struct *vma,
diff --git a/mm/mmap.c b/mm/mmap.c
index 6ce654e33e3b..3167272a77ad 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -46,6 +46,7 @@
 #include <linux/pkeys.h>
 #include <linux/oom.h>
 #include <linux/sched/mm.h>
+#include <linux/sched/task.h>
 #include <linux/ksm.h>
 #include <linux/memfd.h>
 
@@ -1705,7 +1706,8 @@ bool mmap_read_lock_maybe_expand(struct mm_struct *mm,
 	return true;
 }
 
-__latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
+__latent_entropy int dup_mmap(struct task_struct *tsk, struct mm_struct *mm,
+			      struct mm_struct *oldmm)
 {
 	struct vm_area_struct *mpnt, *tmp;
 	int retval;
@@ -1897,6 +1899,16 @@ __latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
 	mmap_write_unlock(mm);
 	flush_tlb_mm(oldmm);
 	mmap_write_unlock(oldmm);
+	if (!retval) {
+		/*
+		 * As soon as dup_mmap_sem is released uprobes can find mm via
+		 * the rmap tree and ask the consumers if they want it. The
+		 * filter of an inherited perf event looks for mm in tsk->mm.
+		 */
+		task_lock(tsk);
+		tsk->mm = mm;
+		task_unlock(tsk);
+	}
 	uprobe_end_dup_mmap();
 	if (!retval)
 		dup_userfaultfd_complete(&uf);
diff --git a/mm/nommu.c b/mm/nommu.c
index 68b05565e15a..7529e03d3f2c 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1885,7 +1885,8 @@ static int __meminit init_admin_reserve(void)
 }
 subsys_initcall(init_admin_reserve);
 
-int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
+int dup_mmap(struct task_struct *tsk, struct mm_struct *mm,
+	     struct mm_struct *oldmm)
 {
 	mmap_write_lock(oldmm);
 	dup_mm_exe_file(mm, oldmm);
-- 
2.55.0