[PATCH v2] fork: avoid inappropriate uprobe access to invalid mm

[Lorenzo Stoakes]

If dup_mmap() encounters an issue, currently uprobe is able to access the
relevant mm via the reverse mapping (in build_map_info()), and if we are
very unlucky with a race window, observe invalid XA_ZERO_ENTRY state which
we establish as part of the fork error path.

This occurs because uprobe_write_opcode() invokes anon_vma_prepare() which
in turn invokes find_mergeable_anon_vma() that uses a VMA iterator,
invoking vma_iter_load() which uses the advanced maple tree API and thus is
able to observe XA_ZERO_ENTRY entries added to dup_mmap() in commit
d24062914837 ("fork: use __mt_dup() to duplicate maple tree in
dup_mmap()").

This change was made on the assumption that only process tear-down code
would actually observe (and make use of) these values. However this very
unlikely but still possible edge case with uprobes exists and unfortunately
does make these observable.

The uprobe operation prevents races against the dup_mmap() operation via
the dup_mmap_sem semaphore, which is acquired via uprobe_start_dup_mmap()
and dropped via uprobe_end_dup_mmap(), and held across
register_for_each_vma() prior to invoking build_map_info() which does the
reverse mapping lookup.

Currently these are acquired and dropped within dup_mmap(), which exposes
the race window prior to error handling in the invoking dup_mm() which
tears down the mm.

We can avoid all this by just moving the invocation of
uprobe_start_dup_mmap() and uprobe_end_dup_mmap() up a level to dup_mm()
and only release this lock once the dup_mmap() operation succeeds or clean
up is done.

This means that the uprobe code can never observe an incompletely
constructed mm and resolves the issue in this case.

Reported-by: syzbot+2d788f4f7cb660dac4b7@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/6756d273.050a0220.2477f.003d.GAE@xxxxxxxxxx/
Fixes: d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
v2:
* Quick fix for silly mistake in error handling in dup_mm() as pointed out by
  Oleg.

v1:
https://lore.kernel.org/linux-mm/20241210163104.55181-1-lorenzo.stoakes@xxxxxxxxxx/

 kernel/fork.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index d532f893e977..0bf377e2892b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -696,11 +696,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	LIST_HEAD(uf);
 	VMA_ITERATOR(vmi, mm, 0);

-	uprobe_start_dup_mmap();
-	if (mmap_write_lock_killable(oldmm)) {
-		retval = -EINTR;
-		goto fail_uprobe_end;
-	}
+	if (mmap_write_lock_killable(oldmm))
+		return -EINTR;
 	flush_cache_dup_mm(oldmm);
 	uprobe_dup_mmap(oldmm, mm);
 	/*
@@ -839,8 +836,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 		dup_userfaultfd_complete(&uf);
 	else
 		dup_userfaultfd_fail(&uf);
-fail_uprobe_end:
-	uprobe_end_dup_mmap();
 	return retval;

 fail_nomem_anon_vma_fork:
@@ -1746,9 +1741,11 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
 	if (!mm_init(mm, tsk, mm->user_ns))
 		goto fail_nomem;

+	uprobe_start_dup_mmap();
 	err = dup_mmap(mm, oldmm);
 	if (err)
 		goto free_pt;
+	uprobe_end_dup_mmap();

 	mm->hiwater_rss = get_mm_rss(mm);
 	mm->hiwater_vm = mm->total_vm;
@@ -1763,6 +1760,8 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
 	mm->binfmt = NULL;
 	mm_init_owner(mm, NULL);
 	mmput(mm);
+	if (err)
+		uprobe_end_dup_mmap();

 fail_nomem:
 	return NULL;
--
2.47.0