[PATCH v2] kernel: Be more careful about dup_mmap() failures and uprobe registering

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>

In the even that there is a failure during dup_mmap(), the maple tree
can be left in an unsafe state for other iterators besides the exit
path.  All the locks are dropped before the exit_mmap() call (in
mm/mmap.c), but the incomplete mm_struct can be reached through (at
least) the rmap finding the vmas which have a pointer back to the
mm_struct.

Up to this point, there have been no issues with being able to find an
mm_struct that was only partially initialised.  Syzbot was able to make
the incomplete mm_struct fail with recent forking changes, so it has
been proven unsafe to use the mm_struct that hasn't been initialised, as
referenced in the link below.

Although 8ac662f5da19f ("fork: avoid inappropriate uprobe access to
invalid mm") fixed the uprobe access, it does not completely remove the
race.

This patch sets the MMF_OOM_SKIP to avoid the iteration of the vmas on
the oom side (even though this is extremely unlikely to be selected as
an oom victim in the race window), and sets MMF_UNSTABLE to avoid other
potential users from using a partially initialised mm_struct.

When registering vmas for uprobe, skip the vmas in an mm that is marked
unstable.  Modifying a vma in an unstable mm may cause issues if the mm
isn't fully initialised.

Link: https://lore.kernel.org/all/6756d273.050a0220.2477f.003d.GAE@google.com/
Fixes: d240629148377 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()")
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---

v1: https://lore.kernel.org/all/20250123205849.793810-1-Liam.Howlett@oracle.com/

Changes since:
v1
 - Added check_stable_address_space() to uprobe code - Thanks Lorenzo
 - Added Oleg & Masami to Cc list.

 kernel/events/uprobes.c |  4 ++++
 kernel/fork.c           | 17 ++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index fa04b14a7d723..90ebcdbad05ca 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -28,6 +28,7 @@
 #include <linux/rcupdate_trace.h>
 #include <linux/workqueue.h>
 #include <linux/srcu.h>
+#include <linux/oom.h>          /* check_stable_address_space */
 
 #include <linux/uprobes.h>
 
@@ -1260,6 +1261,9 @@ register_for_each_vma(struct uprobe *uprobe, struct uprobe_consumer *new)
 		 * returns NULL in find_active_uprobe_rcu().
 		 */
 		mmap_write_lock(mm);
+		if (check_stable_address_space(mm))
+			goto unlock;
+
 		vma = find_vma(mm, info->vaddr);
 		if (!vma || !valid_vma(vma, is_register) ||
 		    file_inode(vma->vm_file) != uprobe->inode)
diff --git a/kernel/fork.c b/kernel/fork.c
index ded49f18cd95c..20b2120f019ca 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -760,7 +760,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 		mt_set_in_rcu(vmi.mas.tree);
 		ksm_fork(mm, oldmm);
 		khugepaged_fork(mm, oldmm);
-	} else if (mpnt) {
+	} else {
+
 		/*
 		 * The entire maple tree has already been duplicated. If the
 		 * mmap duplication fails, mark the failure point with
@@ -768,8 +769,18 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 		 * stop releasing VMAs that have not been duplicated after this
 		 * point.
 		 */
-		mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1);
-		mas_store(&vmi.mas, XA_ZERO_ENTRY);
+		if (mpnt) {
+			mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1);
+			mas_store(&vmi.mas, XA_ZERO_ENTRY);
+			/* Avoid OOM iterating a broken tree */
+			set_bit(MMF_OOM_SKIP, &mm->flags);
+		}
+		/*
+		 * The mm_struct is going to exit, but the locks will be dropped
+		 * first.  Set the mm_struct as unstable is advisable as it is
+		 * not fully initialised.
+		 */
+		set_bit(MMF_UNSTABLE, &mm->flags);
 	}
 out:
 	mmap_write_unlock(mm);
-- 
2.43.0