[WIP] mm/vma: clear dst->anon_vma on anon_vma_clone() failure in dup_anon_vma()

[Sasha Levin <sashal@kernel.org>]

When dup_anon_vma() calls anon_vma_clone() and it fails with -ENOMEM,
dst->anon_vma is left pointing at src->anon_vma without a corresponding
num_active_vmas increment (which only happens on the success path).

The internal cleanup_partial_anon_vmas() correctly frees partially-
allocated AVCs but does not clear dst->anon_vma. Later, when the VMA is
torn down during process exit, unlink_anon_vmas() sees a non-NULL
vma->anon_vma and decrements num_active_vmas without a prior matching
increment, causing an underflow. This eventually triggers:

  WARNING: mm/rmap.c:528 at unlink_anon_vmas+0x68e/0x900 mm/rmap.c:528

First, fault injection in the mlock2 syscall path:

  FAULT_INJECTION: forcing a failure.
  name failslab, interval 1, probability 0, space 0, times 0
  CPU: 3 PID: 4261 Comm: syz.6.96
  Call Trace:
   should_fail_ex.cold+0xd8/0x15d
   should_failslab+0xd4/0x150
   kmem_cache_alloc_noprof+0x60/0x630
   anon_vma_clone+0x2ed/0xcf0
   dup_anon_vma+0x1cb/0x320
   vma_modify+0x16dd/0x2230
   vma_modify_flags+0x1f9/0x350
   mlock_fixup+0x225/0xe10
   apply_vma_lock_flags+0x249/0x360
   do_mlock+0x269/0x7f0
   __x64_sys_mlock2+0xc0/0x100

Followed by the WARNING on the same task during exit:

  WARNING: mm/rmap.c:528 at unlink_anon_vmas+0x68e/0x900
  CPU: 3 PID: 4261 Comm: syz.6.96
  Call Trace:
   free_pgtables+0x312/0x950
   exit_mmap+0x487/0xa80
   __mmput+0x11b/0x540
   exit_mm
   do_exit+0x7b9/0x2c60

Fix this by clearing dst->anon_vma on clone failure, restoring the VMA
to its original unfaulted state. This ensures unlink_anon_vmas() will
correctly bail out early at the !active_anon_vma check.

Other callers of anon_vma_clone() are unaffected: VMA_OP_SPLIT/REMAP
free the dst VMA on error, and VMA_OP_FORK explicitly sets anon_vma to
NULL before cloning.

Fixes: 542eda1a83294 ("mm/rmap: improve anon_vma_clone(), unlink_anon_vmas() comments, add asserts")
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/vma.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/vma.c b/mm/vma.c
index be64f781a3aa7..4cf6a2a05c10a 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -629,8 +629,10 @@ static int dup_anon_vma(struct vm_area_struct *dst,
 		vma_assert_write_locked(dst);
 		dst->anon_vma = src->anon_vma;
 		ret = anon_vma_clone(dst, src, VMA_OP_MERGE_UNFAULTED);
-		if (ret)
+		if (ret) {
+			dst->anon_vma = NULL;
 			return ret;
+		}
 
 		*dup = dst;
 	}
-- 
2.51.0