[PATCH] maple_tree: Add dead node check in mas_dup_alloc()

[Andrew Morton]

On Sat,  3 Jan 2026 17:57:58 +0100 Boudewijn van der Heide <boudewijn at delta-utec.com> wrote:

> The __mt_dup() function is exported and can be called without internal
> locking, relying on the caller to provide appropriate synchronization.
> If a caller fails to hold proper locks, the source tree may be modified
> concurrently, potentially resulting in dead nodes during traversal.
> 
> The call stack is:
>   __mt_dup()
>     → mas_dup_build()
>       → mas_dup_alloc()  [accesses node->slot[]]
> 
> The mas_dup_alloc() function may access node slots without first
> verifying that the node is still alive. If a dead node is encountered,
> its memory layout may have been switched to the RCU union member, making
> slot array access undefined behavior as we would be reading from the
> rcu_head structure instead.
> 
> Add an explicit dead node check to detect concurrent modification during
> duplication. When a dead node is detected, return -EBUSY to indicate that
> the tree is undergoing concurrent modification.
> 
> Signed-off-by: Boudewijn van der Heide <boudewijn at delta-utec.com>
> 
> ---
> 
> Build-tested and boot-tested with QEMU with Buildroot on x86_64. The
> kernel booted and basic commandline operations work correctly. The race
> condition this patch addresses is difficult to reproduce in testing, as
> it requires concurrent tree modifications without proper locking.

Thanks.

What are the worst-case userspace-visible runtime effects when this
happens?

If they're bad then presumably we'll want to backport this fix into
earlier kernels with a Cc: <stable at vger.kernel.org> and, very
preferably a Fixes: line.