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[114.36.226.49]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2aece56c393sm21134195ad.17.2026.03.13.05.50.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Mar 2026 05:50:22 -0700 (PDT) Date: Fri, 13 Mar 2026 20:50:19 +0800 From: Hao-Yu Yang To: Eric Dumazet Cc: security@kernel.org, tglx@kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v1] futex: Use-after-free between futex_key_to_node_opt and vma_replace_policy Message-ID: References: <20260313120529.46754-1-naup96721@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Mar 13, 2026 at 01:47:23PM +0100, Eric Dumazet wrote: > On Fri, Mar 13, 2026 at 1:42 PM Hao-Yu Yang wrote: > > > > On Fri, Mar 13, 2026 at 01:26:21PM +0100, Eric Dumazet wrote: > > > On Fri, Mar 13, 2026 at 1:08 PM Hao-Yu Yang wrote: > > > > > > > > During futex_key_to_node_opt() execution, vma->vm_policy is read under > > > > speculative mmap lock and RCU. Concurrently, mbind() may call > > > > vma_replace_policy() which frees the old mempolicy immediately via > > > > kmem_cache_free(). > > > > > > > > This creates a race where __futex_key_to_node() dereferences a freed > > > > mempolicy pointer, causing a use-after-free read of mpol->mode. > > > > > > > > patch just initial attempt to patch and need more discussion. > > > > I think we need to remove this path or add some lock. > > > > > > > > Fixes: c042c505210d ("futex: Implement FUTEX2_MPOL") > > > > Reported-by: Hao-Yu Yang > > > > Signed-off-by: Hao-Yu Yang > > > > --- > > > > kernel/futex/core.c | 23 ----------------------- > > > > 1 file changed, 23 deletions(-) > > > > > > Good catch ! > > > > > > Adding rcu to __mpol_put() is really a no brainer. > > > > > > Thanks. > > > > > > diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h > > > index 0fe96f3ab3ef02e902e1676e750c2006ecd6147f..65c732d440d2f4e4566204429f1e5e7487ab8f91 > > > 100644 > > > --- a/include/linux/mempolicy.h > > > +++ b/include/linux/mempolicy.h > > > @@ -55,6 +55,7 @@ struct mempolicy { > > > nodemask_t cpuset_mems_allowed; /* relative to these nodes */ > > > nodemask_t user_nodemask; /* nodemask passed by user */ > > > } w; > > > + struct rcu_head rcu; > > > }; > > > > > > /* > > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > > > index 0e5175f1c767d81394276559b9610c24d854f5bc..6dc61a3d4a32f74a06bf005acfd82d4a43112348 > > > 100644 > > > --- a/mm/mempolicy.c > > > +++ b/mm/mempolicy.c > > > @@ -487,7 +487,7 @@ void __mpol_put(struct mempolicy *pol) > > > { > > > if (!atomic_dec_and_test(&pol->refcnt)) > > > return; > > > - kmem_cache_free(policy_cache, pol); > > > + kfree_rcu(pol, rcu); > > > } > > > EXPORT_SYMBOL_FOR_MODULES(__mpol_put, "kvm"); > > > > Thanks for your review. I have send patch2. If this patch is good. I will cc to stable@vger.kernel.org > > Please, always wait ~24 hours before sending a V2, so that other > people can chime in. > > I understand you are super excited to get a patch in linux, but we are > flooded of reports (mostly because of AI), > so please be gentle with us. OK, I know, thanks