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Howlett" Subject: Re: [PATCH v2] futex: Use-after-free between futex_key_to_node_opt and vma_replace_policy Message-ID: <20260325152206.GH3738786@noisy.programming.kicks-ass.net> References: <20260313124756.52461-1-naup96721@gmail.com> <87a4vyihlx.ffs@tglx> <20260324140019.GE3738010@noisy.programming.kicks-ass.net> <87fr5pgp5x.ffs@tglx> <20260324174418.GB1850007@noisy.programming.kicks-ass.net> <20260325151445.GH3738010@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: 883911A0010 X-Stat-Signature: fx53xxnmdfhciwke7mkxud6fdcj9eq3y X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1774452131-116653 X-HE-Meta: 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 En2+Sdo3 jFqSwQp5I3vA38yfFRgfLbSCqlkBa5emwGtg1yp1bt6vivTVLig/VTSERnsRzisrHZeqcxD2gy5raJUph/YV7ahkXPJ9tb6/yjLI3OQJ+6OsmR67P8ZCMe/EUvrupPP3gbqMUTSII4dgnapqlettfmE7RcbT04ODDXhbvRpvgfA6sdP8eC0a/iR8EPJhbvSmA87buA/W71VBuH/py8XtDlglbB2eGRbbnP+CPy3RQoih3G5Pl22VUlB1wvvYr63UwP7svKfio81fUTa0NPNMjaswbJ5Bmw4KE6u/ZS5QTo+l4SOYoIqIYjafidWsEpSpe9ttONA21sWeHmzKTKTzdFBsfnA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Mar 25, 2026 at 08:19:24AM -0700, Eric Dumazet wrote: > On Wed, Mar 25, 2026 at 8:14 AM Peter Zijlstra wrote: > > > > On Tue, Mar 24, 2026 at 09:27:41PM +0100, David Hildenbrand (Arm) wrote: > > > So IIUC, futex_key_to_node_opt() looks up a VMA under RCU, without > > > holding the mmap lock. Concurrent mmap-write lock is detected by using > > > the mmap_lock_speculate_try_begin()/mmap_lock_speculate_retry() seqcount. > > > > > > After looking up the VMA, we access the VMA policy. > > > > > > vma_policy() does a straight vma->vm_policy. > > > > > > What prevents the compiler here to do some load tearing while it is > > > getting modified by mbind()? Or what stops the writer side to to some > > > store tearing? > > > > > > Shouldn't we be using at least READ_ONCE/WRITE_ONCE() etc? > > > > Bah, at that point we might as well RCU the thing like so, I suppose. > > > > --- a/mm/mempolicy.c > > +++ b/mm/mempolicy.c > > @@ -1026,7 +1026,7 @@ static int vma_replace_policy(struct vm_ > > } > > > > old = vma->vm_policy; > > - vma->vm_policy = new; /* protected by mmap_lock */ > > + rcu_assign_pointer(vma->vm_policy, new); /* protected by mmap_lock */ > > mpol_put(old); > > > > return 0; > > diff --git a/kernel/futex/core.c b/kernel/futex/core.c > > index 4bacf5565368..6336a80e3dca 100644 > > --- a/kernel/futex/core.c > > +++ b/kernel/futex/core.c > > @@ -342,7 +342,7 @@ static int __futex_key_to_node(struct mm_struct *mm, unsigned long addr) > > if (!vma) > > return FUTEX_NO_NODE; > > > > - mpol = vma_policy(vma); > > + mpol = rcu_dereference_raw(vma->vm_policy); > > if (!mpol) > > return FUTEX_NO_NODE; > > Yes, but sparse will bite :) Oh gawd, yes, and then people will go 'fix' it and it'll turn into an unholy mess. > READ_ONCE()/WRITE_ONCE() on these two locations seems acceptable. Fair enough. Like so then.. --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -342,7 +342,7 @@ static int __futex_key_to_node(struct mm if (!vma) return FUTEX_NO_NODE; - mpol = vma_policy(vma); + mpol = READ_ONCE(vma->vm_policy); if (!mpol) return FUTEX_NO_NODE; --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -1026,7 +1026,7 @@ static int vma_replace_policy(struct vm_ } old = vma->vm_policy; - vma->vm_policy = new; /* protected by mmap_lock */ + WRITE_ONCE(vma->vm_policy, new); /* protected by mmap_lock */ mpol_put(old); return 0;