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Sun, 12 Apr 2026 10:48:21 -0700 (PDT) MIME-Version: 1.0 References: <20260411062152.2092967-1-lgs201920130244@gmail.com> <20260411142858.85496-1-lance.yang@linux.dev> <848180C7-F98C-44B2-AB1F-579BF9EEA28E@nvidia.com> <3e688ea1-05ba-4e75-9d92-2751ff6f3b7b@linux.dev> <75F536FE-6710-4AE7-B6DB-2997D846237E@nvidia.com> In-Reply-To: <75F536FE-6710-4AE7-B6DB-2997D846237E@nvidia.com> From: Guangshuo Li Date: Mon, 13 Apr 2026 01:48:11 +0800 X-Gm-Features: AQROBzCTa9E1N3xrHdXc-IGVPR5wSvNpHV_7nILnImYBrIIchwBXudfJpaQLPfQ Message-ID: Subject: Re: [PATCH] mm: thp: Fix refcount leak in thpsize_create() error path To: Zi Yan Cc: Lance Yang , akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 0B32940002 X-Stat-Signature: g1zbkcudyqeda4a1kzmmwrk8iqw59nua X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1776016101-932509 X-HE-Meta: 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 xE/4LL3c jX+HCXJMhSmsL1DbNjJZgyBzJlo9Mhpg4bz4qCho6x6Rjc+vaCzPw3/N19j2xWcKabR0gBI90qjIA5ygaHTIvYwu1aoytxpqQJmkYME/0QXSb2Z3cgOEcgDnoxeRWwMkQd47dboM3apa/xpHGOQzs3aSc1DnvVIA/47GyO3BFbHzi+RHdoPN/qsT6sxhpD9q2n8nfp72CUmBDVBfWe+5GWeVmtOJOGANXgztmj0axeFquChS4VA5EX2aqqiVZfwu/SRhKMk8/DRDZWpVlgDcZG4TIYUxPVMkgeFfBiHJezGoYzLO/mAaa0j/XvuSbThxMmzaSRbYPh9/MNPHsxduxGm8k53ug2QxXDcb180UGrvf1uqbY1aPaFQWRqnpRvMxNIGk+Z6IaS/4dPhlMqGW/CD3qNuIYt421V4ztnXt5cUEuEAIQybmyzlBJG6L8AqL7SuuQmM/o9whP/8hmIYLX20JISA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Lance, Zi, Thanks for the review and for pointing this out. You are right =E2=80=94 that was my mistake in the changelog wording. After rechecking the failure path more carefully, I do not think a use-after-free can actually happen here. The real issue is that the error path skips the proper kobject cleanup flow, so the problem is better described as an unbalanced kobject reference / refcount handling issue rather than a potential UAF. I will update the commit message accordingly and send a v2 shortly. Thanks, Guangshuo Zi Yan =E4=BA=8E2026=E5=B9=B44=E6=9C=8812=E6=97=A5=E5=91= =A8=E6=97=A5 21:33=E5=86=99=E9=81=93=EF=BC=9A > > On 11 Apr 2026, at 23:24, Lance Yang wrote: > > > On 2026/4/12 09:49, Zi Yan wrote: > >> On 11 Apr 2026, at 10:28, Lance Yang wrote: > >> > >>> On Sat, Apr 11, 2026 at 02:21:52PM +0800, Guangshuo Li wrote: > >>>> After kobject_init_and_add(), the lifetime of the embedded struct > >>>> kobject is expected to be managed through the kobject core reference > >>>> counting. > >>>> > >>>> In thpsize_create(), if kobject_init_and_add() fails, thpsize is fre= ed > >>>> directly with kfree() rather than releasing the kobject reference wi= th > >>>> kobject_put(). This may leave the reference count of the embedded st= ruct > >>> > >>> Right. As documented for kobject_init_and_add(), once it has been > >>> called, the error path should go through kobject_put(): > >>> > >>> /** > >>> * kobject_init_and_add() - Initialize a kobject structure and add i= t to > >>> * the kobject hierarchy. > >>> ... > >>> * > >>> * This function combines the call to kobject_init() and kobject_add= (). > >>> * > >>> * If this function returns an error, kobject_put() must be called t= o > >>> * properly clean up the memory associated with the object. This is= the > >>> ... > >>> */ > >>> int kobject_init_and_add(struct kobject *kobj, const struct kobj_type= *ktype, > >>> struct kobject *parent, const char *fmt, ...) > >>> > >>>> kobject unbalanced, resulting in a refcount leak and potentially lea= ding > >>>> to a use-after-free. > >>> > >>> IIUC, this looks more like wrong kobject lifetime handling and likely= a > >>> leak, not a clear UAF :) > >> > >> kobject_put() ends up with calling kobj_type->release(), which is just > >> kfree(to_thpsize(kobj)), equivalent to kfree(thpsize) in the old code. > >> IIUC, there is no leak. Let me know if I miss anything. > > > > Right, the fix is correct. I was only commenting on the changelog > > wording, especially: > > > > "resulting in a refcount leak and potentially leading to a use-after-fr= ee" > > > > The old code does skip the required kobject cleanup path, but is > > a UAF actually possible there? > > That is my question too. The original code might not cause any real issue= . > > Guangshuo, let us know if we get it wrong. Thanks. > > > > > Just a wording nit. > > > -- > Best Regards, > Yan, Zi