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([2405:201:682f:383f:efe4:e813:1db6:97e3]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-36dd918f4aasm1035573a91.2.2026.06.01.19.52.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Jun 2026 19:53:01 -0700 (PDT) From: Deepanshu Kartikey To: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, memxor@gmail.com, song@kernel.org, yonghong.song@linux.dev, viro@zeniv.linux.org.uk, jolsa@kernel.org Cc: kafai.wan@linux.dev, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, brauner@kernel.org, jack@suse.cz, Deepanshu Kartikey , syzbot+36e50496c8ac4bcde3f9@syzkaller.appspotmail.com Subject: [PATCH v2] bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs Date: Tue, 2 Jun 2026 08:22:49 +0530 Message-ID: <20260602025249.113828-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" commit 4f375ade6aa9 ("bpf: Avoid RCU context warning when unpinning htab with internal structs") moved inode cleanup from ->free_inode() into ->destroy_inode() to avoid sleeping in RCU context when calling bpf_any_put(). However this removed the RCU delay on freeing the inode itself and the cached symlink body (i_link), both of which can be accessed by RCU pathwalk (pick_link, may_lookup etc.). This causes a use-after-free when a concurrent unlinkat() drops the last inode reference and destroy_inode() frees the inode immediately, while another task is still walking the path in RCU mode and reads inode->i_opflags (offset +2) inside current_time() -> is_mgtime(). KASAN reports: BUG: KASAN: slab-use-after-free in is_mgtime include/linux/fs.h:2313 Read of size 2 at addr ffff8880407e4282 (offset +2 =3D i_opflags) The rules (per Al Viro): ->destroy_inode() called immediately, can sleep, use for blocking cleanup e.g. bpf_any_put() ->free_inode() called after RCU grace period, use for freeing inode and anything RCU-accessible e.g. i_link Fix: split the two concerns properly: - keep bpf_any_put() in bpf_destroy_inode() since it is blocking and needs to run promptly - introduce bpf_free_inode() to handle kfree(i_link) and free_inode_nonrcu() with proper RCU delay, preventing the UAF Fixes: 4f375ade6aa9 ("bpf: Avoid RCU context warning when unpinning htab wi= th internal structs") Reported-by: syzbot+36e50496c8ac4bcde3f9@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3D36e50496c8ac4bcde3f9 Suggested-by: Al Viro Link: https://lore.kernel.org/all/20260423043906.GN3518998@ZenIV/ Link: https://lore.kernel.org/all/20260602002607.110866-1-kartikey406@gmail= .com/T/ [v1] Signed-off-by: Deepanshu Kartikey Acked-by: Al Viro --- Changes in v2: - NAK on v1 fix in fs/namei.c (pick_link) by Al Viro - v1 was papering over the symptom not fixing root cause - real fix is in kernel/bpf/inode.c as suggested by Al Viro - restore ->free_inode() for RCU-delayed inode/symlink freeing - keep ->destroy_inode() only for blocking bpf_any_put() cleanup --- kernel/bpf/inode.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c index 25c06a011825..188c774a469c 100644 --- a/kernel/bpf/inode.c +++ b/kernel/bpf/inode.c @@ -766,10 +766,18 @@ static void bpf_destroy_inode(struct inode *inode) { enum bpf_type type; =20 - if (S_ISLNK(inode->i_mode)) - kfree(inode->i_link); if (!bpf_inode_type(inode, &type)) bpf_any_put(inode->i_private, type); +} + +/* + * Called after RCU grace period - safe to free inode and anything + * that might be accessed by RCU pathwalk (inode fields, i_link). + */ +static void bpf_free_inode(struct inode *inode) +{ + if (S_ISLNK(inode->i_mode)) + kfree(inode->i_link); free_inode_nonrcu(inode); } =20 @@ -778,6 +786,7 @@ const struct super_operations bpf_super_ops =3D { .drop_inode =3D inode_just_drop, .show_options =3D bpf_show_options, .destroy_inode =3D bpf_destroy_inode, + .free_inode =3D bpf_free_inode, }; =20 enum { --=20 2.43.0