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([2405:4802:1d4a:e90:814e:e9d3:767e:9a2a]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-cc72aefa9c0sm797164a12.29.2026.09.19.03.30.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 19 Sep 2026 03:30:03 -0700 (PDT) From: Nguyen Ngoc Thang To: Eric Van Hensbergen , Latchesar Ionkov , Dominique Martinet , Christian Schoenebeck Cc: v9fs@lists.linux.dev, linux-kernel@vger.kernel.org, Nguyen Ngoc Thang , syzbot+de6fd6789748a8aa64a0@syzkaller.appspotmail.com Subject: [PATCH] 9p: bound the xattr size used to allocate the ACL buffer Date: Sat, 19 Sep 2026 17:29:58 +0700 Message-ID: <20260919102958.142460-1-ngocthang2710.1999@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" v9fs_fid_get_acl() sizes its kzalloc() buffer from the xattr length reported by the 9p server. A server, or a syzbot-style fake one, can report an arbitrarily large value. When it exceeds what the page allocator can serve, mounting with posixacl,access=3Dclient trips: WARNING: mm/page_alloc.c:5340 at __alloc_frozen_pages_noprof ___kmalloc_large_node v9fs_fid_get_acl v9fs_get_acl v9fs_inode_from_fid_dotl v9fs_get_tree A valid POSIX ACL always fits in an xattr value, so reject sizes above XATTR_SIZE_MAX. __v9fs_get_acl() already maps this error to -EIO. Reported-by: syzbot+de6fd6789748a8aa64a0@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3Dde6fd6789748a8aa64a0 Fixes: 85ff872d3f4a ("fs/9p: Implement POSIX ACL permission checking functi= on") Signed-off-by: Nguyen Ngoc Thang --- fs/9p/acl.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/fs/9p/acl.c b/fs/9p/acl.c index ae7e7cf7523a..0dd7e72bbdd3 100644 --- a/fs/9p/acl.c +++ b/fs/9p/acl.c @@ -29,6 +29,9 @@ static struct posix_acl *v9fs_fid_get_acl(struct p9_fid *= fid, const char *name) return ERR_PTR(size); if (size =3D=3D 0) return ERR_PTR(-ENODATA); + /* the size is server-controlled; a valid ACL fits in an xattr */ + if (size > XATTR_SIZE_MAX) + return ERR_PTR(-E2BIG); =20 value =3D kzalloc(size, GFP_NOFS); if (!value) --=20 2.43.0