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([2405:201:682f:389d:aa38:46c1:6cd:acc2]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29498e3d2aasm102431035ad.88.2025.10.27.21.29.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Oct 2025 21:29:28 -0700 (PDT) From: Deepanshu Kartikey To: mark@fasheh.com, jlbec@evilplan.org, joseph.qi@linux.alibaba.com, heming.zhao@suse.com Cc: ocfs2-devel@lists.linux.dev, linux-kernel@vger.kernel.org, Deepanshu Kartikey , syzbot+fd8af97c7227fe605d95@syzkaller.appspotmail.com Subject: [PATCH v3] ocfs2: validate cl_bpc in allocator inodes to prevent divide-by-zero Date: Tue, 28 Oct 2025 09:59:19 +0530 Message-ID: <20251028042919.18704-1-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" The chain allocator field cl_bpc (blocks per cluster) is read from disk and used in division operations without validation. A corrupted filesystem image with cl_bpc=3D0 causes a divide-by-zero crash in the kernel: divide error: 0000 [#1] PREEMPT SMP KASAN RIP: 0010:ocfs2_bg_discontig_add_extent fs/ocfs2/suballoc.c:335 [inline] RIP: 0010:ocfs2_block_group_fill+0x5bd/0xa70 fs/ocfs2/suballoc.c:386 Call Trace: ocfs2_block_group_alloc+0x7e9/0x1330 fs/ocfs2/suballoc.c:703 ocfs2_reserve_suballoc_bits+0x20a6/0x4640 fs/ocfs2/suballoc.c:834 ocfs2_reserve_new_inode+0x4f4/0xcc0 fs/ocfs2/suballoc.c:1074 ocfs2_mknod+0x83c/0x2050 fs/ocfs2/namei.c:306 This patch adds validation in ocfs2_validate_inode_block() to ensure cl_bpc matches the expected value calculated from the superblock's cluster size and block size for chain allocator inodes (identified by OCFS2_CHAIN_FL). Moving the validation to inode validation time (rather than allocation time) has several benefits: - Validates once when the inode is read, rather than on every allocation - Protects all code paths that use cl_bpc (allocation, resize, etc.) - Follows the existing pattern of inode validation in OCFS2 - Centralizes validation logic The validation catches both: - Zero values that cause divide-by-zero crashes - Non-zero but incorrect values indicating filesystem corruption or mismatched filesystem geometry With this fix, mounting a corrupted filesystem produces: OCFS2: ERROR (device loop0): ocfs2_validate_inode_block: Inode 74 has corrupted cl_bpc: ondisk=3D0 expected=3D16 Instead of a kernel crash. Link: https://lore.kernel.org/ocfs2-devel/20251026132625.12348-1-kartikey40= 6@gmail.com/T/#u [v1] Link: https://lore.kernel.org/all/20251027124131.10002-1-kartikey406@gmail.= com/T/ [v2] Reported-by: syzbot+fd8af97c7227fe605d95@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3Dfd8af97c7227fe605d95 Tested-by: syzbot+fd8af97c7227fe605d95@syzkaller.appspotmail.com Suggested-by: Joseph Qi Signed-off-by: Deepanshu Kartikey Reviewed-by: Joseph Qi --- Changes in v3: - Removed unnecessary local osb variable as suggested by Joseph Qi - Added blank line before rc =3D 0 for better readability Changes in v2: - Moved validation from ocfs2_block_group_alloc() to ocfs2_validate_inode_b= lock() as suggested by Joseph Qi to benefit all code paths - Added OCFS2_CHAIN_FL check to only validate chain allocator inodes - Updated commit message to reflect the new location --- fs/ocfs2/inode.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c index fcc89856ab95..dbbfe72b1dc7 100644 --- a/fs/ocfs2/inode.c +++ b/fs/ocfs2/inode.c @@ -1502,6 +1502,21 @@ int ocfs2_validate_inode_block(struct super_block *s= b, le16_to_cpu(di->i_suballoc_slot)); goto bail; } + /* Validate cl_bpc for chain allocator inodes */ + if (le32_to_cpu(di->i_flags) & OCFS2_CHAIN_FL) { + struct ocfs2_chain_list *cl =3D &di->id2.i_chain; + u16 cl_bpc =3D le16_to_cpu(cl->cl_bpc); + u16 expected_bpc =3D 1 << (OCFS2_SB(sb)->s_clustersize_bits - + sb->s_blocksize_bits); + + if (cl_bpc !=3D expected_bpc) { + rc =3D ocfs2_error(sb, + "Inode %llu has corrupted cl_bpc: ondisk=3D%u expected=3D%u\n", + (unsigned long long)bh->b_blocknr, + cl_bpc, expected_bpc); + goto bail; + } + } =20 rc =3D 0; =20 --=20 2.43.0