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([2405:201:682f:3094:aa9a:16e7:7fcc:f4d0]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3347224b0f7sm10473322a91.7.2025.09.28.03.09.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 28 Sep 2025 03:09:53 -0700 (PDT) From: Deepanshu Kartikey To: tytso@mit.edu, adilger.kernel@dilger.ca Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, Deepanshu Kartikey , syzbot+038b7bf43423e132b308@syzkaller.appspotmail.com Subject: [PATCH] ext4: validate extent entries before caching in ext4_find_extent() Date: Sun, 28 Sep 2025 15:39:46 +0530 Message-ID: <20250928100946.12445-1-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" syzbot reported a BUG_ON in ext4_es_cache_extent() triggered when opening a verity file on a corrupted ext4 filesystem mounted without a journal. The issue occurs when the extent tree contains out-of-order extents, which can happen in a corrupted filesystem. ext4_find_extent() calls ext4_cache_extents() without validating the extent entries when the tree depth is 0 (leaf level). This allows corrupted extent trees with out-of-order extents to be cached, triggering a BUG_ON in ext4_es_cache_extent() due to integer underflow when calculating hole sizes: If prev =3D 4352 and lblk =3D 1280: lblk - prev =3D 1280 - 4352 =3D -3072 (as signed) =3D 4294964224 (as unsigned) end =3D lblk + len - 1 =3D 4352 + 4294964224 - 1 =3D 1279 (after overflow) BUG_ON(end < lblk) triggers because 1279 < 4352 Fix this by adding extent entry validation using the existing ext4_valid_extent_entries() function before caching. This ensures corrupted extent trees are detected and handled properly through the error path, preventing both the BUG_ON and potential use-after-free issues. Reported-and-tested-by: syzbot+038b7bf43423e132b308@syzkaller.appspotmail.c= om Closes: https://syzkaller.appspot.com/bug?extid=3D038b7bf43423e132b308 Signed-off-by: Deepanshu Kartikey --- fs/ext4/extents.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c index ca5499e9412b..f8e45623f7ea 100644 --- a/fs/ext4/extents.c +++ b/fs/ext4/extents.c @@ -924,8 +924,18 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t bloc= k, path[0].p_bh =3D NULL; =20 i =3D depth; - if (!(flags & EXT4_EX_NOCACHE) && depth =3D=3D 0) + if (!(flags & EXT4_EX_NOCACHE) && depth =3D=3D 0) { + ext4_fsblk_t pblk =3D 0; + + if (!ext4_valid_extent_entries(inode, eh, 0, &pblk, 0)) { + EXT4_ERROR_INODE(inode, + "invalid extent entries, pblk %llu", + pblk); + ret =3D -EFSCORRUPTED; + goto err; + } ext4_cache_extents(inode, eh); + } /* walk through the tree */ while (i) { ext_debug(inode, "depth %d: num %d, max %d\n", --=20 2.43.0