Mounting a corrupted filesystem with directory which contains '.' dir entry with rec_len == block size results in out-of-bounds read (later on, when the corrupted directory is removed).
ext4_empty_dir() assumes every ext4 directory contains at least '.' and '..' as directory entries in the first data block. It first loads the '.' dir entry, performs sanity checks by calling ext4_check_dir_entry() and then uses its rec_len member to compute the location of '..' dir entry (in ext4_next_entry). It assumes the '..' dir entry fits into the same data block.
If the rec_len of '.' is precisely one block (4KB), it slips through the sanity checks (it is considered the last directory entry in the data block) and leaves "struct ext4_dir_entry_2 *de" point exactly past the memory slot allocated to the data block. The following call to ext4_check_dir_entry() on new value of de then dereferences this pointer which results in out-of-bounds mem access.
Fix this by extending __ext4_check_dir_entry() to check for '.' dir entries that reach the end of data block. Make sure to ignore the phony dir entries for checksum (by checking name_len for non-zero).
Note: This is reported by KASAN as use-after-free in case another structure was recently freed from the slot past the bound, but it is really an OOB read.
This issue was found by syzkaller tool.
Call Trace: [ 38.594108] BUG: KASAN: slab-use-after-free in __ext4_check_dir_entry+0x67e/0x710 [ 38.594649] Read of size 2 at addr ffff88802b41a004 by task syz-executor/5375 [ 38.595158] [ 38.595288] CPU: 0 UID: 0 PID: 5375 Comm: syz-executor Not tainted 6.14.0-rc7 #1 [ 38.595298] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 [ 38.595304] Call Trace: [ 38.595308] <TASK> [ 38.595311] dump_stack_lvl+0xa7/0xd0 [ 38.595325] print_address_description.constprop.0+0x2c/0x3f0 [ 38.595339] ? __ext4_check_dir_entry+0x67e/0x710 [ 38.595349] print_report+0xaa/0x250 [ 38.595359] ? __ext4_check_dir_entry+0x67e/0x710 [ 38.595368] ? kasan_addr_to_slab+0x9/0x90 [ 38.595378] kasan_report+0xab/0xe0 [ 38.595389] ? __ext4_check_dir_entry+0x67e/0x710 [ 38.595400] __ext4_check_dir_entry+0x67e/0x710 [ 38.595410] ext4_empty_dir+0x465/0x990 [ 38.595421] ? __pfx_ext4_empty_dir+0x10/0x10 [ 38.595432] ext4_rmdir.part.0+0x29a/0xd10 [ 38.595441] ? __dquot_initialize+0x2a7/0xbf0 [ 38.595455] ? __pfx_ext4_rmdir.part.0+0x10/0x10 [ 38.595464] ? __pfx___dquot_initialize+0x10/0x10 [ 38.595478] ? down_write+0xdb/0x140 [ 38.595487] ? __pfx_down_write+0x10/0x10 [ 38.595497] ext4_rmdir+0xee/0x140 [ 38.595506] vfs_rmdir+0x209/0x670 [ 38.595517] ? lookup_one_qstr_excl+0x3b/0x190 [ 38.595529] do_rmdir+0x363/0x3c0 [ 38.595537] ? __pfx_do_rmdir+0x10/0x10 [ 38.595544] ? strncpy_from_user+0x1ff/0x2e0 [ 38.595561] __x64_sys_unlinkat+0xf0/0x130 [ 38.595570] do_syscall_64+0x5b/0x180 [ 38.595583] entry_SYSCALL_64_after_hwframe+0x76/0x7e
Fixes: ac27a0ec112a0 ("[PATCH] ext4: initial copy of files from ext3") Signed-off-by: Jakub Acs acsjakub@amazon.com Cc: "Theodore Ts'o" tytso@mit.edu Cc: Andreas Dilger adilger.kernel@dilger.ca Cc: linux-ext4@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: Mahmoud Adam mngyadam@amazon.com Cc: stable@vger.kernel.org Cc: security@kernel.org --- If not fixed, this potentially leaks information from kernel data structures allocated after the data block.
I based the check on the assumption that every ext4 directory has '.' followed by at least one entry ('..') in the first data block. (the code in ext4_empty_dir seems to operate on this assumption)
..and it is also supported by claim in https://www.kernel.org/doc/html/latest/filesystems/ext4/directory.html: "By ext2 custom, the '.' and '..' entries must appear at the beginning of this first block"
Please confirm that this is correct and there are no valid ext4 directories that have '.' as the last directory entry. If this assumption is wrong, I would fix the caller rather than the callee.
Testing: I booted the kernel in AL2023 EC2 Instance and ran ext4 tests from git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git:
<setup with loop devices> ./check ext4/* [..] ext4/002 [not run] This test requires a valid $SCRATCH_LOGDEV ext4/004 [not run] dump utility required, skipped this test ext4/006 [not run] Couldn't find e2fuzz ext4/029 [not run] This test requires a valid $SCRATCH_LOGDEV ext4/030 [not run] mount /dev/loop1 with dax failed ext4/031 [not run] mount /dev/loop1 with dax failed ext4/047 [not run] mount /dev/loop1 with dax=always failed ext4/055 [not run] fsgqa user not defined. ext4/057 [not run] UUID ioctls are not supported by kernel. Not run: ext4/002 ext4/004 ext4/006 ext4/029 ext4/030 ext4/031 ext4/047 ext4/055 ext4/057 Passed all 69 tests (please let me know if any of the skipped tests need to be run)
Thanks, Jakub --- fs/ext4/dir.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c index 02d47a64e8d1..d157a6c0eff6 100644 --- a/fs/ext4/dir.c +++ b/fs/ext4/dir.c @@ -104,6 +104,9 @@ int __ext4_check_dir_entry(const char *function, unsigned int line, else if (unlikely(le32_to_cpu(de->inode) > le32_to_cpu(EXT4_SB(dir->i_sb)->s_es->s_inodes_count))) error_msg = "inode out of bounds"; + else if (unlikely(de->name_len > 0 && strcmp(".", de->name) == 0 && + next_offset == size)) + error_msg = "'.' directory cannot be the last in data block"; else return 0;