Re: [PATCH] ext4: fix use-after-free in ext4_xattr_set_entry

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在 2022/6/14 17:54, Jan Kara 写道:
On Thu 02-06-22 19:46:51, Baokun Li wrote:
Hulk Robot reported a issue:
==================================================================
BUG: KASAN: use-after-free in ext4_xattr_set_entry+0x18ab/0x3500
Write of size 4105 at addr ffff8881675ef5f4 by task syz-executor.0/7092

CPU: 1 PID: 7092 Comm: syz-executor.0 Not tainted 4.19.90-dirty #17
Call Trace:
[...]
  memcpy+0x34/0x50 mm/kasan/kasan.c:303
  ext4_xattr_set_entry+0x18ab/0x3500 fs/ext4/xattr.c:1747
  ext4_xattr_ibody_inline_set+0x86/0x2a0 fs/ext4/xattr.c:2205
  ext4_xattr_set_handle+0x940/0x1300 fs/ext4/xattr.c:2386
  ext4_xattr_set+0x1da/0x300 fs/ext4/xattr.c:2498
  __vfs_setxattr+0x112/0x170 fs/xattr.c:149
  __vfs_setxattr_noperm+0x11b/0x2a0 fs/xattr.c:180
  __vfs_setxattr_locked+0x17b/0x250 fs/xattr.c:238
  vfs_setxattr+0xed/0x270 fs/xattr.c:255
  setxattr+0x235/0x330 fs/xattr.c:520
  path_setxattr+0x176/0x190 fs/xattr.c:539
  __do_sys_lsetxattr fs/xattr.c:561 [inline]
  __se_sys_lsetxattr fs/xattr.c:557 [inline]
  __x64_sys_lsetxattr+0xc2/0x160 fs/xattr.c:557
  do_syscall_64+0xdf/0x530 arch/x86/entry/common.c:298
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x459fe9
RSP: 002b:00007fa5e54b4c08 EFLAGS: 00000246 ORIG_RAX: 00000000000000bd
RAX: ffffffffffffffda RBX: 000000000051bf60 RCX: 0000000000459fe9
RDX: 00000000200003c0 RSI: 0000000020000180 RDI: 0000000020000140
RBP: 000000000051bf60 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000001009 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffc73c93fc0 R14: 000000000051bf60 R15: 00007fa5e54b4d80
[...]
==================================================================

Above issue may happen as follows:
-------------------------------------
ext4_xattr_set
   ext4_xattr_set_handle
     ext4_xattr_ibody_find
       >> s->end < s->base
       >> no EXT4_STATE_XATTR
       >> xattr_check_inode is not executed
     ext4_xattr_ibody_set
       ext4_xattr_set_entry
        >> size_t min_offs = s->end - s->base
        >> UAF in memcpy

we can easily reproduce this problem with the following commands:
     mkfs.ext4 -F /dev/sda
     mount -o debug_want_extra_isize=128 /dev/sda /mnt
     touch /mnt/file
     setfattr -n user.cat -v `seq -s z 4096|tr -d '[:digit:]'` /mnt/file

In ext4_xattr_ibody_find, we have the following assignment logic:
   header = IHDR(inode, raw_inode)
          = raw_inode + EXT4_GOOD_OLD_INODE_SIZE + i_extra_isize
   is->s.base = IFIRST(header)
              = header + sizeof(struct ext4_xattr_ibody_header)
   is->s.end = raw_inode + s_inode_size

Obviously, when the inode does not have EXT4_status_XATTR and its
i_extra_isize is large, is->s.end may be larger than is->s.base.
In this case, the above issue may be triggered.

    EXT4_GOOD_OLD_INODE_SIZE  extra_isize  header   entry   pad   data
|---------------------------|------------|------|---------|---|--------|

As shown above, when adding an xattr to an inode, we must ensure that the
inode_size is not less than EXT4_GOOD_OLD_INODE_SIZE + extra_isize + pad.

Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
Thanks for the fix! It looks good to me. Just one small nit:

+#define INODE_HAVE_XATTR_SPACE(inode)					\
+	((EXT4_I(inode)->i_extra_isize != 0) &&				\
+	 (EXT4_GOOD_OLD_INODE_SIZE + EXT4_I(inode)->i_extra_isize +	\
+	  sizeof(struct ext4_xattr_ibody_header) + EXT4_XATTR_PAD <=	\
+	  EXT4_INODE_SIZE((inode)->i_sb)))
+
We should have ext4 in the name of the above macro so something like:

EXT4_INODE_HAS_XATTR_SPACE()

With that fixed feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

Thank you for your review!

I will send a patch V2 with the changes suggested by you. Thanks again!

--
With Best Regards,
Baokun Li

.




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