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

[Jan Kara <jack@suse.cz>]

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@huawei.com>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

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@suse.cz>

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR