Re: [PATCH v3 2/4] ext4: fix use-after-free in ext4_xattr_set_entry

[Ritesh Harjani]

On 22/06/16 10:13AM, 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

Thanks for sharing the test case. Indeed this results into UAF.

>
> 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
>
> In ext4_xattr_set_entry
>   min_offs = s->end - s->base
>            = s_inode_size - EXT4_GOOD_OLD_INODE_SIZE - i_extra_isize -
> 	     sizeof(struct ext4_xattr_ibody_header)
>   last = s->first
>   free = min_offs - ((void *)last - s->base) - sizeof(__u32)
>        = s_inode_size - EXT4_GOOD_OLD_INODE_SIZE - i_extra_isize -
>          sizeof(struct ext4_xattr_ibody_header) - sizeof(__u32)
>
> In the calculation formula, all values except s_inode_size and
> i_extra_size are fixed values. When i_extra_size is the maximum value
> s_inode_size - EXT4_GOOD_OLD_INODE_SIZE, min_offs is -4 and free is -8.
> The value overflows. As a result, the preceding issue is triggered when
> memcpy is executed.
>
> Therefore, when finding xattr or setting xattr, check whether
> there is space for storing xattr in the inode to resolve this issue.

Sounds right. Thanks for fixing it and providing detailed analysis.

Feel free to add -

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>


>
> Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> ---
>  fs/ext4/xattr.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 042325349098..c3c3194f3ee1 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -2176,8 +2176,9 @@ int ext4_xattr_ibody_find(struct inode *inode, struct ext4_xattr_info *i,
>  	struct ext4_inode *raw_inode;
>  	int error;
>
> -	if (EXT4_I(inode)->i_extra_isize == 0)
> +	if (!EXT4_INODE_HAS_XATTR_SPACE(inode))
>  		return 0;
> +
>  	raw_inode = ext4_raw_inode(&is->iloc);
>  	header = IHDR(inode, raw_inode);
>  	is->s.base = is->s.first = IFIRST(header);
> @@ -2205,8 +2206,9 @@ int ext4_xattr_ibody_set(handle_t *handle, struct inode *inode,
>  	struct ext4_xattr_search *s = &is->s;
>  	int error;
>
> -	if (EXT4_I(inode)->i_extra_isize == 0)
> +	if (!EXT4_INODE_HAS_XATTR_SPACE(inode))
>  		return -ENOSPC;
> +
>  	error = ext4_xattr_set_entry(i, s, handle, inode, false /* is_block */);
>  	if (error)
>  		return error;
> --
> 2.31.1
>