Re: [PATCH 2/3] ntfs: fix out-of-bounds read in ntfs_attr_find()

[Dan Carpenter]

On Wed, Aug 31, 2022 at 10:43:36AM +0800, Hawkins Jiawei wrote:
> Kernel will iterates over ATTR_RECORDs in mft record in ntfs_attr_find().
> To ensure access on these ATTR_RECORDs are within bounds, kernel will
> do some checking during iteration.
> 
> The problem is that during checking whether ATTR_RECORD's name is within
> bounds, kernel will dereferences the ATTR_RECORD name_offset field,
> before checking this ATTR_RECORD strcture is within bounds. This problem
> may result out-of-bounds read in ntfs_attr_find(), reported by
> Syzkaller:
> 
> ==================================================================
> BUG: KASAN: use-after-free in ntfs_attr_find+0xc02/0xce0 fs/ntfs/attrib.c:597
> Read of size 2 at addr ffff88807e352009 by task syz-executor153/3607
> 
> [...]
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
>  print_address_description mm/kasan/report.c:317 [inline]
>  print_report.cold+0x2ba/0x719 mm/kasan/report.c:433
>  kasan_report+0xb1/0x1e0 mm/kasan/report.c:495
>  ntfs_attr_find+0xc02/0xce0 fs/ntfs/attrib.c:597
>  ntfs_attr_lookup+0x1056/0x2070 fs/ntfs/attrib.c:1193
>  ntfs_read_inode_mount+0x89a/0x2580 fs/ntfs/inode.c:1845
>  ntfs_fill_super+0x1799/0x9320 fs/ntfs/super.c:2854
>  mount_bdev+0x34d/0x410 fs/super.c:1400
>  legacy_get_tree+0x105/0x220 fs/fs_context.c:610
>  vfs_get_tree+0x89/0x2f0 fs/super.c:1530
>  do_new_mount fs/namespace.c:3040 [inline]
>  path_mount+0x1326/0x1e20 fs/namespace.c:3370
>  do_mount fs/namespace.c:3383 [inline]
>  __do_sys_mount fs/namespace.c:3591 [inline]
>  __se_sys_mount fs/namespace.c:3568 [inline]
>  __x64_sys_mount+0x27f/0x300 fs/namespace.c:3568
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>  [...]
>  </TASK>
> 
> The buggy address belongs to the physical page:
> page:ffffea0001f8d400 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7e350
> head:ffffea0001f8d400 order:3 compound_mapcount:0 compound_pincount:0
> flags: 0xfff00000010200(slab|head|node=0|zone=1|lastcpupid=0x7ff)
> raw: 00fff00000010200 0000000000000000 dead000000000122 ffff888011842140
> raw: 0000000000000000 0000000000040004 00000001ffffffff 0000000000000000
> page dumped because: kasan: bad access detected
> Memory state around the buggy address:
>  ffff88807e351f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff88807e351f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >ffff88807e352000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                       ^
>  ffff88807e352080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff88807e352100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> 
> This patch solves it by moving the ATTR_RECORD strcture's bounds
> checking earlier, then checking whether ATTR_RECORD's name
> is within bounds. What's more, this patch also add some comments
> to improve its maintainability.
> 
> Reported-and-tested-by: syzbot+5f8dcabe4a3b2c51c607@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: chenxiaosong (A) <chenxiaosong2@xxxxxxxxxx> 
> Link: https://lore.kernel.org/all/1636796c-c85e-7f47-e96f-e074fee3c7d3@xxxxxxxxxx/
> Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx> 
> Link: https://groups.google.com/g/syzkaller-bugs/c/t_XdeKPGTR4/m/LECAuIGcBgAJ
> Signed-off-by: Hawkins Jiawei <yin31149@xxxxxxxxx>
> ---
>  fs/ntfs/attrib.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ntfs/attrib.c b/fs/ntfs/attrib.c
> index 52615e6090e1..904734e34507 100644
> --- a/fs/ntfs/attrib.c
> +++ b/fs/ntfs/attrib.c
> @@ -594,11 +594,23 @@ static int ntfs_attr_find(const ATTR_TYPE type, const ntfschar *name,
>  	for (;;	a = (ATTR_RECORD*)((u8*)a + le32_to_cpu(a->length))) {
>  		u8 *mrec_end = (u8 *)ctx->mrec +
>  		               le32_to_cpu(ctx->mrec->bytes_allocated);
> -		u8 *name_end = (u8 *)a + le16_to_cpu(a->name_offset) +
> -			       a->name_length * sizeof(ntfschar);
> -		if ((u8*)a < (u8*)ctx->mrec || (u8*)a > mrec_end ||
> -		    name_end > mrec_end)
> +		u8 *name_end, *arec_head_end;
> +
> +		/* check for wrap around */
> +		if ((u8 *)a < (u8 *)ctx->mrec)
> +			break;
> +
> +		/* check whether Attribute Record Header is within bounds */
> +		arec_head_end = (u8 *)a + sizeof(ATTR_RECORD);
> +		if (arec_head_end < (u8 *)a || arec_head_end > mrec_end)

This works but I feel like it would be more natural to just check if
a was valid and if a +  sizeof(ATTR_RECORD) was also valid.

	if (a > mrec_end || (u8 *)a + sizeof(ATTR_RECORD) > mrec_end)

But what you have also works so if you want to go with that then that's
fine.

regards,
dan carpenter

>  			break;
> +
> +		/* check whether ATTR_RECORD's name is within bounds */
> +		name_end = (u8 *)a + le16_to_cpu(a->name_offset) +
> +			   a->name_length * sizeof(ntfschar);
> +		if (name_end > mrec_end)
> +			break;
> +
>  		ctx->attr = a;
>  		if (unlikely(le32_to_cpu(a->type) > le32_to_cpu(type) ||
>  				a->type == AT_END))