Out of bounds write vulnerability in the sqfs_readdir() function

Out of bounds write vulnerability in the sqfs_readdir() function

[Jincheng Wang]

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Hello u-boot list,

I found the sqfs_readdir() function is vulnerable to Out-of-Bound write,
which will cause arbitrary code execution.

```
int sqfs_readdir(struct fs_dir_stream *fs_dirs, struct fs_dirent **dentp)
{
......
/* Set entry name */

strncpy(dent->name, dirs->entry->name, dirs->entry->name_size + 1);
dent->name[dirs->entry->name_size + 1] = '\0';

offset = dirs->entry->name_size + 1 + SQFS_ENTRY_BASE_LENGTH;
dirs->entry_count--;
.......
}


struct squashfs_dir_stream {
struct fs_dir_stream fs_dirs;
struct fs_dirent dentp;
size_t size;
int entry_count;
struct squashfs_directory_header *dir_header;
struct squashfs_directory_entry *entry;
......
};


static int sqfs_search_dir(struct squashfs_dir_stream *dirs, char
**token_list,
  int token_count, u32 *m_list, int m_count)
{
......
while (!sqfs_readdir(dirsp, &dent)) {
ret = strcmp(dent->name, token_list[j]);
if (!ret)
break;
free(dirs->entry);
dirs->entry = NULL;
}
......
}

```

The sqfs_readdir() function  use strncpy to  set entry name, while the type
of dirs->entry->name_size is defined as "u16" in the struct
squashfs_directory_entry
and dent->name
is defined as "char[256]" in the struct fs_dirent.

We can overwrite *dirs_header and *entry in the struct squashfs_dir_stream,
so that  we can use the sqfs_search_dir() function to free a fake
chunk which causes arbitrary code execution.
You can see the Poc in the attachment.

     host bind 0 test4.sqfs
     ls host 0 /dirs

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Re: Out of bounds write vulnerability in the sqfs_readdir() function

[Tom Rini]

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On Thu, May 26, 2022 at 04:28:07PM +0800, Jincheng Wang wrote:

> Hello u-boot list,
> 
> I found the sqfs_readdir() function is vulnerable to Out-of-Bound write,
> which will cause arbitrary code execution.
> 
> ```
> int sqfs_readdir(struct fs_dir_stream *fs_dirs, struct fs_dirent **dentp)
> {
> ......
> /* Set entry name */
> 
> strncpy(dent->name, dirs->entry->name, dirs->entry->name_size + 1);
> dent->name[dirs->entry->name_size + 1] = '\0';
> 
> offset = dirs->entry->name_size + 1 + SQFS_ENTRY_BASE_LENGTH;
> dirs->entry_count--;
> .......
> }
> 
> 
> struct squashfs_dir_stream {
> struct fs_dir_stream fs_dirs;
> struct fs_dirent dentp;
> size_t size;
> int entry_count;
> struct squashfs_directory_header *dir_header;
> struct squashfs_directory_entry *entry;
> ......
> };
> 
> 
> static int sqfs_search_dir(struct squashfs_dir_stream *dirs, char
> **token_list,
>   int token_count, u32 *m_list, int m_count)
> {
> ......
> while (!sqfs_readdir(dirsp, &dent)) {
> ret = strcmp(dent->name, token_list[j]);
> if (!ret)
> break;
> free(dirs->entry);
> dirs->entry = NULL;
> }
> ......
> }
> 
> ```
> 
> The sqfs_readdir() function  use strncpy to  set entry name, while the type
> of dirs->entry->name_size is defined as "u16" in the struct
> squashfs_directory_entry
> and dent->name
> is defined as "char[256]" in the struct fs_dirent.
> 
> We can overwrite *dirs_header and *entry in the struct squashfs_dir_stream,
> so that  we can use the sqfs_search_dir() function to free a fake
> chunk which causes arbitrary code execution.
> You can see the Poc in the attachment.
> 
>      host bind 0 test4.sqfs
>      ls host 0 /dirs

Adding the listed maintainers...

-- 
Tom

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