[PATCH] fs/squashfs: sqfs_read: Prevent arbitrary code execution

[Jincheng Wang]

To be honest, I don't have experience....

Reference to the article "
https://events19.linuxfoundation.org/wp-content/uploads/2018/07/may_be_maintainer-2.pdf"
, I tried to do some work.

1. checkpatch  (√)
2. no new compile warnings (√)
3. if bug fix, verify the bug is fixed (√)
     I tried some vul-samples, and the patch solved them well.
4. if new feature, feature matches documentation (×)
     I try some normal samples, and there is a bug. The dir name or
filename loses a byte. You can use the attachment to check it.
     I use command " mksquashfs $dir normal.sqfs -noI -noD -noF -noX" to
create the squashfs sample.


```before patch
=> ls host 0
BTRFS: superblock end 69632 is larger than device size 4096
BTRFS: superblock end 69632 is larger than device size 4096
       11   hello
BTRFS: superblock end 69632 is larger than device size 4096
BTRFS: superblock end 69632 is larger than device size 4096
1 file(s), 0 dir(s)
```


```after patch
=> ls host 0
BTRFS: superblock end 69632 is larger than device size 4096
BTRFS: superblock end 69632 is larger than device size 4096
       11   hell
BTRFS: superblock end 69632 is larger than device size 4096
BTRFS: superblock end 69632 is larger than device size 4096

1 file(s), 0 dir(s)
 ```






Tom Rini <trini at konsulko.com> 于2022年6月7日周二 21:43写道：

> On Tue, Jun 07, 2022 at 06:00:38PM +0800, Jincheng Wang wrote:
>
> > It works well, thanks for your work.
>
> Can you please provide a Tested-by?  Thanks!
>
> >
> >
> > Miquel Raynal <miquel.raynal at bootlin.com> 于2022年6月3日周五 23:26写道：
> > >
> > > Following Jincheng's report, an out-of-band write leading to arbitrary
> > > code execution is possible because on one side the squashfs logic
> > > accepts directory names up to 65535 bytes (u16), while U-Boot fs logic
> > > accepts directory names up to 255 bytes long.
> > >
> > > Prevent such an exploit from happening by capping directory name sizes
> > > to 255. Use a define for this purpose so that developers can link the
> > > limitation to its source and eventually kill it some day by dynamically
> > > allocating this array (if ever desired).
> > >
> > > Link:
> https://lore.kernel.org/all/CALO=DHFB+yBoXxVr5KcsK0iFdg+e7ywko4-e+72kjbcS8JBfPw@mail.gmail.com
> > > Reported-by: Jincheng Wang <jc.w4ng at gmail.com>
> > > Signed-off-by: Miquel Raynal <miquel.raynal at bootlin.com>
> > > ---
> > >
> > > Hello Jincheng, can you please give this fix a try?
> > >
> > > Thanks!
> > > Miquèl
> > >
> > >  fs/squashfs/sqfs.c | 8 +++++---
> > >  include/fs.h       | 4 +++-
> > >  2 files changed, 8 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/fs/squashfs/sqfs.c b/fs/squashfs/sqfs.c
> > > index b4484fa17f5..ee6ac8b3e4d 100644
> > > --- a/fs/squashfs/sqfs.c
> > > +++ b/fs/squashfs/sqfs.c
> > > @@ -976,6 +976,7 @@ int sqfs_readdir(struct fs_dir_stream *fs_dirs,
> struct fs_dirent **dentp)
> > >         int i_number, offset = 0, ret;
> > >         struct fs_dirent *dent;
> > >         unsigned char *ipos;
> > > +       u16 name_size;
> > >
> > >         dirs = (struct squashfs_dir_stream *)fs_dirs;
> > >         if (!dirs->size) {
> > > @@ -1058,9 +1059,10 @@ int sqfs_readdir(struct fs_dir_stream *fs_dirs,
> struct fs_dirent **dentp)
> > >                 return -SQFS_STOP_READDIR;
> > >         }
> > >
> > > -       /* Set entry name */
> > > -       strncpy(dent->name, dirs->entry->name, dirs->entry->name_size
> + 1);
> > > -       dent->name[dirs->entry->name_size + 1] = '\0';
> > > +       /* Set entry name (capped at FS_DIRENT_NAME_LEN which is a
> U-Boot limitation) */
> > > +       name_size = min_t(u16, dirs->entry->name_size,
> FS_DIRENT_NAME_LEN - 1);
> > > +       strncpy(dent->name, dirs->entry->name, name_size);
> > > +       dent->name[name_size] = '\0';
> > >
> > >         offset = dirs->entry->name_size + 1 + SQFS_ENTRY_BASE_LENGTH;
> > >         dirs->entry_count--;
> > > diff --git a/include/fs.h b/include/fs.h
> > > index b43f16a692f..2195dc172ec 100644
> > > --- a/include/fs.h
> > > +++ b/include/fs.h
> > > @@ -174,6 +174,8 @@ int fs_write(const char *filename, ulong addr,
> loff_t offset, loff_t len,
> > >  #define FS_DT_REG  8         /* regular file */
> > >  #define FS_DT_LNK  10        /* symbolic link */
> > >
> > > +#define FS_DIRENT_NAME_LEN 256
> > > +
> > >  /**
> > >   * struct fs_dirent - directory entry
> > >   *
> > > @@ -194,7 +196,7 @@ struct fs_dirent {
> > >         /** change_time:        time of last modification */
> > >         struct rtc_time change_time;
> > >         /** name:               file name */
> > > -       char name[256];
> > > +       char name[FS_DIRENT_NAME_LEN];
> > >  };
> > >
> > >  /* Note: fs_dir_stream should be treated as opaque to the user of fs
> layer */
> > > --
> > > 2.34.1
> > >
>
> --
> Tom
>
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