[PATCH v2] fs/squashfs: sqfs_read: Prevent arbitrary code execution

[PATCH v2] fs/squashfs: sqfs_read: Prevent arbitrary code execution

[Miquel Raynal]

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@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---

Changes in v2:
* Jincheng reported in private that there was a problem with small name
  sizes, the last byte was lost. The reason is, dirs->entry->name_size
  contains the length of the string minus one (and excluding the
  trailing '\0'). The previous implementation had this handled correctly
  but my initial fix did not kept the "+ 1" in place because it felt
  wrong but is actually necessary. This information is actually
  available in a comment a bit above in this file.
 

 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..3f1030057c4 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 + 1, 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

Re: [PATCH v2] fs/squashfs: sqfs_read: Prevent arbitrary code execution

[Jincheng Wang]

It works well.

Tested-by: Jincheng Wang <jc.w4ng@gmail.com>

Miquel Raynal <miquel.raynal@bootlin.com> 于2022年6月9日周四 22:02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@gmail.com>
> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> ---
>
> Changes in v2:
> * Jincheng reported in private that there was a problem with small name
>   sizes, the last byte was lost. The reason is, dirs->entry->name_size
>   contains the length of the string minus one (and excluding the
>   trailing '\0'). The previous implementation had this handled correctly
>   but my initial fix did not kept the "+ 1" in place because it felt
>   wrong but is actually necessary. This information is actually
>   available in a comment a bit above in this file.
>
>
>  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..3f1030057c4 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 + 1,
> 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
>
>

Re: [PATCH v2] fs/squashfs: sqfs_read: Prevent arbitrary code execution

[Tom Rini]

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On Thu, Jun 09, 2022 at 04:02:06PM +0200, Miquel Raynal wrote:

> 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@gmail.com>
> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> Tested-by: Jincheng Wang <jc.w4ng@gmail.com>

Applied to u-boot/master, thanks!

-- 
Tom

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