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

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+72kjbcS8JB... 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