ext4: Fix integer overflow in ext4fs_read_symlink()

While zalloc() takes a size_t type, adding 1 to the le32 variable
will overflow.
A carefully crafted ext4 filesystem can exhibit an inode size of 0xffffffff
and as consequence zalloc() will do a zero allocation.

Later in the function the inode size is again used for copying data.
So an attacker can overwrite memory.

Avoid the overflow by using the __builtin_add_overflow() helper.

Signed-off-by: Richard Weinberger <richard@nod.at>

diff --git a/fs/ext4/ext4_common.c b/fs/ext4/ext4_common.c
index 7cf0160c408d5882550b625245dd5d2f93cf78e5..76f7102456e331f7724b2fed2303721805818fc9 100644 (file)
--- a/fs/ext4/ext4_common.c
+++ b/fs/ext4/ext4_common.c
@@ -2181,13 +2181,18 @@ static char *ext4fs_read_symlink(struct ext2fs_node *node)
        struct ext2fs_node *diro = node;
        int status;
        loff_t actread;
+       size_t alloc_size;
 
        if (!diro->inode_read) {
                status = ext4fs_read_inode(diro->data, diro->ino, &diro->inode);
                if (status == 0)
                        return NULL;
        }
-       symlink = zalloc(le32_to_cpu(diro->inode.size) + 1);
+
+       if (__builtin_add_overflow(le32_to_cpu(diro->inode.size), 1, &alloc_size))
+               return NULL;
+
+       symlink = zalloc(alloc_size);
        if (!symlink)
                return NULL;