Re: [PATCH] ecryptfs: fix memory corruption when storing crypto info in xattrs

Michael Halcrow
2008-10-22T19:27:19Z

On Wed, Oct 22, 2008 at 12:14:00PM -0500, Eric Sandeen wrote:
> When ecryptfs allocates space to write crypto headers into, before
> copying it out to file headers or to xattrs, it looks at the value of
> crypt_stat->num_header_bytes_at_front to determine how much space it
> needs. This is also used as the file offset to the actual encrypted
> data, so for xattr-stored crypto info, the value was zero.
>
> So, we kzalloc'd 0 bytes, and then ran off to write to that memory.
> (Which returned as ZERO_SIZE_PTR, so we explode quickly).
>
> The right answer is to always allocate a page to write into; the current
> code won't ever write more than that (this is enforced by the
> (PAGE_CACHE_SIZE - offset) length in the call to
> ecryptfs_generate_key_packet_set). To be explicit about this, we now
> send in a "max" parameter, rather than magically using PAGE_CACHE_SIZE
> there.
>
> Also, since the pointer we pass down the callchain eventually gets the
> virt_to_page() treatment, we should be using a alloc_page variant, not
> kzalloc (see also 7fcba054373d5dfc43d26e243a5c9b92069972ee)
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>

The removal of the memset is okay because it is header information,
which is written to the disk in the clear anyway and is not
sensitive.

Acked-by: Michael Halcrow <mhalcrow@us.ibm.com>

> ---
>
> Index: linux-2.6.27.x86_64/fs/ecryptfs/crypto.c
> ===================================================================
> --- linux-2.6.27.x86_64.orig/fs/ecryptfs/crypto.c
> +++ linux-2.6.27.x86_64/fs/ecryptfs/crypto.c
> @@ -1251,6 +1251,7 @@ struct kmem_cache *ecryptfs_header_cache
> /**
> * ecryptfs_write_headers_virt
> * @page_virt: The virtual address to write the headers to
> + * @max: The size of memory allocated at page_virt
> * @size: Set to the number of bytes written by this function
> * @crypt_stat: The cryptographic context
> * @ecryptfs_dentry: The eCryptfs dentry
> @@ -1278,7 +1279,8 @@ struct kmem_cache *ecryptfs_header_cache
> *
> * Returns zero on success
> */
> -static int ecryptfs_write_headers_virt(char *page_virt, size_t *size,
> +static int ecryptfs_write_headers_virt(char *page_virt, size_t max,
> + size_t *size,
> struct ecryptfs_crypt_stat *crypt_stat,
> struct dentry *ecryptfs_dentry)
> {
> @@ -1296,7 +1298,7 @@ static int ecryptfs_write_headers_virt(c
> offset += written;
> rc = ecryptfs_generate_key_packet_set((page_virt + offset), crypt_stat,
> ecryptfs_dentry, &written,
> - PAGE_CACHE_SIZE - offset);
> + max - offset);
> if (rc)
> ecryptfs_printk(KERN_WARNING, "Error generating key packet "
> "set; rc = [%d]\n", rc);
> @@ -1368,14 +1370,14 @@ int ecryptfs_write_metadata(struct dentr
> goto out;
> }
> /* Released in this function */
> - virt = kzalloc(crypt_stat->num_header_bytes_at_front, GFP_KERNEL);
> + virt = (char *)get_zeroed_page(GFP_KERNEL);
> if (!virt) {
> printk(KERN_ERR "%s: Out of memory\n", __func__);
> rc = -ENOMEM;
> goto out;
> }
> - rc = ecryptfs_write_headers_virt(virt, &size, crypt_stat,
> - ecryptfs_dentry);
> + rc = ecryptfs_write_headers_virt(virt, PAGE_CACHE_SIZE, &size,
> + crypt_stat, ecryptfs_dentry);
> if (unlikely(rc)) {
> printk(KERN_ERR "%s: Error whilst writing headers; rc = [%d]\n",
> __func__, rc);
> @@ -1393,8 +1395,7 @@ int ecryptfs_write_metadata(struct dentr
> goto out_free;
> }
> out_free:
> - memset(virt, 0, crypt_stat->num_header_bytes_at_front);
> - kfree(virt);
> + free_page((unsigned long)virt);
> out:
> return rc;
> }
>