Re: [PATCH 1/2] SUNRPC: Fix buffer handling of GSS MIC with less slack

Benjamin Coddington
2019-09-13T17:39:33Z

On 13 Sep 2019, at 12:05, Chuck Lever wrote:

> Hi Ben-
>
> A few review comments below.
>
>
>> On Sep 12, 2019, at 1:07 PM, Benjamin Coddington
>> <bcodding@redhat.com> wrote:
>>
>> The GSS Message Integrity Check data for krb5i may lie partially in
>> the XDR
>> reply buffer's pages and tail. If so, we try to copy the entire MIC
>> into
>> free space in the tail. But as the estimations of the slack space
>> required
>> for authentication and verification have improved there may be less
>> free
>> space in the tail to complete this copy -- see commit 2c94b8eca1a2
>> ("SUNRPC: Use au_rslack when computing reply buffer size"). In fact,
>> there
>> may only be room in the tail for a single copy of the MIC, and not
>> part of
>> the MIC and then another complete copy.
>>
>> The real world failure reported is that `ls` of a directory on NFS
>> may
>> sometimes return -EIO, which can be traced back to
>> xdr_buf_read_netobj()
>> failing to find available free space in the tail to copy the MIC.
>>
>> Fix this by checking for the case of the MIC crossing the boundaries
>> of
>> head, pages, and tail. If so, shift the buffer until the MIC is
>> contained
>> completely within the pages or tail. This allows the remainder of
>> the
>> function to create a sub buffer that directly address the complete
>> MIC.
>>
>> Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
>> Cc: stable@vger.kernel.org
>> ---
>> net/sunrpc/xdr.c | 45 +++++++++++++++++++++++++++------------------
>> 1 file changed, 27 insertions(+), 18 deletions(-)
>>
>> diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
>> index 48c93b9e525e..6e05a9693568 100644
>> --- a/net/sunrpc/xdr.c
>> +++ b/net/sunrpc/xdr.c
>> @@ -1237,39 +1237,48 @@ xdr_encode_word(struct xdr_buf *buf, unsigned
>> int base, u32 obj)
>> EXPORT_SYMBOL_GPL(xdr_encode_word);
>>
>> /* If the netobj starting offset bytes from the start of xdr_buf is
>> contained
>> - * entirely in the head or the tail, set object to point to it;
>> otherwise
>> - * try to find space for it at the end of the tail, copy it there,
>> and
>> - * set obj to point to it. */
>> + * entirely in the head, pages, or tail, set object to point to it;
>> otherwise
>> + * shift the buffer until it is contained entirely within the pages
>> or tail.
>> + */
>
> Nit: I would explicitly note in this comment that there is no need
> for the caller to free @obj, and perhaps it should be noted that
> the organization of @buf can be changed by this function.
>
> Maybe more appropriate for 2/2.

Ok.. yes.. though I don't feel strongly about noting that *obj doesn't
need to be freed.

>> int xdr_buf_read_netobj(struct xdr_buf *buf, struct xdr_netobj *obj,
>> unsigned int offset)
>> {
>> struct xdr_buf subbuf;
>> + unsigned int len_to_boundary;
>>
>> if (xdr_decode_word(buf, offset, &obj->len))
>> return -EFAULT;
>> - if (xdr_buf_subsegment(buf, &subbuf, offset + 4, obj->len))
>> +
>> + offset += 4;
>
> Nit: No blank line before "offset += 4;" would help me understand
> how the offset bump is related to xdr_decode_word(). It took me
> a few blinks to see.
>
>
>> +
>> + /* Is the obj partially in the head? */
>> + len_to_boundary = buf->head->iov_len - offset;
>> + if (len_to_boundary > 0 && len_to_boundary < obj->len)
>
> I'm not especially excited about the integer underflow when offset
> is larger than buf->head->iov_len. This might be more explicit:
>
> if (offset < buf->head[0].iov_len &&
> offset + obj->len > buf->head[0].iov_len)

Yep, makes sense - and I prefer the clarity.

>> + xdr_shift_buf(buf, len_to_boundary);
>> +
>> + /* Is the obj partially in the pages? */
>> + len_to_boundary = buf->head->iov_len + buf->page_len - offset;
>> + if (len_to_boundary > 0 && len_to_boundary < obj->len)
>
> Ditto.
>
>
>> + xdr_shrink_pagelen(buf, len_to_boundary);
>> +
>> + if (xdr_buf_subsegment(buf, &subbuf, offset, obj->len))
>> return -EFAULT;
>>
>> - /* Is the obj contained entirely in the head? */
>> - obj->data = subbuf.head[0].iov_base;
>> - if (subbuf.head[0].iov_len == obj->len)
>> - return 0;
>> - /* ..or is the obj contained entirely in the tail? */
>> + /* Most likely: is the obj contained entirely in the tail? */
>> obj->data = subbuf.tail[0].iov_base;
>> if (subbuf.tail[0].iov_len == obj->len)
>> return 0;
>>
>> - /* use end of tail as storage for obj:
>> - * (We don't copy to the beginning because then we'd have
>> - * to worry about doing a potentially overlapping copy.
>> - * This assumes the object is at most half the length of the
>> - * tail.) */
>> + /* ..or is the obj contained entirely in the head? */
>> + obj->data = subbuf.head[0].iov_base;
>> + if (subbuf.head[0].iov_len == obj->len)
>> + return 0;
>
> It looks like you're reversing these two tests as a
> micro-optimization?
> Maybe that should be left for another patch, since this is supposed to
> be a narrow fix.

Yes, if not done here - is it even worth another patch?

> Also, I found the new comments confusing: here they refer to the head
> and tail of @subbuf; above they refer to head and tail of @buf. Note
> for
> 2/2, I guess.

I can clarify in 2/2.

>> +
>> + /* obj is in the pages: move to tail */
>> if (obj->len > buf->buflen - buf->len)
>> return -ENOMEM;
>> - if (buf->tail[0].iov_len != 0)
>> - obj->data = buf->tail[0].iov_base + buf->tail[0].iov_len;
>> - else
>> - obj->data = buf->head[0].iov_base + buf->head[0].iov_len;
>> + obj->data = buf->head[0].iov_base + buf->head[0].iov_len;
>
> Not sure this is a safe change. It's possible that the head buffer
> and tail buffer are not contiguous, which is what the
> buf->tail.iov_len
> check is looking for, IMO. Can this hunk be left out?

That's something I missed somehow, thanks for pointing it out. I see
now
that the transport can allocate them any way it likes.

Thanks for the review, I'll send a v2.

Ben