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

Benjamin Coddington
2019-09-15T14:08:52Z

On 13 Sep 2019, at 13:39, Benjamin Coddington wrote:

> 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.

Just looking at this again today -- we can definitely keep the check,
but
the second half of the statement also assumes a contiguous head/tail
range.
I think it's safe to just remove the test altogether and place the
netobj at
the end of the tail. Then in 2/2, we'll just place it at the beginning
of
the tail because the function is specialized for the mic.

Ben