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iflib: prevent possible infinite loop in iflib_encap

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From Jake:
iflib_encap calls bus_dmamap_load_mbuf_sg. Upon it returning EFBIG, an
m_collapse and an m_defrag are attempted to shrink the mbuf cluster to
fit within the DMA segment limitations.

However, if we call m_defrag, and then bus_dmamap_load_mbuf_sg returns
EFBIG on the now defragmented mbuf, we will continuously re-call
bus_dmamap_load_mbuf_sg over and over.

This happens because m_head isn't NULL, and remap is >1, so we don't try
to m_collapse or m_defrag again. The only way we exit the loop is if
m_head is NULL. However, m_head can't be modified by the call to
bus_dmamap_load_mbuf_sg, because we don't pass it as a double pointer.

I believe this will be an incredibly rare occurrence, because it is
unlikely that bus_dmamap_load_mbuf_sg will actually fail on the second
defragment with an EFBIG error. However, it still seems like
a possibility that we should account for.

Fix the exit check to ensure that if remap is >1, we will also exit,
even if m_head is not NULL.

Submitted by:	Jacob Keller <jacob.e.keller@intel.com>
Reviewed by:	shurd@, gallatin@
MFC after:	1 week
Sponsored by:	Intel Corporation
Differential Revision:	https://reviews.freebsd.org/D19468
This commit is contained in:
Eric Joyner 2019-03-19 17:49:03 +00:00
parent 938b7a44d9
commit 3e8d1bae5f
1 changed files with 7 additions and 2 deletions
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9
sys/net/iflib.c
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@ -3276,9 +3276,14 @@ iflib_encap(iflib_txq_t txq, struct mbuf **m_headp)
txq->ift_mbuf_defrag++;
m_head = m_defrag(*m_headp, M_NOWAIT);
}
remap++;
if (__predict_false(m_head == NULL))
/*
* remap should never be >1 unless bus_dmamap_load_mbuf_sg
* failed to map an mbuf that was run through m_defrag
*/
MPASS(remap <= 1);
if (__predict_false(m_head == NULL || remap > 1))
goto defrag_failed;
remap++;
*m_headp = m_head;
goto retry;
break;