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| author | Eric Dumazet <edumazet@google.com> | 2015-12-14 14:08:53 -0800 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2015-12-14 23:52:00 -0500 |
| commit | 5037e9ef9454917b047f9f3a19b4dd179fbf7cd4 (patch) | |
| tree | dc12a109e95664b9b2d3f0dbbf97f195c57595f0 /include/net/dst.h | |
| parent | 2274d3753f6c5a885be4cfdf8b39ae2045ba6e30 (diff) | |
| download | linux-5037e9ef9454917b047f9f3a19b4dd179fbf7cd4.tar.gz | |
net: fix IP early demux races
David Wilder reported crashes caused by dst reuse. <quote David> I am seeing a crash on a distro V4.2.3 kernel caused by a double release of a dst_entry. In ipv4_dst_destroy() the call to list_empty() finds a poisoned next pointer, indicating the dst_entry has already been removed from the list and freed. The crash occurs 18 to 24 hours into a run of a network stress exerciser. </quote> Thanks to his detailed report and analysis, we were able to understand the core issue. IP early demux can associate a dst to skb, after a lookup in TCP/UDP sockets. When socket cache is not properly set, we want to store into sk->sk_dst_cache the dst for future IP early demux lookups, by acquiring a stable refcount on the dst. Problem is this acquisition is simply using an atomic_inc(), which works well, unless the dst was queued for destruction from dst_release() noticing dst refcount went to zero, if DST_NOCACHE was set on dst. We need to make sure current refcount is not zero before incrementing it, or risk double free as David reported. This patch, being a stable candidate, adds two new helpers, and use them only from IP early demux problematic paths. It might be possible to merge in net-next skb_dst_force() and skb_dst_force_safe(), but I prefer having the smallest patch for stable kernels : Maybe some skb_dst_force() callers do not expect skb->dst can suddenly be cleared. Can probably be backported back to linux-3.6 kernels Reported-by: David J. Wilder <dwilder@us.ibm.com> Tested-by: David J. Wilder <dwilder@us.ibm.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include/net/dst.h')
| -rw-r--r-- | include/net/dst.h | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/include/net/dst.h b/include/net/dst.h index 1279f9b09791..c7329dcd90cc 100644 --- a/include/net/dst.h +++ b/include/net/dst.h @@ -322,6 +322,39 @@ static inline void skb_dst_force(struct sk_buff *skb) } } +/** + * dst_hold_safe - Take a reference on a dst if possible + * @dst: pointer to dst entry + * + * This helper returns false if it could not safely + * take a reference on a dst. + */ +static inline bool dst_hold_safe(struct dst_entry *dst) +{ + if (dst->flags & DST_NOCACHE) + return atomic_inc_not_zero(&dst->__refcnt); + dst_hold(dst); + return true; +} + +/** + * skb_dst_force_safe - makes sure skb dst is refcounted + * @skb: buffer + * + * If dst is not yet refcounted and not destroyed, grab a ref on it. + */ +static inline void skb_dst_force_safe(struct sk_buff *skb) +{ + if (skb_dst_is_noref(skb)) { + struct dst_entry *dst = skb_dst(skb); + + if (!dst_hold_safe(dst)) + dst = NULL; + + skb->_skb_refdst = (unsigned long)dst; + } +} + /** * __skb_tunnel_rx - prepare skb for rx reinsert |