6.12-stable review patch. If anyone has any objections, please let me know.
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From: Jiayuan Chen jiayuan.chen@linux.dev
[ Upstream commit 1adaea51c61b52e24e7ab38f7d3eba023b2d050d ]
On PREEMPT_RT kernels, after rt6_get_pcpu_route() returns NULL, the current task can be preempted. Another task running on the same CPU may then execute rt6_make_pcpu_route() and successfully install a pcpu_rt entry. When the first task resumes execution, its cmpxchg() in rt6_make_pcpu_route() will fail because rt6i_pcpu is no longer NULL, triggering the BUG_ON(prev). It's easy to reproduce it by adding mdelay() after rt6_get_pcpu_route().
Using preempt_disable/enable is not appropriate here because ip6_rt_pcpu_alloc() may sleep.
Fix this by handling the cmpxchg() failure gracefully on PREEMPT_RT: free our allocation and return the existing pcpu_rt installed by another task. The BUG_ON is replaced by WARN_ON_ONCE for non-PREEMPT_RT kernels where such races should not occur.
Link: https://syzkaller.appspot.com/bug?extid=9b35e9bc0951140d13e6 Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.") Reported-by: syzbot+9b35e9bc0951140d13e6@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6918cd88.050a0220.1c914e.0045.GAE@google.com/T/ Signed-off-by: Jiayuan Chen jiayuan.chen@linux.dev Link: https://patch.msgid.link/20251223051413.124687-1-jiayuan.chen@linux.dev Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv6/route.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/net/ipv6/route.c b/net/ipv6/route.c index 22866444efc0..276fa74af206 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -1470,7 +1470,18 @@ static struct rt6_info *rt6_make_pcpu_route(struct net *net,
p = this_cpu_ptr(res->nh->rt6i_pcpu); prev = cmpxchg(p, NULL, pcpu_rt); - BUG_ON(prev); + if (unlikely(prev)) { + /* + * Another task on this CPU already installed a pcpu_rt. + * This can happen on PREEMPT_RT where preemption is possible. + * Free our allocation and return the existing one. + */ + WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_RT)); + + dst_dev_put(&pcpu_rt->dst); + dst_release(&pcpu_rt->dst); + return prev; + }
if (res->f6i->fib6_destroying) { struct fib6_info *from;