[PATCH net-next v2] rds: synchronize info callbacks with module unload

[Chengfeng Ye <nicoyip.dev@gmail.com>]

rds_info_getsockopt() reads a callback from rds_info_funcs and invokes it
without protecting the callback's lifetime.  Transport modules register
functions stored in this array.  For example, rds_tcp.ko registers
rds_tcp_tc_info() for RDS_INFO_TCP_SOCKETS.

This permits the following interleaving:

  CPU0                             CPU1
  rds_info_getsockopt()
    func = rds_tcp_tc_info
                                   rmmod rds_tcp
                                     rds_tcp_exit()
                                       rds_info_deregister_func()
                                         rds_info_funcs[offset] = NULL
                                     free rds_tcp module text
    func()

The reader can therefore branch to an address in unloaded module text.

Protect callback invocation with SRCU.  Enter the SRCU read-side critical
section before loading the callback and leave it only after the callback
returns.  Clear the callback with release semantics and call
synchronize_srcu() before deregistration returns, preventing module unload
from freeing its text while an old reader is still executing it.  SRCU is
required because callbacks such as RDS_INFO_COUNTERS can sleep.

Keep the callback array unannotated and use acquire and release operations
for publication so sparse does not have to apply __rcu through the
function-pointer typedef.  Replace the two callback-slot BUG_ON() checks
with WARN_ON_ONCE() and return without changing the slot on mismatch.

Fixes: 70041088e3b9 ("RDS: Add TCP transport to RDS")
Link: https://lore.kernel.org/netdev/20260720184955.3008978-1-nicoyip.dev@gmail.com/
Suggested-by: Allison Henderson <achender@kernel.org>
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---

Changes in v2:
- Split the info callback/module-unload race from the independent
  inc->i_conn lifetime bug.
- Keep the callback array unannotated and use smp_load_acquire() and
  smp_store_release() to resolve the sparse errors.
- Replace the two touched callback-slot BUG_ON() checks with
  WARN_ON_ONCE() error paths.
- Rewrite the commit message for the callback race and omit the unrelated
  inc->i_conn KASAN report.

Link: https://lore.kernel.org/netdev/20260720184955.3008978-1-nicoyip.dev@gmail.com/ [v1]

 net/rds/info.c | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/net/rds/info.c b/net/rds/info.c
index 21b32eb16559..408a4f0d04a3 100644
--- a/net/rds/info.c
+++ b/net/rds/info.c
@@ -32,6 +32,7 @@
  */
 #include <linux/percpu.h>
 #include <linux/seq_file.h>
+#include <linux/srcu.h>
 #include <linux/slab.h>
 #include <linux/proc_fs.h>
 #include <linux/export.h>
@@ -68,6 +69,7 @@ struct rds_info_iterator {
 	unsigned long offset;
 };
 
+DEFINE_STATIC_SRCU(rds_info_srcu);
 static DEFINE_SPINLOCK(rds_info_lock);
 static rds_info_func rds_info_funcs[RDS_INFO_LAST - RDS_INFO_FIRST + 1];
 
@@ -78,8 +80,13 @@ void rds_info_register_func(int optname, rds_info_func func)
 	BUG_ON(optname < RDS_INFO_FIRST || optname > RDS_INFO_LAST);
 
 	spin_lock(&rds_info_lock);
-	BUG_ON(rds_info_funcs[offset]);
-	rds_info_funcs[offset] = func;
+	/* Validate the latest published slot value. */
+	if (WARN_ON_ONCE(smp_load_acquire(&rds_info_funcs[offset]))) {
+		spin_unlock(&rds_info_lock);
+		return;
+	}
+	/* Pair with lockless callback lookup. */
+	smp_store_release(&rds_info_funcs[offset], func);
 	spin_unlock(&rds_info_lock);
 }
 EXPORT_SYMBOL_GPL(rds_info_register_func);
@@ -91,9 +98,15 @@ void rds_info_deregister_func(int optname, rds_info_func func)
 	BUG_ON(optname < RDS_INFO_FIRST || optname > RDS_INFO_LAST);
 
 	spin_lock(&rds_info_lock);
-	BUG_ON(rds_info_funcs[offset] != func);
-	rds_info_funcs[offset] = NULL;
+	/* Validate the latest published slot value. */
+	if (WARN_ON_ONCE(smp_load_acquire(&rds_info_funcs[offset]) != func)) {
+		spin_unlock(&rds_info_lock);
+		return;
+	}
+	/* Hide the callback before waiting for old readers. */
+	smp_store_release(&rds_info_funcs[offset], NULL);
 	spin_unlock(&rds_info_lock);
+	synchronize_srcu(&rds_info_srcu);
 }
 EXPORT_SYMBOL_GPL(rds_info_deregister_func);
 
@@ -165,6 +178,7 @@ int rds_info_getsockopt(struct socket *sock, int optname, sockopt_t *opt)
 	int npages = 0;
 	int ret;
 	int len;
+	int srcu_idx;
 	int total;
 
 	len = opt->optlen;
@@ -214,8 +228,11 @@ int rds_info_getsockopt(struct socket *sock, int optname, sockopt_t *opt)
 	rdsdebug("len %d nr_pages %lu\n", len, nr_pages);
 
 call_func:
-	func = rds_info_funcs[optname - RDS_INFO_FIRST];
+	srcu_idx = srcu_read_lock(&rds_info_srcu);
+	/* Pair with callback slot publication and removal. */
+	func = smp_load_acquire(&rds_info_funcs[optname - RDS_INFO_FIRST]);
 	if (!func) {
+		srcu_read_unlock(&rds_info_srcu, srcu_idx);
 		ret = -ENOPROTOOPT;
 		goto out;
 	}
@@ -225,6 +242,7 @@ int rds_info_getsockopt(struct socket *sock, int optname, sockopt_t *opt)
 	iter.offset = offset0;
 
 	func(sock, len, &iter, &lens);
+	srcu_read_unlock(&rds_info_srcu, srcu_idx);
 	BUG_ON(lens.each == 0);
 
 	total = lens.nr * lens.each;
-- 
2.43.0