[PATCH bpf v4] bpf: Fix theoretical prog_array UAF in __uprobe_perf_func()

[Jann Horn <jannh@google.com>]

Currently, the pointer stored in call->prog_array is loaded in
__uprobe_perf_func(), with no RCU annotation and no immediately visible
RCU protection, so it looks as if the loaded pointer can immediately be
dangling.
Later, bpf_prog_run_array_uprobe() starts a RCU-trace read-side critical
section, but this is too late. It then uses rcu_dereference_check(), but
this use of rcu_dereference_check() does not actually dereference anything.

Fix it by aligning the semantics to bpf_prog_run_array(): Let the caller
provide rcu_read_lock_trace() protection and then load call->prog_array
with rcu_dereference_check().

This issue seems to be theoretical: I don't know of any way to reach this
code without having handle_swbp() further up the stack, which is already
holding a rcu_read_lock_trace() lock, so where we take
rcu_read_lock_trace() in __uprobe_perf_func()/bpf_prog_run_array_uprobe()
doesn't actually have any effect.

Fixes: 8c7dcb84e3b7 ("bpf: implement sleepable uprobes by chaining gps")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jann Horn <jannh@google.com>
---
Changes in v4:
- re-add !array check that I wrongly removed (Andrii)
- style: remove one empty line I added that doesn't exist in bpf_prog_run_array
- Link to v3: https://lore.kernel.org/r/20241210-bpf-fix-uprobe-uaf-v3-1-ce50ae2a2f0f@google.com

Changes in v3:
- align semantics with bpf_prog_run_array()
- correct commit message: the issue is theoretical
- remove stable CC
- Link to v2: https://lore.kernel.org/r/20241206-bpf-fix-uprobe-uaf-v2-1-4c75c54fe424@google.com

Changes in v2:
- remove diff chunk in patch notes that confuses git
- Link to v1: https://lore.kernel.org/r/20241206-bpf-fix-uprobe-uaf-v1-1-6869c8a17258@google.com
---
 include/linux/bpf.h         | 13 +++++--------
 kernel/trace/trace_uprobe.c |  6 +++++-
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index eaee2a819f4c150a34a7b1075584711609682e4c..cfc415632cad7401d1ab66410f095c28f9293c93 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2193,26 +2193,25 @@ bpf_prog_run_array(const struct bpf_prog_array *array,
  * rcu-protected dynamically sized maps.
  */
 static __always_inline u32
-bpf_prog_run_array_uprobe(const struct bpf_prog_array __rcu *array_rcu,
+bpf_prog_run_array_uprobe(const struct bpf_prog_array *array,
 			  const void *ctx, bpf_prog_run_fn run_prog)
 {
 	const struct bpf_prog_array_item *item;
 	const struct bpf_prog *prog;
-	const struct bpf_prog_array *array;
 	struct bpf_run_ctx *old_run_ctx;
 	struct bpf_trace_run_ctx run_ctx;
 	u32 ret = 1;
 
 	might_fault();
+	RCU_LOCKDEP_WARN(!rcu_read_lock_trace_held(), "no rcu lock held");
+
+	if (unlikely(!array))
+		return ret;
 
-	rcu_read_lock_trace();
 	migrate_disable();
 
 	run_ctx.is_uprobe = true;
 
-	array = rcu_dereference_check(array_rcu, rcu_read_lock_trace_held());
-	if (unlikely(!array))
-		goto out;
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
 	item = &array->items[0];
 	while ((prog = READ_ONCE(item->prog))) {
@@ -2227,9 +2226,7 @@ bpf_prog_run_array_uprobe(const struct bpf_prog_array __rcu *array_rcu,
 			rcu_read_unlock();
 	}
 	bpf_reset_run_ctx(old_run_ctx);
-out:
 	migrate_enable();
-	rcu_read_unlock_trace();
 	return ret;
 }
 
diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index fed382b7881b82ee3c334ea77860cce77581a74d..4875e7f5de3db249af34c539c079fbedd38f4107 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -1402,9 +1402,13 @@ static void __uprobe_perf_func(struct trace_uprobe *tu,
 
 #ifdef CONFIG_BPF_EVENTS
 	if (bpf_prog_array_valid(call)) {
+		const struct bpf_prog_array *array;
 		u32 ret;
 
-		ret = bpf_prog_run_array_uprobe(call->prog_array, regs, bpf_prog_run);
+		rcu_read_lock_trace();
+		array = rcu_dereference_check(call->prog_array, rcu_read_lock_trace_held());
+		ret = bpf_prog_run_array_uprobe(array, regs, bpf_prog_run);
+		rcu_read_unlock_trace();
 		if (!ret)
 			return;
 	}

---
base-commit: 509df676c2d79c985ec2eaa3e3a3bbe557645861
change-id: 20241206-bpf-fix-uprobe-uaf-53d928bab3d0

-- 
Jann Horn <jannh@google.com>