bpf: annotate file argument as __nullable in bpf_lsm_mmap_file

As reported in [0], anonymous memory mappings are not backed by a
struct file instance. Consequently, the struct file pointer passed to
the security_mmap_file() LSM hook is NULL in such cases.

The BPF verifier is currently unaware of this, allowing BPF LSM
programs to dereference this struct file pointer without needing to
perform an explicit NULL check. This leads to potential NULL pointer
dereference and a kernel crash.

Add a strong override for bpf_lsm_mmap_file() which annotates the
struct file pointer parameter with the __nullable suffix. This
explicitly informs the BPF verifier that this pointer (PTR_MAYBE_NULL)
can be NULL, forcing BPF LSM programs to perform a check on it before
dereferencing it.

[0] https://lore.kernel.org/bpf/5e460d3c.4c3e9.19adde547d8.Coremail.kaiyanm@hust.edu.cn/

Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Closes: https://lore.kernel.org/bpf/5e460d3c.4c3e9.19adde547d8.Coremail.kaiyanm@hust.edu.cn/
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20251216133000.3690723-1-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

MAINTAINERS

@@ -4848,6 +4848,7 @@ S:	Maintained
 F:	Documentation/bpf/prog_lsm.rst
 F:	include/linux/bpf_lsm.h
 F:	kernel/bpf/bpf_lsm.c
+F:	kernel/bpf/bpf_lsm_proto.c
 F:	kernel/trace/bpf_trace.c
 F:	security/bpf/
 

kernel/bpf/Makefile

@@ -42,7 +42,17 @@ endif
 ifeq ($(CONFIG_BPF_JIT),y)
 obj-$(CONFIG_BPF_SYSCALL) += bpf_struct_ops.o
 obj-$(CONFIG_BPF_SYSCALL) += cpumask.o
-obj-${CONFIG_BPF_LSM} += bpf_lsm.o
+# bpf_lsm_proto.o must precede bpf_lsm.o. The current pahole logic
+# deduplicates function prototypes within
+# btf_encoder__add_saved_func() by keeping the first instance seen. We
+# need the function prototype(s) in bpf_lsm_proto.o to take precedence
+# over those within bpf_lsm.o. Having bpf_lsm_proto.o precede
+# bpf_lsm.o ensures its DWARF CU is processed early, forcing the
+# generated BTF to contain the overrides.
+#
+# Notably, this is a temporary workaround whilst the deduplication
+# semantics within pahole are revisited accordingly.
+obj-${CONFIG_BPF_LSM} += bpf_lsm_proto.o bpf_lsm.o
 endif
 ifneq ($(CONFIG_CRYPTO),)
 obj-$(CONFIG_BPF_SYSCALL) += crypto.o

kernel/bpf/bpf_lsm.c

@@ -18,10 +18,11 @@
 #include <linux/bpf-cgroup.h>
 
 /* For every LSM hook that allows attachment of BPF programs, declare a nop
- * function where a BPF program can be attached.
+ * function where a BPF program can be attached. Notably, we qualify each with
+ * weak linkage such that strong overrides can be implemented if need be.
  */
 #define LSM_HOOK(RET, DEFAULT, NAME, ...)	\
-noinline RET bpf_lsm_##NAME(__VA_ARGS__)	\
+__weak noinline RET bpf_lsm_##NAME(__VA_ARGS__)	\
 {						\
 	return DEFAULT;				\
 }

kernel/bpf/bpf_lsm_proto.c

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright 2025 Google LLC.
+ */
+
+#include <linux/fs.h>
+#include <linux/bpf_lsm.h>
+
+/*
+ * Strong definition of the mmap_file() BPF LSM hook. The __nullable suffix on
+ * the struct file pointer parameter name marks it as PTR_MAYBE_NULL. This
+ * explicitly enforces that BPF LSM programs check for NULL before attempting to
+ * dereference it.
+ */
+int bpf_lsm_mmap_file(struct file *file__nullable, unsigned long reqprot,
+		      unsigned long prot, unsigned long flags)
+{
+	return 0;
+}