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Subject[PATCH] bpf: validate bpf_func when BPF_JIT is enabled
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With CONFIG_BPF_JIT, the kernel makes indirect calls to dynamically
generated code. This change adds basic sanity checking to ensure
we are jumping to a valid location, which narrows down the attack
surface on the stored pointer. This also prepares the code for future
Control-Flow Integrity (CFI) checking, which adds indirect call
validation to call targets that can be determined at compile-time, but
cannot validate calls to jited functions.

In addition, this change adds a weak arch_bpf_jit_check_func function,
which architectures that implement BPF JIT can override to perform
additional validation, such as verifying that the pointer points to
the correct memory region.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
include/linux/filter.h | 26 ++++++++++++++++++++++++--
kernel/bpf/core.c | 25 +++++++++++++++++++++++++
2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 92c6e31fb008..abfb0e1b21a8 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -511,7 +511,10 @@ struct sock_fprog_kern {
struct sock_filter *filter;
};

+#define BPF_BINARY_HEADER_MAGIC 0x05de0e82
+
struct bpf_binary_header {
+ u32 magic;
u32 pages;
/* Some arches need word alignment for their instructions */
u8 image[] __aligned(4);
@@ -553,20 +556,39 @@ struct sk_filter {

DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);

+#ifdef CONFIG_BPF_JIT
+/*
+ * With JIT, the kernel makes an indirect call to dynamically generated
+ * code. Use bpf_call_func to perform additional validation of the call
+ * target to narrow down attack surface. Architectures implementing BPF
+ * JIT can override arch_bpf_jit_check_func for arch-specific checking.
+ */
+extern unsigned int bpf_call_func(const struct bpf_prog *prog,
+ const void *ctx);
+
+extern bool arch_bpf_jit_check_func(const struct bpf_prog *prog);
+#else
+static inline unsigned int bpf_call_func(const struct bpf_prog *prog,
+ const void *ctx)
+{
+ return prog->bpf_func(ctx, prog->insnsi);
+}
+#endif
+
#define BPF_PROG_RUN(prog, ctx) ({ \
u32 ret; \
cant_sleep(); \
if (static_branch_unlikely(&bpf_stats_enabled_key)) { \
struct bpf_prog_stats *stats; \
u64 start = sched_clock(); \
- ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi); \
+ ret = bpf_call_func(prog, ctx); \
stats = this_cpu_ptr(prog->aux->stats); \
u64_stats_update_begin(&stats->syncp); \
stats->cnt++; \
stats->nsecs += sched_clock() - start; \
u64_stats_update_end(&stats->syncp); \
} else { \
- ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi); \
+ ret = bpf_call_func(prog, ctx); \
} \
ret; })

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 66088a9e9b9e..7aad58f67105 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -792,6 +792,30 @@ void __weak bpf_jit_free_exec(void *addr)
module_memfree(addr);
}

+#ifdef CONFIG_BPF_JIT
+bool __weak arch_bpf_jit_check_func(const struct bpf_prog *prog)
+{
+ return true;
+}
+
+unsigned int bpf_call_func(const struct bpf_prog *prog, const void *ctx)
+{
+ const struct bpf_binary_header *hdr = bpf_jit_binary_hdr(prog);
+
+ if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
+ return prog->bpf_func(ctx, prog->insnsi);
+
+ if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
+ !arch_bpf_jit_check_func(prog))) {
+ WARN(1, "attempt to jump to an invalid address");
+ return 0;
+ }
+
+ return prog->bpf_func(ctx, prog->insnsi);
+}
+EXPORT_SYMBOL_GPL(bpf_call_func);
+#endif
+
struct bpf_binary_header *
bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
unsigned int alignment,
@@ -818,6 +842,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
/* Fill space with illegal/arch-dep instructions. */
bpf_fill_ill_insns(hdr, size);

+ hdr->magic = BPF_BINARY_HEADER_MAGIC;
hdr->pages = pages;
hole = min_t(unsigned int, size - (proglen + sizeof(*hdr)),
PAGE_SIZE - sizeof(*hdr));
--
2.23.0.162.g0b9fbb3734-goog
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