bpf: Reject programs that try to load __percpu memory.

With the introduction of the btf_type_tag "percpu", we can add a
MEM_PERCPU to identify those pointers that point to percpu memory.
The ability of differetiating percpu pointers from regular memory
pointers have two benefits:

 1. It forbids unexpected use of percpu pointers, such as direct loads.
    In kernel, there are special functions used for accessing percpu
    memory. Directly loading percpu memory is meaningless. We already
    have BPF helpers like bpf_per_cpu_ptr() and bpf_this_cpu_ptr() that
    wrap the kernel percpu functions. So we can now convert percpu
    pointers into regular pointers in a safe way.

 2. Previously, bpf_per_cpu_ptr() and bpf_this_cpu_ptr() only work on
    PTR_TO_PERCPU_BTF_ID, a special reg_type which describes static
    percpu variables in kernel (we rely on pahole to encode them into
    vmlinux BTF). Now, since we can identify __percpu tagged pointers,
    we can also identify dynamically allocated percpu memory as well.
    It means we can use bpf_xxx_cpu_ptr() on dynamic percpu memory.
    This would be very convenient when accessing fields like
    "cgroup->rstat_cpu".

Signed-off-by: Hao Luo <haoluo@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220304191657.981240-4-haoluo@google.com

1 files changed, 9 insertions, 2 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f19abc59b6cd..88449fbbe063 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -334,7 +334,15 @@ enum bpf_type_flag {
 	/* MEM is in user address space. */
 	MEM_USER		= BIT(3 + BPF_BASE_TYPE_BITS),
 
-	__BPF_TYPE_LAST_FLAG	= MEM_USER,
+	/* MEM is a percpu memory. MEM_PERCPU tags PTR_TO_BTF_ID. When tagged
+	 * with MEM_PERCPU, PTR_TO_BTF_ID _cannot_ be directly accessed. In
+	 * order to drop this tag, it must be passed into bpf_per_cpu_ptr()
+	 * or bpf_this_cpu_ptr(), which will return the pointer corresponding
+	 * to the specified cpu.
+	 */
+	MEM_PERCPU		= BIT(4 + BPF_BASE_TYPE_BITS),
+
+	__BPF_TYPE_LAST_FLAG	= MEM_PERCPU,
 };
 
 /* Max number of base types. */
@@ -516,7 +524,6 @@ enum bpf_reg_type {
 	 */
 	PTR_TO_MEM,		 /* reg points to valid memory region */
 	PTR_TO_BUF,		 /* reg points to a read/write buffer */
-	PTR_TO_PERCPU_BTF_ID,	 /* reg points to a percpu kernel variable */
 	PTR_TO_FUNC,		 /* reg points to a bpf program function */
 	__BPF_REG_TYPE_MAX,