bpf: Introduce bpf_sysctl_{get,set}_new_value helpers

Add helpers to work with new value being written to sysctl by user
space.

bpf_sysctl_get_new_value() copies value being written to sysctl into
provided buffer.

bpf_sysctl_set_new_value() overrides new value being written by user
space with a one from provided buffer. Buffer should contain string
representation of the value, similar to what can be seen in /proc/sys/.

Both helpers can be used only on sysctl write.

File position matters and can be managed by an interface that will be
introduced separately. E.g. if user space calls sys_write to a file in
/proc/sys/ at file position = X, where X > 0, then the value set by
bpf_sysctl_set_new_value() will be written starting from X. If program
wants to override whole value with specified buffer, file position has
to be set to zero.

Documentation for the new helpers is provided in bpf.h UAPI.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 80 insertions, 1 deletions

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index c6b2cf29a54b..ba4e21986760 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -778,6 +778,13 @@ const struct bpf_verifier_ops cg_dev_verifier_ops = {
  * @head: sysctl table header
  * @table: sysctl table
  * @write: sysctl is being read (= 0) or written (= 1)
+ * @buf: pointer to buffer passed by user space
+ * @pcount: value-result argument: value is size of buffer pointed to by @buf,
+ *	result is size of @new_buf if program set new value, initial value
+ *	otherwise
+ * @new_buf: pointer to pointer to new buffer that will be allocated if program
+ *	overrides new value provided by user space on sysctl write
+ *	NOTE: it's caller responsibility to free *new_buf if it was set
  * @type: type of program to be executed
  *
  * Program is run when sysctl is being accessed, either read or written, and
@@ -788,7 +795,8 @@ const struct bpf_verifier_ops cg_dev_verifier_ops = {
  */
 int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
 				   struct ctl_table *table, int write,
-				   enum bpf_attach_type type)
+				   void __user *buf, size_t *pcount,
+				   void **new_buf, enum bpf_attach_type type)
 {
 	struct bpf_sysctl_kern ctx = {
 		.head = head,
@@ -796,6 +804,9 @@ int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
 		.write = write,
 		.cur_val = NULL,
 		.cur_len = PAGE_SIZE,
+		.new_val = NULL,
+		.new_len = 0,
+		.new_updated = 0,
 	};
 	struct cgroup *cgrp;
 	int ret;
@@ -818,6 +829,18 @@ int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
 		ctx.cur_len = 0;
 	}
 
+	if (write && buf && *pcount) {
+		/* BPF program should be able to override new value with a
+		 * buffer bigger than provided by user.
+		 */
+		ctx.new_val = kmalloc_track_caller(PAGE_SIZE, GFP_KERNEL);
+		ctx.new_len = min(PAGE_SIZE, *pcount);
+		if (!ctx.new_val ||
+		    copy_from_user(ctx.new_val, buf, ctx.new_len))
+			/* Let BPF program decide how to proceed. */
+			ctx.new_len = 0;
+	}
+
 	rcu_read_lock();
 	cgrp = task_dfl_cgroup(current);
 	ret = BPF_PROG_RUN_ARRAY(cgrp->bpf.effective[type], &ctx, BPF_PROG_RUN);
@@ -825,6 +848,13 @@ int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
 
 	kfree(ctx.cur_val);
 
+	if (ret == 1 && ctx.new_updated) {
+		*new_buf = ctx.new_val;
+		*pcount = ctx.new_len;
+	} else {
+		kfree(ctx.new_val);
+	}
+
 	return ret == 1 ? 0 : -EPERM;
 }
 EXPORT_SYMBOL(__cgroup_bpf_run_filter_sysctl);
@@ -932,6 +962,51 @@ static const struct bpf_func_proto bpf_sysctl_get_current_value_proto = {
 	.arg3_type	= ARG_CONST_SIZE,
 };
 
+BPF_CALL_3(bpf_sysctl_get_new_value, struct bpf_sysctl_kern *, ctx, char *, buf,
+	   size_t, buf_len)
+{
+	if (!ctx->write) {
+		if (buf && buf_len)
+			memset(buf, '\0', buf_len);
+		return -EINVAL;
+	}
+	return copy_sysctl_value(buf, buf_len, ctx->new_val, ctx->new_len);
+}
+
+static const struct bpf_func_proto bpf_sysctl_get_new_value_proto = {
+	.func		= bpf_sysctl_get_new_value,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_PTR_TO_UNINIT_MEM,
+	.arg3_type	= ARG_CONST_SIZE,
+};
+
+BPF_CALL_3(bpf_sysctl_set_new_value, struct bpf_sysctl_kern *, ctx,
+	   const char *, buf, size_t, buf_len)
+{
+	if (!ctx->write || !ctx->new_val || !ctx->new_len || !buf || !buf_len)
+		return -EINVAL;
+
+	if (buf_len > PAGE_SIZE - 1)
+		return -E2BIG;
+
+	memcpy(ctx->new_val, buf, buf_len);
+	ctx->new_len = buf_len;
+	ctx->new_updated = 1;
+
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_sysctl_set_new_value_proto = {
+	.func		= bpf_sysctl_set_new_value,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_PTR_TO_MEM,
+	.arg3_type	= ARG_CONST_SIZE,
+};
+
 static const struct bpf_func_proto *
 sysctl_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -940,6 +1015,10 @@ sysctl_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_sysctl_get_name_proto;
 	case BPF_FUNC_sysctl_get_current_value:
 		return &bpf_sysctl_get_current_value_proto;
+	case BPF_FUNC_sysctl_get_new_value:
+		return &bpf_sysctl_get_new_value_proto;
+	case BPF_FUNC_sysctl_set_new_value:
+		return &bpf_sysctl_set_new_value_proto;
 	default:
 		return cgroup_base_func_proto(func_id, prog);
 	}