[RFC v1 11/14] krsi: Pin argument pages in bprm_check_security hook

[KP Singh]

From: KP Singh <kpsingh at google.com>

Pin the memory allocated to the the argv + envv for the new
process and passes it in the context to the eBPF programs attached to
the hook.

The get_user_pages_remote cannot be called from an eBPF helper because
the helpers run in atomic context and the get_user_pages_remote function
can sleep.

The following heuristics can be added as an optimization:

- Don't pin the pages if no eBPF programs are attached.
- Don't pin the pages if none of the eBPF programs depend on the
  information. This would require introspection of the byte-code and
  checking if certain helpers are called.

Signed-off-by: KP Singh <kpsingh at google.com>
---
 security/krsi/include/krsi_init.h |  3 ++
 security/krsi/krsi.c              | 56 +++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+)

diff --git a/security/krsi/include/krsi_init.h b/security/krsi/include/krsi_init.h
index 4e17ecacd4ed..6152847c3b08 100644
--- a/security/krsi/include/krsi_init.h
+++ b/security/krsi/include/krsi_init.h
@@ -16,6 +16,9 @@ extern int krsi_fs_initialized;
 
 struct krsi_bprm_ctx {
 	struct linux_binprm *bprm;
+	char *arg_pages;
+	unsigned long num_arg_pages;
+	unsigned long max_arg_offset;
 };
 
 /*
diff --git a/security/krsi/krsi.c b/security/krsi/krsi.c
index d3a4a361c192..00a7150c1b22 100644
--- a/security/krsi/krsi.c
+++ b/security/krsi/krsi.c
@@ -4,6 +4,8 @@
 #include <linux/filter.h>
 #include <linux/bpf.h>
 #include <linux/binfmts.h>
+#include <linux/highmem.h>
+#include <linux/mm.h>
 
 #include "krsi_init.h"
 
@@ -17,6 +19,53 @@ struct krsi_hook krsi_hooks_list[] = {
 	#undef KRSI_HOOK_INIT
 };
 
+static int pin_arg_pages(struct krsi_bprm_ctx *ctx)
+{
+	int ret = 0;
+	char *kaddr;
+	struct page *page;
+	unsigned long i, pos, num_arg_pages;
+	struct linux_binprm *bprm = ctx->bprm;
+	char *buf;
+
+	/*
+	 * The bprm->vma_pages does not have the correct count
+	 * for execution that is done by a kernel thread using the UMH.
+	 * vm_pages is updated in acct_arg_size and bails
+	 * out if current->mm is NULL (which is the case for a kernel thread).
+	 * It's safer to use vma_pages(struct linux_binprm*) to get the
+	 * actual number
+	 */
+	num_arg_pages = vma_pages(bprm->vma);
+	if (!num_arg_pages)
+		return -ENOMEM;
+
+	buf = kmalloc_array(num_arg_pages, PAGE_SIZE, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	for (i = 0; i < num_arg_pages; i++) {
+		pos = ALIGN_DOWN(bprm->p, PAGE_SIZE) + i * PAGE_SIZE;
+		ret = get_user_pages_remote(current, bprm->mm, pos, 1,
+					    FOLL_FORCE, &page, NULL, NULL);
+		if (ret <= 0) {
+			kfree(buf);
+			return -ENOMEM;
+		}
+
+		kaddr = kmap(page);
+		memcpy(buf + i * PAGE_SIZE, kaddr, PAGE_SIZE);
+		kunmap(page);
+		put_page(page);
+	}
+
+	ctx->arg_pages = buf;
+	ctx->num_arg_pages = num_arg_pages;
+	ctx->max_arg_offset = num_arg_pages * PAGE_SIZE;
+
+	return 0;
+}
+
 static int krsi_process_execution(struct linux_binprm *bprm)
 {
 	int ret;
@@ -26,7 +75,14 @@ static int krsi_process_execution(struct linux_binprm *bprm)
 		.bprm = bprm,
 	};
 
+	ret = pin_arg_pages(&ctx.bprm_ctx);
+	if (ret < 0)
+		goto out_arg_pages;
+
 	ret = krsi_run_progs(PROCESS_EXECUTION, &ctx);
+	kfree(ctx.bprm_ctx.arg_pages);
+
+out_arg_pages:
 	return ret;
 }
 
-- 
2.20.1