[PATCH v4 1/3] liveupdate: prevent double management of files

[Pasha Tatashin <pasha.tatashin@soleen.com>]

Currently, LUO does not prevent the same file from being managed twice
across different active sessions.

Use a global xarray luo_preserved_files to keep track of file
identifiers being preserved by LUO. Update luo_preserve_file() to
check and insert the file identifier into this xarray when it is
preserved, and erase it in luo_file_unpreserve_files() when it is
released.

To allow handlers to define what constitutes a "unique" file (e.g.,
different struct file objects pointing to the same hardware resource),
add a get_id() callback to struct liveupdate_file_ops. If not
provided, the default identifier is the struct file pointer itself.

This ensures that the same file (or resource) cannot be managed by
multiple sessions. If another session attempts to preserve an already
managed file, it will now fail with -EBUSY.

Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/liveupdate.h   |  2 ++
 kernel/liveupdate/luo_file.c | 32 ++++++++++++++++++++++++++++++--
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/include/linux/liveupdate.h b/include/linux/liveupdate.h
index dd11fdc76a5f..61325ad26526 100644
--- a/include/linux/liveupdate.h
+++ b/include/linux/liveupdate.h
@@ -63,6 +63,7 @@ struct liveupdate_file_op_args {
  *                finish, in order to do successful finish calls for all
  *                resources in the session.
  * @finish:       Required. Final cleanup in the new kernel.
+ * @get_id:       Optional. Returns a unique identifier for the file.
  * @owner:        Module reference
  *
  * All operations (except can_preserve) receive a pointer to a
@@ -78,6 +79,7 @@ struct liveupdate_file_ops {
 	int (*retrieve)(struct liveupdate_file_op_args *args);
 	bool (*can_finish)(struct liveupdate_file_op_args *args);
 	void (*finish)(struct liveupdate_file_op_args *args);
+	unsigned long (*get_id)(struct file *file);
 	struct module *owner;
 };
 
diff --git a/kernel/liveupdate/luo_file.c b/kernel/liveupdate/luo_file.c
index 5acee4174bf0..b02e2891cdb8 100644
--- a/kernel/liveupdate/luo_file.c
+++ b/kernel/liveupdate/luo_file.c
@@ -110,10 +110,14 @@
 #include <linux/sizes.h>
 #include <linux/slab.h>
 #include <linux/string.h>
+#include <linux/xarray.h>
 #include "luo_internal.h"
 
 static LIST_HEAD(luo_file_handler_list);
 
+/* Keep track of files being preserved by LUO */
+static DEFINE_XARRAY(luo_preserved_files);
+
 /* 2 4K pages, give space for 128 files per file_set */
 #define LUO_FILE_PGCNT		2ul
 #define LUO_FILE_MAX							\
@@ -203,6 +207,12 @@ static void luo_free_files_mem(struct luo_file_set *file_set)
 	file_set->files = NULL;
 }
 
+static unsigned long luo_get_id(struct liveupdate_file_handler *fh,
+				struct file *file)
+{
+	return fh->ops->get_id ? fh->ops->get_id(file) : (unsigned long)file;
+}
+
 static bool luo_token_is_used(struct luo_file_set *file_set, u64 token)
 {
 	struct luo_file *iter;
@@ -248,6 +258,7 @@ static bool luo_token_is_used(struct luo_file_set *file_set, u64 token)
  * Context: Can be called from an ioctl handler during normal system operation.
  * Return: 0 on success. Returns a negative errno on failure:
  *         -EEXIST if the token is already used.
+ *         -EBUSY if the file descriptor is already preserved by another session.
  *         -EBADF if the file descriptor is invalid.
  *         -ENOSPC if the file_set is full.
  *         -ENOENT if no compatible handler is found.
@@ -288,10 +299,15 @@ int luo_preserve_file(struct luo_file_set *file_set, u64 token, int fd)
 	if (err)
 		goto err_free_files_mem;
 
-	err = luo_flb_file_preserve(fh);
+	err = xa_insert(&luo_preserved_files, luo_get_id(fh, file),
+			file, GFP_KERNEL);
 	if (err)
 		goto err_free_files_mem;
 
+	err = luo_flb_file_preserve(fh);
+	if (err)
+		goto err_erase_xa;
+
 	luo_file = kzalloc_obj(*luo_file);
 	if (!luo_file) {
 		err = -ENOMEM;
@@ -320,6 +336,8 @@ int luo_preserve_file(struct luo_file_set *file_set, u64 token, int fd)
 	kfree(luo_file);
 err_flb_unpreserve:
 	luo_flb_file_unpreserve(fh);
+err_erase_xa:
+	xa_erase(&luo_preserved_files, luo_get_id(fh, file));
 err_free_files_mem:
 	luo_free_files_mem(file_set);
 err_fput:
@@ -363,6 +381,8 @@ void luo_file_unpreserve_files(struct luo_file_set *file_set)
 		luo_file->fh->ops->unpreserve(&args);
 		luo_flb_file_unpreserve(luo_file->fh);
 
+		xa_erase(&luo_preserved_files,
+			 luo_get_id(luo_file->fh, luo_file->file));
 		list_del(&luo_file->list);
 		file_set->count--;
 
@@ -606,6 +626,11 @@ int luo_retrieve_file(struct luo_file_set *file_set, u64 token,
 	luo_file->file = args.file;
 	/* Get reference so we can keep this file in LUO until finish */
 	get_file(luo_file->file);
+
+	WARN_ON(xa_insert(&luo_preserved_files,
+			  luo_get_id(luo_file->fh, luo_file->file),
+			  luo_file->file, GFP_KERNEL));
+
 	*filep = luo_file->file;
 	luo_file->retrieve_status = 1;
 
@@ -701,8 +726,11 @@ int luo_file_finish(struct luo_file_set *file_set)
 
 		luo_file_finish_one(file_set, luo_file);
 
-		if (luo_file->file)
+		if (luo_file->file) {
+			xa_erase(&luo_preserved_files,
+				 luo_get_id(luo_file->fh, luo_file->file));
 			fput(luo_file->file);
+		}
 		list_del(&luo_file->list);
 		file_set->count--;
 		mutex_destroy(&luo_file->mutex);
-- 
2.43.0