liveupdate: prevent double management of files

Patch series "liveupdate: prevent double preservation", v4.

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

Because LUO preserves files of absolutely different types: memfd, and
upcoming vfiofd [1], iommufd [2], guestmefd (and possible kvmfd/cpufd).
There is no common private data or guarantee on how to prevent that the
same file is not preserved twice beside using inode or some slower and
expensive method like hashtables.

This patch (of 4)

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.

Link: https://lore.kernel.org/20260326163943.574070-1-pasha.tatashin@soleen.com
Link: https://lore.kernel.org/20260326163943.574070-2-pasha.tatashin@soleen.com
Link: https://lore.kernel.org/all/20260129212510.967611-1-dmatlack@google.com
Link: https://lore.kernel.org/all/20260203220948.2176157-1-skhawaja@google.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: David Matlack <dmatlack@google.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/liveupdate.h b/include/linux/liveupdate.h
index dd11fdc76a5f2734ecfdafcbac6853bda6df33d0..61325ad265265498297c52aacd7959bcfa04ec4c 100644 (file)
--- 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 5acee4174bf00f9af895c732624caa94e06a52b8..09103cf81107bb4718e7b3d202001f1eff4c0f0a 100644 (file)
--- a/kernel/liveupdate/luo_file.c
+++ b/kernel/liveupdate/luo_file.c
@@ -108,12 +108,16 @@
 #include <linux/liveupdate.h>
 #include <linux/module.h>
 #include <linux/sizes.h>
+#include <linux/xarray.h>
 #include <linux/slab.h>
 #include <linux/string.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 @@ err_kfree:
        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);