mm: memfd_luo: preserve file seals

File seals are used on memfd for making shared memory communication with
untrusted peers safer and simpler.  Seals provide a guarantee that certain
operations won't be allowed on the file such as writes or truncations.
Maintaining these guarantees across a live update will help keeping such
use cases secure.

These guarantees will also be needed for IOMMUFD preservation with LUO.
Normally when IOMMUFD maps a memfd, it pins all its pages to make sure any
truncation operations on the memfd don't lead to IOMMUFD using freed
memory.  This doesn't work with LUO since the preserved memfd might have
completely different pages after a live update, and mapping them back to
the IOMMUFD will cause all sorts of problems.  Using and preserving the
seals allows IOMMUFD preservation logic to trust the memfd.

Since the uABI defines seals as an int, preserve them by introducing a new
u32 field.  There are currently only 6 possible seals, so the extra bits
are unused and provide room for future expansion.  Since the seals are
uABI, it is safe to use them directly in the ABI.  While at it, also add a
u32 flags field.  It makes sure the struct is nicely aligned, and can be
used later to support things like MFD_CLOEXEC.

Since the serialization structure is changed, bump the version number to
"memfd-v2".

It is important to note that the memfd-v2 version only supports seals that
existed when this version was defined.  This set is defined by
MEMFD_LUO_ALL_SEALS.  Any new seal might bring a completely different
semantic with it and the parser for memfd-v2 cannot be expected to deal
with that.  If there are any future seals added, they will need another
version bump.

Link: https://lkml.kernel.org/r/20260216185946.1215770-3-pratyush@kernel.org
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Tested-by: Samiullah Khawaja <skhawaja@google.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/kho/abi/memfd.h b/include/linux/kho/abi/memfd.h
index 68cb6303b846eb2a4728695b157f60f9e17f99bf..08b10fea2afc845293726cc0caaa6daf0466de3c 100644 (file)
--- a/include/linux/kho/abi/memfd.h
+++ b/include/linux/kho/abi/memfd.h
@@ -56,10 +56,24 @@ struct memfd_luo_folio_ser {
        u64 index;
 } __packed;
 
+/*
+ * The set of seals this version supports preserving. If support for any new
+ * seals is needed, add it here and bump version.
+ */
+#define MEMFD_LUO_ALL_SEALS (F_SEAL_SEAL | \
+                            F_SEAL_SHRINK | \
+                            F_SEAL_GROW | \
+                            F_SEAL_WRITE | \
+                            F_SEAL_FUTURE_WRITE | \
+                            F_SEAL_EXEC)
+
 /**
  * struct memfd_luo_ser - Main serialization structure for a memfd.
  * @pos:       The file's current position (f_pos).
  * @size:      The total size of the file in bytes (i_size).
+ * @seals:     The seals present on the memfd. The seals are uABI so it is safe
+ *             to directly use them in the ABI.
+ * @flags:     Flags for the file. Unused flag bits must be set to 0.
  * @nr_folios: Number of folios in the folios array.
  * @folios:    KHO vmalloc descriptor pointing to the array of
  *             struct memfd_luo_folio_ser.
@@ -67,11 +81,13 @@ struct memfd_luo_folio_ser {
 struct memfd_luo_ser {
        u64 pos;
        u64 size;
+       u32 seals;
+       u32 flags;
        u64 nr_folios;
        struct kho_vmalloc folios;
 } __packed;
 
 /* The compatibility string for memfd file handler */
-#define MEMFD_LUO_FH_COMPATIBLE        "memfd-v1"
+#define MEMFD_LUO_FH_COMPATIBLE        "memfd-v2"
 
 #endif /* _LINUX_KHO_ABI_MEMFD_H */

diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
index b8edb9f981d7ff5f5baa1418f27f320c87435b40..bc7f4f045edff7cfc82b390a0052780f0cdbcaf5 100644 (file)
--- a/mm/memfd_luo.c
+++ b/mm/memfd_luo.c
@@ -79,6 +79,8 @@
 #include <linux/shmem_fs.h>
 #include <linux/vmalloc.h>
 #include <linux/memfd.h>
+#include <uapi/linux/memfd.h>
+
 #include "internal.h"
 
 static int memfd_luo_preserve_folios(struct file *file,
@@ -259,7 +261,7 @@ static int memfd_luo_preserve(struct liveupdate_file_op_args *args)
        struct memfd_luo_folio_ser *folios_ser;
        struct memfd_luo_ser *ser;
        u64 nr_folios;
-       int err = 0;
+       int err = 0, seals;
 
        inode_lock(inode);
        shmem_freeze(inode, true);
@@ -271,8 +273,21 @@ static int memfd_luo_preserve(struct liveupdate_file_op_args *args)
                goto err_unlock;
        }
 
+       seals = memfd_get_seals(args->file);
+       if (seals < 0) {
+               err = seals;
+               goto err_free_ser;
+       }
+
+       /* Make sure the file only has the seals supported by this version. */
+       if (seals & ~MEMFD_LUO_ALL_SEALS) {
+               err = -EOPNOTSUPP;
+               goto err_free_ser;
+       }
+
        ser->pos = args->file->f_pos;
        ser->size = i_size_read(inode);
+       ser->seals = seals;
 
        err = memfd_luo_preserve_folios(args->file, &ser->folios,
                                        &folios_ser, &nr_folios);
@@ -486,13 +501,29 @@ static int memfd_luo_retrieve(struct liveupdate_file_op_args *args)
        if (!ser)
                return -EINVAL;
 
-       file = memfd_alloc_file("", 0);
+       /* Make sure the file only has seals supported by this version. */
+       if (ser->seals & ~MEMFD_LUO_ALL_SEALS) {
+               err = -EOPNOTSUPP;
+               goto free_ser;
+       }
+
+       /*
+        * The seals are preserved. Allow sealing here so they can be added
+        * later.
+        */
+       file = memfd_alloc_file("", MFD_ALLOW_SEALING);
        if (IS_ERR(file)) {
                pr_err("failed to setup file: %pe\n", file);
                err = PTR_ERR(file);
                goto free_ser;
        }
 
+       err = memfd_add_seals(file, ser->seals);
+       if (err) {
+               pr_err("failed to add seals: %pe\n", ERR_PTR(err));
+               goto put_file;
+       }
+
        vfs_setpos(file, ser->pos, MAX_LFS_FILESIZE);
        file->f_inode->i_size = ser->size;