fat: handle idmapped mounts

Let fat handle idmapped mounts. This allows to have the same fat mount
appear in multiple locations with different id mappings. This allows to
expose a vfat formatted USB stick to multiple user with different ids on
the host or in user namespaces allowing for dac permissions:

mount -o uid=1000,gid=1000 /dev/sdb /mnt

u1001@f2-vm:/lower1$ ls -ln /mnt/
total 4
-rwxr-xr-x 1 1000 1000 4 Oct 28 03:44 aaa
-rwxr-xr-x 1 1000 1000 0 Oct 28 01:09 bbb
-rwxr-xr-x 1 1000 1000 0 Oct 28 01:10 ccc
-rwxr-xr-x 1 1000 1000 0 Oct 28 03:46 ddd
-rwxr-xr-x 1 1000 1000 0 Oct 28 04:01 eee

mount-idmapped --map-mount b:1000:1001:1

u1001@f2-vm:/lower1$ ls -ln /lower1/
total 4
-rwxr-xr-x 1 1001 1001 4 Oct 28 03:44 aaa
-rwxr-xr-x 1 1001 1001 0 Oct 28 01:09 bbb
-rwxr-xr-x 1 1001 1001 0 Oct 28 01:10 ccc
-rwxr-xr-x 1 1001 1001 0 Oct 28 03:46 ddd
-rwxr-xr-x 1 1001 1001 0 Oct 28 04:01 eee

u1001@f2-vm:/lower1$ touch /lower1/fff

u1001@f2-vm:/lower1$ ls -ln /lower1/fff
-rwxr-xr-x 1 1001 1001 0 Oct 28 04:03 /lower1/fff

u1001@f2-vm:/lower1$ ls -ln /mnt/fff
-rwxr-xr-x 1 1000 1000 0 Oct 28 04:03 /mnt/fff

Link: https://lore.kernel.org/r/20210121131959.646623-38-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/fs/fat/file.c b/fs/fat/file.c
index dd73d1b..da7c562 100644 (file)
--- a/fs/fat/file.c
+++ b/fs/fat/file.c
@@ -398,7 +398,7 @@ int fat_getattr(struct user_namespace *mnt_userns, const struct path *path,
                struct kstat *stat, u32 request_mask, unsigned int flags)
 {
        struct inode *inode = d_inode(path->dentry);
-       generic_fillattr(&init_user_ns, inode, stat);
+       generic_fillattr(mnt_userns, inode, stat);
        stat->blksize = MSDOS_SB(inode->i_sb)->cluster_size;
 
        if (MSDOS_SB(inode->i_sb)->options.nfs == FAT_NFS_NOSTALE_RO) {
@@ -447,12 +447,13 @@ static int fat_sanitize_mode(const struct msdos_sb_info *sbi,
        return 0;
 }
 
-static int fat_allow_set_time(struct msdos_sb_info *sbi, struct inode *inode)
+static int fat_allow_set_time(struct user_namespace *mnt_userns,
+                             struct msdos_sb_info *sbi, struct inode *inode)
 {
        umode_t allow_utime = sbi->options.allow_utime;
 
-       if (!uid_eq(current_fsuid(), inode->i_uid)) {
-               if (in_group_p(inode->i_gid))
+       if (!uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode))) {
+               if (in_group_p(i_gid_into_mnt(mnt_userns, inode)))
                        allow_utime >>= 3;
                if (allow_utime & MAY_WRITE)
                        return 1;
@@ -477,11 +478,11 @@ int fat_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
        /* Check for setting the inode time. */
        ia_valid = attr->ia_valid;
        if (ia_valid & TIMES_SET_FLAGS) {
-               if (fat_allow_set_time(sbi, inode))
+               if (fat_allow_set_time(mnt_userns, sbi, inode))
                        attr->ia_valid &= ~TIMES_SET_FLAGS;
        }
 
-       error = setattr_prepare(&init_user_ns, dentry, attr);
+       error = setattr_prepare(mnt_userns, dentry, attr);
        attr->ia_valid = ia_valid;
        if (error) {
                if (sbi->options.quiet)
@@ -551,7 +552,7 @@ int fat_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
                fat_truncate_time(inode, &attr->ia_mtime, S_MTIME);
        attr->ia_valid &= ~(ATTR_ATIME|ATTR_CTIME|ATTR_MTIME);
 
-       setattr_copy(&init_user_ns, inode, attr);
+       setattr_copy(mnt_userns, inode, attr);
        mark_inode_dirty(inode);
 out:
        return error;

diff --git a/fs/fat/namei_msdos.c b/fs/fat/namei_msdos.c
index a8f3375..efba301 100644 (file)
--- a/fs/fat/namei_msdos.c
+++ b/fs/fat/namei_msdos.c
@@ -667,7 +667,7 @@ static struct file_system_type msdos_fs_type = {
        .name           = "msdos",
        .mount          = msdos_mount,
        .kill_sb        = kill_block_super,
-       .fs_flags       = FS_REQUIRES_DEV,
+       .fs_flags       = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("msdos");
 

diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 23936ec..5369d82 100644 (file)
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -1063,7 +1063,7 @@ static struct file_system_type vfat_fs_type = {
        .name           = "vfat",
        .mount          = vfat_mount,
        .kill_sb        = kill_block_super,
-       .fs_flags       = FS_REQUIRES_DEV,
+       .fs_flags       = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("vfat");