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Merge tag 'vfs-6.14-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs
Pull pidfs updates from Christian Brauner: - Rework inode number allocation Recently we received a patchset that aims to enable file handle encoding and decoding via name_to_handle_at(2) and open_by_handle_at(2). A crucical step in the patch series is how to go from inode number to struct pid without leaking information into unprivileged contexts. The issue is that in order to find a struct pid the pid number in the initial pid namespace must be encoded into the file handle via name_to_handle_at(2). This can be used by containers using a separate pid namespace to learn what the pid number of a given process in the initial pid namespace is. While this is a weak information leak it could be used in various exploits and in general is an ugly wart in the design. To solve this problem a new way is needed to lookup a struct pid based on the inode number allocated for that struct pid. The other part is to remove the custom inode number allocation on 32bit systems that is also an ugly wart that should go away. Allocate unique identifiers for struct pid by simply incrementing a 64 bit counter and insert each struct pid into the rbtree so it can be looked up to decode file handles avoiding to leak actual pids across pid namespaces in file handles. On both 64 bit and 32 bit the same 64 bit identifier is used to lookup struct pid in the rbtree. On 64 bit the unique identifier for struct pid simply becomes the inode number. Comparing two pidfds continues to be as simple as comparing inode numbers. On 32 bit the 64 bit number assigned to struct pid is split into two 32 bit numbers. The lower 32 bits are used as the inode number and the upper 32 bits are used as the inode generation number. Whenever a wraparound happens on 32 bit the 64 bit number will be incremented by 2 so inode numbering starts at 2 again. When a wraparound happens on 32 bit multiple pidfds with the same inode number are likely to exist. This isn't a problem since before pidfs pidfds used the anonymous inode meaning all pidfds had the same inode number. On 32 bit sserspace can thus reconstruct the 64 bit identifier by retrieving both the inode number and the inode generation number to compare, or use file handles. This gives the same guarantees on both 32 bit and 64 bit. - Implement file handle support This is based on custom export operation methods which allows pidfs to implement permission checking and opening of pidfs file handles cleanly without hacking around in the core file handle code too much. - Support bind-mounts Allow bind-mounting pidfds. Similar to nsfs let's allow bind-mounts for pidfds. This allows pidfds to be safely recovered and checked for process recycling. Instead of checking d_ops for both nsfs and pidfs we could in a follow-up patch add a flag argument to struct dentry_operations that functions similar to file_operations->fop_flags. * tag 'vfs-6.14-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: selftests: add pidfd bind-mount tests pidfs: allow bind-mounts pidfs: lookup pid through rbtree selftests/pidfd: add pidfs file handle selftests pidfs: check for valid ioctl commands pidfs: implement file handle support exportfs: add permission method fhandle: pull CAP_DAC_READ_SEARCH check into may_decode_fh() exportfs: add open method fhandle: simplify error handling pseudofs: add support for export_ops pidfs: support FS_IOC_GETVERSION pidfs: remove 32bit inode number handling pidfs: rework inode number allocation