Re: [RFC PATCH v1] LSM: Remove double path_rename hook calls for RENAME_EXCHANGE

[John Johansen <john.johansen@canonical.com>]

Looks good to me. Of course now I am going to have to create a follow-up
patch to optimize the apparmor mediation.

Acked-by: John Johansen <john.johansen@canonical.com>

On 3/23/22 01:40, Mickaël Salaün wrote:
> Any comment? John, Tetsuo, does it look OK for AppArmor and Tomoyo?
> 
> On 22/02/2022 18:53, Mickaël Salaün wrote:
>> From: Mickaël Salaün <mic@linux.microsoft.com>
>>
>> In order to be able to identify a file exchange with renameat2(2) and
>> RENAME_EXCHANGE, which will be useful for Landlock [1], propagate the
>> rename flags to LSMs.  This may also improve performance because of the
>> switch from two set of LSM hook calls to only one, and because LSMs
>> using this hook may optimize the double check (e.g. only one lock,
>> reduce the number of path walks).
>>
>> AppArmor, Landlock and Tomoyo are updated to leverage this change.  This
>> should not change the current behavior (same check order), except
>> (different level of) speed boosts.
>>
>> [1] https://lore.kernel.org/r/20220221212522.320243-1-mic@digikod.net
>>
>> Cc: James Morris <jmorris@namei.org>
>> Cc: John Johansen <john.johansen@canonical.com>
>> Cc: Kentaro Takeda <takedakn@nttdata.co.jp>
>> Cc: Serge E. Hallyn <serge@hallyn.com>
>> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
>> Link: https://lore.kernel.org/r/20220222175332.384545-1-mic@digikod.net
>> ---
>>   include/linux/lsm_hook_defs.h |  2 +-
>>   include/linux/lsm_hooks.h     |  1 +
>>   security/apparmor/lsm.c       | 30 +++++++++++++++++++++++++-----
>>   security/landlock/fs.c        | 12 ++++++++++--
>>   security/security.c           |  9 +--------
>>   security/tomoyo/tomoyo.c      | 11 ++++++++++-
>>   6 files changed, 48 insertions(+), 17 deletions(-)
>>
>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
>> index 819ec92dc2a8..d8b49c9c3a8a 100644
>> --- a/include/linux/lsm_hook_defs.h
>> +++ b/include/linux/lsm_hook_defs.h
>> @@ -100,7 +100,7 @@ LSM_HOOK(int, 0, path_link, struct dentry *old_dentry,
>>        const struct path *new_dir, struct dentry *new_dentry)
>>   LSM_HOOK(int, 0, path_rename, const struct path *old_dir,
>>        struct dentry *old_dentry, const struct path *new_dir,
>> -     struct dentry *new_dentry)
>> +     struct dentry *new_dentry, unsigned int flags)
>>   LSM_HOOK(int, 0, path_chmod, const struct path *path, umode_t mode)
>>   LSM_HOOK(int, 0, path_chown, const struct path *path, kuid_t uid, kgid_t gid)
>>   LSM_HOOK(int, 0, path_chroot, const struct path *path)
>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
>> index 3bf5c658bc44..32cd2a7fe9fc 100644
>> --- a/include/linux/lsm_hooks.h
>> +++ b/include/linux/lsm_hooks.h
>> @@ -358,6 +358,7 @@
>>    *    @old_dentry contains the dentry structure of the old link.
>>    *    @new_dir contains the path structure for parent of the new link.
>>    *    @new_dentry contains the dentry structure of the new link.
>> + *    @flags may contain rename options such as RENAME_EXCHANGE.
>>    *    Return 0 if permission is granted.
>>    * @path_chmod:
>>    *    Check for permission to change a mode of the file @path. The new
>> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
>> index 4f0eecb67dde..900bc540656a 100644
>> --- a/security/apparmor/lsm.c
>> +++ b/security/apparmor/lsm.c
>> @@ -354,13 +354,16 @@ static int apparmor_path_link(struct dentry *old_dentry, const struct path *new_
>>   }
>>     static int apparmor_path_rename(const struct path *old_dir, struct dentry *old_dentry,
>> -                const struct path *new_dir, struct dentry *new_dentry)
>> +                const struct path *new_dir, struct dentry *new_dentry,
>> +                const unsigned int flags)
>>   {
>>       struct aa_label *label;
>>       int error = 0;
>>         if (!path_mediated_fs(old_dentry))
>>           return 0;
>> +    if ((flags & RENAME_EXCHANGE) && !path_mediated_fs(new_dentry))
>> +        return 0;
>>         label = begin_current_label_crit_section();
>>       if (!unconfined(label)) {
>> @@ -374,10 +377,27 @@ static int apparmor_path_rename(const struct path *old_dir, struct dentry *old_d
>>               d_backing_inode(old_dentry)->i_mode
>>           };
>>   -        error = aa_path_perm(OP_RENAME_SRC, label, &old_path, 0,
>> -                     MAY_READ | AA_MAY_GETATTR | MAY_WRITE |
>> -                     AA_MAY_SETATTR | AA_MAY_DELETE,
>> -                     &cond);
>> +        if (flags & RENAME_EXCHANGE) {
>> +            struct path_cond cond_exchange = {
>> +                i_uid_into_mnt(mnt_userns, d_backing_inode(new_dentry)),
>> +                d_backing_inode(new_dentry)->i_mode
>> +            };
>> +
>> +            error = aa_path_perm(OP_RENAME_SRC, label, &new_path, 0,
>> +                         MAY_READ | AA_MAY_GETATTR | MAY_WRITE |
>> +                         AA_MAY_SETATTR | AA_MAY_DELETE,
>> +                         &cond_exchange);
>> +            if (!error)
>> +                error = aa_path_perm(OP_RENAME_DEST, label, &old_path,
>> +                             0, MAY_WRITE | AA_MAY_SETATTR |
>> +                             AA_MAY_CREATE, &cond_exchange);
>> +        }
>> +
>> +        if (!error)
>> +            error = aa_path_perm(OP_RENAME_SRC, label, &old_path, 0,
>> +                         MAY_READ | AA_MAY_GETATTR | MAY_WRITE |
>> +                         AA_MAY_SETATTR | AA_MAY_DELETE,
>> +                         &cond);
>>           if (!error)
>>               error = aa_path_perm(OP_RENAME_DEST, label, &new_path,
>>                            0, MAY_WRITE | AA_MAY_SETATTR |
>> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
>> index 97b8e421f617..7e57fca6e814 100644
>> --- a/security/landlock/fs.c
>> +++ b/security/landlock/fs.c
>> @@ -574,10 +574,12 @@ static inline u32 maybe_remove(const struct dentry *const dentry)
>>   static int hook_path_rename(const struct path *const old_dir,
>>           struct dentry *const old_dentry,
>>           const struct path *const new_dir,
>> -        struct dentry *const new_dentry)
>> +        struct dentry *const new_dentry,
>> +        const unsigned int flags)
>>   {
>>       const struct landlock_ruleset *const dom =
>>           landlock_get_current_domain();
>> +    u32 exchange_access = 0;
>>         if (!dom)
>>           return 0;
>> @@ -585,11 +587,17 @@ static int hook_path_rename(const struct path *const old_dir,
>>       if (old_dir->dentry != new_dir->dentry)
>>           /* Gracefully forbids reparenting. */
>>           return -EXDEV;
>> +    if (flags & RENAME_EXCHANGE) {
>> +        if (unlikely(d_is_negative(new_dentry)))
>> +            return -ENOENT;
>> +        exchange_access =
>> +            get_mode_access(d_backing_inode(new_dentry)->i_mode);
>> +    }
>>       if (unlikely(d_is_negative(old_dentry)))
>>           return -ENOENT;
>>       /* RENAME_EXCHANGE is handled because directories are the same. */
>>       return check_access_path(dom, old_dir, maybe_remove(old_dentry) |
>> -            maybe_remove(new_dentry) |
>> +            maybe_remove(new_dentry) | exchange_access |
>>               get_mode_access(d_backing_inode(old_dentry)->i_mode));
>>   }
>>   diff --git a/security/security.c b/security/security.c
>> index 22261d79f333..8634da4cfd46 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -1184,15 +1184,8 @@ int security_path_rename(const struct path *old_dir, struct dentry *old_dentry,
>>                (d_is_positive(new_dentry) && IS_PRIVATE(d_backing_inode(new_dentry)))))
>>           return 0;
>>   -    if (flags & RENAME_EXCHANGE) {
>> -        int err = call_int_hook(path_rename, 0, new_dir, new_dentry,
>> -                    old_dir, old_dentry);
>> -        if (err)
>> -            return err;
>> -    }
>> -
>>       return call_int_hook(path_rename, 0, old_dir, old_dentry, new_dir,
>> -                new_dentry);
>> +                new_dentry, flags);
>>   }
>>   EXPORT_SYMBOL(security_path_rename);
>>   diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
>> index b6a31901f289..71e82d855ebf 100644
>> --- a/security/tomoyo/tomoyo.c
>> +++ b/security/tomoyo/tomoyo.c
>> @@ -264,17 +264,26 @@ static int tomoyo_path_link(struct dentry *old_dentry, const struct path *new_di
>>    * @old_dentry: Pointer to "struct dentry".
>>    * @new_parent: Pointer to "struct path".
>>    * @new_dentry: Pointer to "struct dentry".
>> + * @flags: Rename options.
>>    *
>>    * Returns 0 on success, negative value otherwise.
>>    */
>>   static int tomoyo_path_rename(const struct path *old_parent,
>>                     struct dentry *old_dentry,
>>                     const struct path *new_parent,
>> -                  struct dentry *new_dentry)
>> +                  struct dentry *new_dentry,
>> +                  const unsigned int flags)
>>   {
>>       struct path path1 = { .mnt = old_parent->mnt, .dentry = old_dentry };
>>       struct path path2 = { .mnt = new_parent->mnt, .dentry = new_dentry };
>>   +    if (flags & RENAME_EXCHANGE) {
>> +        const int err = tomoyo_path2_perm(TOMOYO_TYPE_RENAME, &path2,
>> +                &path1);
>> +
>> +        if (err)
>> +            return err;
>> +    }
>>       return tomoyo_path2_perm(TOMOYO_TYPE_RENAME, &path1, &path2);
>>   }
>>  
>> base-commit: cfb92440ee71adcc2105b0890bb01ac3cddb8507