[PATCH v6 1/5] security: create file_truncate hook from path_truncate hook
Mickaël Salaün
mic at digikod.net
Fri Sep 16 17:30:24 UTC 2022
We may indeed need to change fs/open.c:vfs_truncate() because of these
different call sites. I'm not sure how these subsystems work though.
On 08/09/2022 22:28, Günther Noack wrote:
> Adding Namjae Jeon and David Howells as authors of the respective
> files in fs/ksmbd and fs/cachefiles -- do you happen to know whether
> these vfs_truncate() calls are using 'struct file's that are opened by
> normal userspace processes, where LSM policies may apply?
>
> P.S. In this patch I have looked for all places where the
> security_path_truncate() hook was called, to see which of these should
> rather use security_file_truncate() (and I made sure that it does the
> same thing for all the LSMs that use it).
>
> I'm confident that this does the right thing when truncate() or
> ftruncate() are called from userspace, but one of the places that
> still calls the path-based hook is vfs_truncate(), and this is called
> from more places in the kernel than just from userspace:
>
> init/initramfs.c
> 387: vfs_truncate(&wfile->f_path, body_len);
>
> security/keys/big_key.c
> 172: vfs_truncate(&payload->path, 0);
>
> fs/cachefiles/interface.c
> 242: ret = vfs_truncate(&file->f_path, dio_size);
>
> fs/cachefiles/namei.c
> 497: ret = vfs_truncate(&path, ni_size); >
> fs/ksmbd/smb2pdu.c
> 2350: int rc = vfs_truncate(path, 0);
>
> fs/ksmbd/vfs.c
> 874: err = vfs_truncate(&filp->f_path, size);
>
> I suspect that these are benign but am not familiar with all of these
> corners of the codebase. -- The question is: Some of these call
> vfs_truncate() on the f_path of an existing struct file -- should
> these rather be calling the security_file_truncate() than the
> security_path_truncate() hook to authorize the truncation?
>
> Specifically, I think:
>
> * initramfs happens at system startup and LSMs should not interfere at
> this point yet
> * security/keys does not use an opened struct file, so calling the
> path-based hook through vfs_truncate() is correct
> * fs/cachefiles and fs/ksmbd use the file system from the kernel to
> expose it as another file system (in a cached form for cachefiles,
> and over the network for ksmbd). I suspect that these file systems
> are not handling 'struct file's which are opened in contexts where a
> LSM applies? It that a reasonable assumption?
I think you're right but I have some doubts about the cachefiles subsystem.
I don't know how ksmb deals with these file descriptors but changing
such call sites (where there is a struct file) could improve API
consistency though.
Any though?
>
> Thanks,
> Günther
>
> On Thu, Sep 08, 2022 at 09:58:01PM +0200, Günther Noack wrote:
>> Like path_truncate, the file_truncate hook also restricts file
>> truncation, but is called in the cases where truncation is attempted
>> on an already-opened file.
>>
>> This is required in a subsequent commit to handle ftruncate()
>> operations differently to truncate() operations.
>>
>> Signed-off-by: Günther Noack <gnoack3000 at gmail.com>
>> ---
>> fs/namei.c | 6 +++---
>> fs/open.c | 4 ++--
>> include/linux/lsm_hook_defs.h | 1 +
>> include/linux/security.h | 6 ++++++
>> security/apparmor/lsm.c | 6 ++++++
>> security/security.c | 5 +++++
>> security/tomoyo/tomoyo.c | 13 +++++++++++++
>> 7 files changed, 36 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 53b4bc094db2..52105873d1f8 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -53,8 +53,8 @@
>> * The new code replaces the old recursive symlink resolution with
>> * an iterative one (in case of non-nested symlink chains). It does
>> * this with calls to <fs>_follow_link().
>> - * As a side effect, dir_namei(), _namei() and follow_link() are now
>> - * replaced with a single function lookup_dentry() that can handle all
>> + * As a side effect, dir_namei(), _namei() and follow_link() are now
>> + * replaced with a single function lookup_dentry() that can handle all
>> * the special cases of the former code.
>> *
>> * With the new dcache, the pathname is stored at each inode, at least as
>> @@ -3211,7 +3211,7 @@ static int handle_truncate(struct user_namespace *mnt_userns, struct file *filp)
>> if (error)
>> return error;
>>
>> - error = security_path_truncate(path);
>> + error = security_file_truncate(filp);
>> if (!error) {
>> error = do_truncate(mnt_userns, path->dentry, 0,
>> ATTR_MTIME|ATTR_CTIME|ATTR_OPEN,
>> diff --git a/fs/open.c b/fs/open.c
>> index 8a813fa5ca56..0831433e493a 100644
>> --- a/fs/open.c
>> +++ b/fs/open.c
>> @@ -188,7 +188,7 @@ long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
>> if (IS_APPEND(file_inode(f.file)))
>> goto out_putf;
>> sb_start_write(inode->i_sb);
>> - error = security_path_truncate(&f.file->f_path);
>> + error = security_file_truncate(f.file);
>> if (!error)
>> error = do_truncate(file_mnt_user_ns(f.file), dentry, length,
>> ATTR_MTIME | ATTR_CTIME, f.file);
>> @@ -1271,7 +1271,7 @@ struct file *filp_open(const char *filename, int flags, umode_t mode)
>> {
>> struct filename *name = getname_kernel(filename);
>> struct file *file = ERR_CAST(name);
>> -
>> +
>> if (!IS_ERR(name)) {
>> file = file_open_name(name, flags, mode);
>> putname(name);
>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
>> index 60fff133c0b1..dee35ab253ba 100644
>> --- a/include/linux/lsm_hook_defs.h
>> +++ b/include/linux/lsm_hook_defs.h
>> @@ -177,6 +177,7 @@ LSM_HOOK(int, 0, file_send_sigiotask, struct task_struct *tsk,
>> struct fown_struct *fown, int sig)
>> LSM_HOOK(int, 0, file_receive, struct file *file)
>> LSM_HOOK(int, 0, file_open, struct file *file)
>> +LSM_HOOK(int, 0, file_truncate, struct file *file)
>> LSM_HOOK(int, 0, task_alloc, struct task_struct *task,
>> unsigned long clone_flags)
>> LSM_HOOK(void, LSM_RET_VOID, task_free, struct task_struct *task)
>> diff --git a/include/linux/security.h b/include/linux/security.h
>> index 7bd0c490703d..f80b23382dd9 100644
>> --- a/include/linux/security.h
>> +++ b/include/linux/security.h
>> @@ -394,6 +394,7 @@ int security_file_send_sigiotask(struct task_struct *tsk,
>> struct fown_struct *fown, int sig);
>> int security_file_receive(struct file *file);
>> int security_file_open(struct file *file);
>> +int security_file_truncate(struct file *file);
>> int security_task_alloc(struct task_struct *task, unsigned long clone_flags);
>> void security_task_free(struct task_struct *task);
>> int security_cred_alloc_blank(struct cred *cred, gfp_t gfp);
>> @@ -1011,6 +1012,11 @@ static inline int security_file_open(struct file *file)
>> return 0;
>> }
>>
>> +static inline int security_file_truncate(struct file *file)
>> +{
>> + return 0;
>> +}
>> +
>> static inline int security_task_alloc(struct task_struct *task,
>> unsigned long clone_flags)
>> {
>> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
>> index e29cade7b662..98ecb7f221b8 100644
>> --- a/security/apparmor/lsm.c
>> +++ b/security/apparmor/lsm.c
>> @@ -329,6 +329,11 @@ static int apparmor_path_truncate(const struct path *path)
>> return common_perm_cond(OP_TRUNC, path, MAY_WRITE | AA_MAY_SETATTR);
>> }
>>
>> +static int apparmor_file_truncate(struct file *file)
>> +{
>> + return apparmor_path_truncate(&file->f_path);
>> +}
>> +
>> static int apparmor_path_symlink(const struct path *dir, struct dentry *dentry,
>> const char *old_name)
>> {
>> @@ -1232,6 +1237,7 @@ static struct security_hook_list apparmor_hooks[] __lsm_ro_after_init = {
>> LSM_HOOK_INIT(mmap_file, apparmor_mmap_file),
>> LSM_HOOK_INIT(file_mprotect, apparmor_file_mprotect),
>> LSM_HOOK_INIT(file_lock, apparmor_file_lock),
>> + LSM_HOOK_INIT(file_truncate, apparmor_file_truncate),
>>
>> LSM_HOOK_INIT(getprocattr, apparmor_getprocattr),
>> LSM_HOOK_INIT(setprocattr, apparmor_setprocattr),
>> diff --git a/security/security.c b/security/security.c
>> index 4b95de24bc8d..e491120c48ba 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -1210,6 +1210,11 @@ int security_path_truncate(const struct path *path)
>> return call_int_hook(path_truncate, 0, path);
>> }
>>
>> +int security_file_truncate(struct file *file)
>> +{
>> + return call_int_hook(file_truncate, 0, file);
>> +}
>> +
>> int security_path_chmod(const struct path *path, umode_t mode)
>> {
>> if (unlikely(IS_PRIVATE(d_backing_inode(path->dentry))))
>> diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
>> index 71e82d855ebf..af04a7b7eb28 100644
>> --- a/security/tomoyo/tomoyo.c
>> +++ b/security/tomoyo/tomoyo.c
>> @@ -134,6 +134,18 @@ static int tomoyo_path_truncate(const struct path *path)
>> return tomoyo_path_perm(TOMOYO_TYPE_TRUNCATE, path, NULL);
>> }
>>
>> +/**
>> + * tomoyo_file_truncate - Target for security_file_truncate().
>> + *
>> + * @file: Pointer to "struct file".
>> + *
>> + * Returns 0 on success, negative value otherwise.
>> + */
>> +static int tomoyo_file_truncate(struct file *file)
>> +{
>> + return tomoyo_path_truncate(&file->f_path);
>> +}
>> +
>> /**
>> * tomoyo_path_unlink - Target for security_path_unlink().
>> *
>> @@ -545,6 +557,7 @@ static struct security_hook_list tomoyo_hooks[] __lsm_ro_after_init = {
>> LSM_HOOK_INIT(bprm_check_security, tomoyo_bprm_check_security),
>> LSM_HOOK_INIT(file_fcntl, tomoyo_file_fcntl),
>> LSM_HOOK_INIT(file_open, tomoyo_file_open),
>> + LSM_HOOK_INIT(file_truncate, tomoyo_file_truncate),
>> LSM_HOOK_INIT(path_truncate, tomoyo_path_truncate),
>> LSM_HOOK_INIT(path_unlink, tomoyo_path_unlink),
>> LSM_HOOK_INIT(path_mkdir, tomoyo_path_mkdir),
>> --
>> 2.37.3
>>
>
> --
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