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Date: Mon, 2 Oct 2023 09:52:57 -0400
From: Stefan Berger <stefanb@...ux.ibm.com>
To: Amir Goldstein <amir73il@...il.com>,
Stefan Berger <stefanb@...ux.vnet.ibm.com>
Cc: linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-unionfs@...r.kernel.org, miklos@...redi.hu,
syzbot+a67fc5321ffb4b311c98@...kaller.appspotmail.com,
Alexander Viro <viro@...iv.linux.org.uk>,
linux-fsdevel@...r.kernel.org, Tyler Hicks <code@...icks.com>,
Mimi Zohar <zohar@...ux.ibm.com>,
Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH] fs: Pass AT_GETATTR_NOSEC flag to getattr interface
function
On 10/2/23 09:22, Amir Goldstein wrote:
> On Mon, Oct 2, 2023 at 3:57 PM Stefan Berger <stefanb@...ux.vnet.ibm.com> wrote:
>> From: Stefan Berger <stefanb@...ux.ibm.com>
>>
>> When vfs_getattr_nosec() calls a filesystem's getattr interface function
>> then the 'nosec' should propagate into this function so that
>> vfs_getattr_nosec() can again be called from the filesystem's gettattr
>> rather than vfs_getattr(). The latter would add unnecessary security
>> checks that the initial vfs_getattr_nosec() call wanted to avoid.
>> Therefore, introduce the getattr flag GETATTR_NOSEC and allow to pass
>> with the new getattr_flags parameter to the getattr interface function.
>> In overlayfs and ecryptfs use this flag to determine which one of the
>> two functions to call.
>>
>> In a recent code change introduced to IMA vfs_getattr_nosec() ended up
>> calling vfs_getattr() in overlayfs, which in turn called
>> security_inode_getattr() on an exiting process that did not have
>> current->fs set anymore, which then caused a kernel NULL pointer
>> dereference. With this change the call to security_inode_getattr() can
>> be avoided, thus avoiding the NULL pointer dereference.
>>
>> Reported-by: syzbot+a67fc5321ffb4b311c98@...kaller.appspotmail.com
>> Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version")
>> Cc: Alexander Viro <viro@...iv.linux.org.uk>
>> Cc: linux-fsdevel@...r.kernel.org
>> Cc: Miklos Szeredi <miklos@...redi.hu>
>> Cc: Amir Goldstein <amir73il@...il.com>
>> Cc: Tyler Hicks <code@...icks.com>
>> Cc: Mimi Zohar <zohar@...ux.ibm.com>
>> Suggested-by: Christian Brauner <brauner@...nel.org>
>> Co-developed-by: Amir Goldstein <amir73il@...il.com>
>> Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
>> ---
> Reviewed-by: Amir Goldstein <amir73il@...il.com>
>
> Now let's see what vfs maintainers think about this...
Thanks.
For the other reviewers, the original syzbot message with the 'final
oops' is here:
https://lore.kernel.org/linux-integrity/bed99e92-cb7c-868d-94f3-ddf53e2b262a@linux.ibm.com/T/#m616371f8ac1a316be13865e1699ac59ccf8ce6ef
Regards,
Stefan
>
> Thanks,
> Amir.
>
>> fs/ecryptfs/inode.c | 12 ++++++++++--
>> fs/overlayfs/inode.c | 10 +++++-----
>> fs/overlayfs/overlayfs.h | 8 ++++++++
>> fs/stat.c | 6 +++++-
>> include/uapi/linux/fcntl.h | 3 +++
>> 5 files changed, 31 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/ecryptfs/inode.c b/fs/ecryptfs/inode.c
>> index 992d9c7e64ae..5ab4b87888a7 100644
>> --- a/fs/ecryptfs/inode.c
>> +++ b/fs/ecryptfs/inode.c
>> @@ -998,6 +998,14 @@ static int ecryptfs_getattr_link(struct mnt_idmap *idmap,
>> return rc;
>> }
>>
>> +static int ecryptfs_do_getattr(const struct path *path, struct kstat *stat,
>> + u32 request_mask, unsigned int flags)
>> +{
>> + if (flags & AT_GETATTR_NOSEC)
>> + return vfs_getattr_nosec(path, stat, request_mask, flags);
>> + return vfs_getattr(path, stat, request_mask, flags);
>> +}
>> +
>> static int ecryptfs_getattr(struct mnt_idmap *idmap,
>> const struct path *path, struct kstat *stat,
>> u32 request_mask, unsigned int flags)
>> @@ -1006,8 +1014,8 @@ static int ecryptfs_getattr(struct mnt_idmap *idmap,
>> struct kstat lower_stat;
>> int rc;
>>
>> - rc = vfs_getattr(ecryptfs_dentry_to_lower_path(dentry), &lower_stat,
>> - request_mask, flags);
>> + rc = ecryptfs_do_getattr(ecryptfs_dentry_to_lower_path(dentry),
>> + &lower_stat, request_mask, flags);
>> if (!rc) {
>> fsstack_copy_attr_all(d_inode(dentry),
>> ecryptfs_inode_to_lower(d_inode(dentry)));
>> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
>> index 83ef66644c21..fca29dba7b14 100644
>> --- a/fs/overlayfs/inode.c
>> +++ b/fs/overlayfs/inode.c
>> @@ -171,7 +171,7 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
>>
>> type = ovl_path_real(dentry, &realpath);
>> old_cred = ovl_override_creds(dentry->d_sb);
>> - err = vfs_getattr(&realpath, stat, request_mask, flags);
>> + err = ovl_do_getattr(&realpath, stat, request_mask, flags);
>> if (err)
>> goto out;
>>
>> @@ -196,8 +196,8 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
>> (!is_dir ? STATX_NLINK : 0);
>>
>> ovl_path_lower(dentry, &realpath);
>> - err = vfs_getattr(&realpath, &lowerstat,
>> - lowermask, flags);
>> + err = ovl_do_getattr(&realpath, &lowerstat, lowermask,
>> + flags);
>> if (err)
>> goto out;
>>
>> @@ -249,8 +249,8 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
>>
>> ovl_path_lowerdata(dentry, &realpath);
>> if (realpath.dentry) {
>> - err = vfs_getattr(&realpath, &lowerdatastat,
>> - lowermask, flags);
>> + err = ovl_do_getattr(&realpath, &lowerdatastat,
>> + lowermask, flags);
>> if (err)
>> goto out;
>> } else {
>> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
>> index 9817b2dcb132..09ca82ed0f8c 100644
>> --- a/fs/overlayfs/overlayfs.h
>> +++ b/fs/overlayfs/overlayfs.h
>> @@ -397,6 +397,14 @@ static inline bool ovl_open_flags_need_copy_up(int flags)
>> return ((OPEN_FMODE(flags) & FMODE_WRITE) || (flags & O_TRUNC));
>> }
>>
>> +static inline int ovl_do_getattr(const struct path *path, struct kstat *stat,
>> + u32 request_mask, unsigned int flags)
>> +{
>> + if (flags & AT_GETATTR_NOSEC)
>> + return vfs_getattr_nosec(path, stat, request_mask, flags);
>> + return vfs_getattr(path, stat, request_mask, flags);
>> +}
>> +
>> /* util.c */
>> int ovl_want_write(struct dentry *dentry);
>> void ovl_drop_write(struct dentry *dentry);
>> diff --git a/fs/stat.c b/fs/stat.c
>> index d43a5cc1bfa4..5375be5f97cc 100644
>> --- a/fs/stat.c
>> +++ b/fs/stat.c
>> @@ -133,7 +133,8 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
>> idmap = mnt_idmap(path->mnt);
>> if (inode->i_op->getattr)
>> return inode->i_op->getattr(idmap, path, stat,
>> - request_mask, query_flags);
>> + request_mask,
>> + query_flags | AT_GETATTR_NOSEC);
>>
>> generic_fillattr(idmap, request_mask, inode, stat);
>> return 0;
>> @@ -166,6 +167,9 @@ int vfs_getattr(const struct path *path, struct kstat *stat,
>> {
>> int retval;
>>
>> + if (WARN_ON_ONCE(query_flags & AT_GETATTR_NOSEC))
>> + return -EPERM;
>> +
>> retval = security_inode_getattr(path);
>> if (retval)
>> return retval;
>> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
>> index 6c80f96049bd..282e90aeb163 100644
>> --- a/include/uapi/linux/fcntl.h
>> +++ b/include/uapi/linux/fcntl.h
>> @@ -116,5 +116,8 @@
>> #define AT_HANDLE_FID AT_REMOVEDIR /* file handle is needed to
>> compare object identity and may not
>> be usable to open_by_handle_at(2) */
>> +#if defined(__KERNEL__)
>> +#define AT_GETATTR_NOSEC 0x80000000
>> +#endif
>>
>> #endif /* _UAPI_LINUX_FCNTL_H */
>> --
>> 2.40.1
>>
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