Re: [PATCH v8] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

From: Jeff Layton
Date: Tue Aug 08 2023 - 13:57:25 EST

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In reply to: Christian Brauner: "Re: [PATCH v8] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing"
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On Tue, 2023-08-08 at 10:53 +0200, Christian Brauner wrote:
> On Mon, Aug 07, 2023 at 10:09:21AM -0400, Jeff Layton wrote:
> > On Mon, 2023-08-07 at 15:38 +0200, Christian Brauner wrote:
> > > On Mon, Aug 07, 2023 at 09:18:01AM -0400, Jeff Layton wrote:
> > > > From: David Howells <dhowells@xxxxxxxxxx>
> > > >
> > > > When NFS superblocks are created by automounting, their LSM parameters
> > > > aren't set in the fs_context struct prior to sget_fc() being called,
> > > > leading to failure to match existing superblocks.
> > > >
> > > > This bug leads to messages like the following appearing in dmesg when
> > > > fscache is enabled:
> > > >
> > > > NFS: Cache volume key already in use (nfs,4.2,2,108,106a8c0,1,,,,100000,100000,2ee,3a98,1d4c,3a98,1)
> > > >
> > > > Fix this by adding a new LSM hook to load fc->security for submount
> > > > creation.
> > > >
> > > > Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > Fixes: 9bc61ab18b1d ("vfs: Introduce fs_context, switch vfs_kern_mount() to it.")
> > > > Fixes: 779df6a5480f ("NFS: Ensure security label is set for root inode)
> > > > Tested-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> > > > Acked-by: "Christian Brauner (Microsoft)" <brauner@xxxxxxxxxx>
> > > > Link: https://lore.kernel.org/r/165962680944.3334508.6610023900349142034.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1
> > > > Link: https://lore.kernel.org/r/165962729225.3357250.14350728846471527137.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v2
> > > > Link: https://lore.kernel.org/r/165970659095.2812394.6868894171102318796.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v3
> > > > Link: https://lore.kernel.org/r/166133579016.3678898.6283195019480567275.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v4
> > > > Link: https://lore.kernel.org/r/217595.1662033775@xxxxxxxxxxxxxxxxxxxxxx/ # v5
> > > > ---
> > > > ver #2)
> > > > - Added Smack support
> > > > - Made LSM parameter extraction dependent on reference != NULL.
> > > >
> > > > ver #3)
> > > > - Made LSM parameter extraction dependent on fc->purpose ==
> > > > FS_CONTEXT_FOR_SUBMOUNT. Shouldn't happen on FOR_RECONFIGURE.
> > > >
> > > > ver #4)
> > > > - When doing a FOR_SUBMOUNT mount, don't set the root label in SELinux or Smack.
> > > >
> > > > ver #5)
> > > > - Removed unused variable.
> > > > - Only allocate smack_mnt_opts if we're dealing with a submount.
> > > >
> > > > ver #6)
> > > > - Rebase onto v6.5.0-rc4
> > > > - Link to v6: https://lore.kernel.org/r/20230802-master-v6-1-45d48299168b@xxxxxxxxxx
> > > >
> > > > ver #7)
> > > > - Drop lsm_set boolean
> > > > - Link to v7: https://lore.kernel.org/r/20230804-master-v7-1-5d4e48407298@xxxxxxxxxx
> > > >
> > > > ver #8)
> > > > - Remove spurious semicolon in smack_fs_context_init
> > > > - Make fs_context_init take a superblock as reference instead of dentry
> > > > - WARN_ON_ONCE's when fc->purpose != FS_CONTEXT_FOR_SUBMOUNT
> > > > - Call the security hook from fs_context_for_submount instead of alloc_fs_context
> > > > ---
> > > > fs/fs_context.c | 23 +++++++++++++++++-
> > > > include/linux/lsm_hook_defs.h | 1 +
> > > > include/linux/security.h | 6 +++++
> > > > security/security.c | 14 +++++++++++
> > > > security/selinux/hooks.c | 25 ++++++++++++++++++++
> > > > security/smack/smack_lsm.c | 54 +++++++++++++++++++++++++++++++++++++++++++
> > > > 6 files changed, 122 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/fs/fs_context.c b/fs/fs_context.c
> > > > index 851214d1d013..a76d7c82e091 100644
> > > > --- a/fs/fs_context.c
> > > > +++ b/fs/fs_context.c
> > > > @@ -315,10 +315,31 @@ struct fs_context *fs_context_for_reconfigure(struct dentry *dentry,
> > > > }
> > > > EXPORT_SYMBOL(fs_context_for_reconfigure);
> > > >
> > > > +/**
> > > > + * fs_context_for_submount: allocate a new fs_context for a submount
> > > > + * @type: file_system_type of the new context
> > > > + * @reference: reference dentry from which to copy relevant info
> > > > + *
> > > > + * Allocate a new fs_context suitable for a submount. This also ensures that
> > > > + * the fc->security object is inherited from @reference (if needed).
> > > > + */
> > > > struct fs_context *fs_context_for_submount(struct file_system_type *type,
> > > > struct dentry *reference)
> > > > {
> > > > - return alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
> > > > + struct fs_context *fc;
> > > > + int ret;
> > > > +
> > > > + fc = alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
> > > > + if (IS_ERR(fc))
> > > > + return fc;
> > > > +
> > > > + ret = security_fs_context_init(fc, reference->d_sb);
> > > > + if (ret) {
> > > > + put_fs_context(fc);
> > > > + return ERR_PTR(ret);
> > > > + }
> > > > +
> > > > + return fc;
> > > > }
> > > > EXPORT_SYMBOL(fs_context_for_submount);
> > > >
> > > > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> > > > index 7308a1a7599b..2876dd6114c0 100644
> > > > --- a/include/linux/lsm_hook_defs.h
> > > > +++ b/include/linux/lsm_hook_defs.h
> > > > @@ -54,6 +54,7 @@ LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, struct file *f
> > > > LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
> > > > LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
> > > > LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, struct linux_binprm *bprm)
> > > > +LSM_HOOK(int, 0, fs_context_init, struct fs_context *fc, struct super_block *reference)
> > > > LSM_HOOK(int, 0, fs_context_dup, struct fs_context *fc,
> > > > struct fs_context *src_sc)
> > > > LSM_HOOK(int, -ENOPARAM, fs_context_parse_param, struct fs_context *fc,
> > > > diff --git a/include/linux/security.h b/include/linux/security.h
> > > > index 32828502f09e..fe9bf5e805ee 100644
> > > > --- a/include/linux/security.h
> > > > +++ b/include/linux/security.h
> > > > @@ -293,6 +293,7 @@ int security_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file);
> > > > int security_bprm_check(struct linux_binprm *bprm);
> > > > void security_bprm_committing_creds(struct linux_binprm *bprm);
> > > > void security_bprm_committed_creds(struct linux_binprm *bprm);
> > > > +int security_fs_context_init(struct fs_context *fc, struct super_block *reference);
> > > > int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc);
> > > > int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param);
> > > > int security_sb_alloc(struct super_block *sb);
> > > > @@ -629,6 +630,11 @@ static inline void security_bprm_committed_creds(struct linux_binprm *bprm)
> > > > {
> > > > }
> > > >
> > > > +static inline int security_fs_context_init(struct fs_context *fc,
> > > > + struct super_block *reference)
> > > > +{
> > > > + return 0;
> > > > +}
> > >
> > > Sorry, my point is we shouldn't be adding a generic
> > > security_fs_context_init() hook at all. Pre superblock creation we have
> > > a hook during parameter parsing for LSMs and another one during actual
> > > superblock creation in vfs_get_tree() and yet another one for fs_context
> > > duplicaton. We don't need another generic one during fs_context
> > > allocation.
> > >
> > > Yes, we may need a hook for submount allocation but then we'll add one
> > > exactly for that. And then for fs_context_for_submount @sb can't be
> > > empty so there's also no point in checking whether it is empty because
> > > you've already crashed in fs_context_for_submount(). All the checks
> > > below for !reference and fc->purpose != FS_CONTEXT_FOR_SUBMOUNT can go
> > > away then as well.
> > >
> > > So we end up with something easier and stricter.
> > >
> >
> > I'm not sure I understand what you're asking for here. fc->security
> > field initialization is dependent on the LSM in use. Where will we do
> > this initialization if we don't add this new hook? Are you suggesting
> > that we fold this into an existing LSM hook? If so, which one?
>
> I'm sorry, I didn't mean to confuse. I just mean the following:
> * renaming security_fs_context_init() to security_fs_context_submount()
> to reflect that it is only called during submount allocation now
> * Now that security_fs_context_submount() is called in
> fs_context_for_submount() you can remove all the checks for @reference
> and fc->purpose == FS_CONTEXT_FOR_SUBMOUNT checks because they can't
> be unset otherwise you would've a bug in fs_context_for_submount().
>
> These two things were missing after you did move the LSM call into
> fs_context_for_submount(). That's basically it. So folded into what you
> sent out this amounts to:
>

Thanks, I'll fold those changes into this patch and resend. There's also
a bug that the kernel test robot spotted in SMACK code that I'll fix as
well. I'll send v9 after I've had a chance to retest the changes.

> fs/fs_context.c | 23 +++++++++++++++-
> include/linux/lsm_hook_defs.h | 1 +
> include/linux/security.h | 6 +++++
> security/security.c | 14 ++++++++++
> security/selinux/hooks.c | 22 +++++++++++++++
> security/smack/smack_lsm.c | 51 +++++++++++++++++++++++++++++++++++
> 6 files changed, 116 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 851214d1d013..375023e40161 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -315,10 +315,31 @@ struct fs_context *fs_context_for_reconfigure(struct dentry *dentry,
> }
> EXPORT_SYMBOL(fs_context_for_reconfigure);
>
> +/**
> + * fs_context_for_submount: allocate a new fs_context for a submount
> + * @type: file_system_type of the new context
> + * @reference: reference dentry from which to copy relevant info
> + *
> + * Allocate a new fs_context suitable for a submount. This also ensures that
> + * the fc->security object is inherited from @reference (if needed).
> + */
> struct fs_context *fs_context_for_submount(struct file_system_type *type,
> struct dentry *reference)
> {
> - return alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
> + struct fs_context *fc;
> + int ret;
> +
> + fc = alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
> + if (IS_ERR(fc))
> + return fc;
> +
> + ret = security_fs_context_submount(fc, reference->d_sb);
> + if (ret) {
> + put_fs_context(fc);
> + return ERR_PTR(ret);
> + }
> +
> + return fc;
> }
> EXPORT_SYMBOL(fs_context_for_submount);
>
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 7308a1a7599b..2876dd6114c0 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -54,6 +54,7 @@ LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, struct file *f
> LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
> LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
> LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, struct linux_binprm *bprm)
> +LSM_HOOK(int, 0, fs_context_init, struct fs_context *fc, struct super_block *reference)
> LSM_HOOK(int, 0, fs_context_dup, struct fs_context *fc,
> struct fs_context *src_sc)
> LSM_HOOK(int, -ENOPARAM, fs_context_parse_param, struct fs_context *fc,
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 32828502f09e..02b41f4d05d1 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -293,6 +293,7 @@ int security_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file);
> int security_bprm_check(struct linux_binprm *bprm);
> void security_bprm_committing_creds(struct linux_binprm *bprm);
> void security_bprm_committed_creds(struct linux_binprm *bprm);
> +int security_fs_context_submount(struct fs_context *fc, struct super_block *reference);
> int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc);
> int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param);
> int security_sb_alloc(struct super_block *sb);
> @@ -629,6 +630,11 @@ static inline void security_bprm_committed_creds(struct linux_binprm *bprm)
> {
> }
>
> +static inline int security_fs_context_submount(struct fs_context *fc,
> + struct super_block *reference)
> +{
> + return 0;
> +}
> static inline int security_fs_context_dup(struct fs_context *fc,
> struct fs_context *src_fc)
> {
> diff --git a/security/security.c b/security/security.c
> index b720424ca37d..3611ebdf31d9 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1138,6 +1138,20 @@ void security_bprm_committed_creds(struct linux_binprm *bprm)
> call_void_hook(bprm_committed_creds, bprm);
> }
>
> +/**
> + * security_fs_context_submount() - Initialise fc->security
> + * @fc: new filesystem context
> + * @reference: dentry reference for submount/remount
> + *
> + * Fill out the ->security field for a new fs_context.
> + *
> + * Return: Returns 0 on success or negative error code on failure.
> + */
> +int security_fs_context_submount(struct fs_context *fc, struct super_block *reference)
> +{
> + return call_int_hook(fs_context_init, 0, fc, reference);
> +}
> +
> /**
> * security_fs_context_dup() - Duplicate a fs_context LSM blob
> * @fc: destination filesystem context
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index d06e350fedee..8b950274138b 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2745,6 +2745,27 @@ static int selinux_umount(struct vfsmount *mnt, int flags)
> FILESYSTEM__UNMOUNT, NULL);
> }
>
> +static int selinux_fs_context_submount(struct fs_context *fc,
> + struct super_block *reference)
> +{
> + const struct superblock_security_struct *sbsec;
> + struct selinux_mnt_opts *opts;
> +
> + opts = kzalloc(sizeof(*opts), GFP_KERNEL);
> + if (!opts)
> + return -ENOMEM;
> +
> + sbsec = selinux_superblock(reference);
> + if (sbsec->flags & FSCONTEXT_MNT)
> + opts->fscontext_sid = sbsec->sid;
> + if (sbsec->flags & CONTEXT_MNT)
> + opts->context_sid = sbsec->mntpoint_sid;
> + if (sbsec->flags & DEFCONTEXT_MNT)
> + opts->defcontext_sid = sbsec->def_sid;
> + fc->security = opts;
> + return 0;
> +}
> +
> static int selinux_fs_context_dup(struct fs_context *fc,
> struct fs_context *src_fc)
> {
> @@ -7182,6 +7203,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
> /*
> * PUT "CLONING" (ACCESSING + ALLOCATING) HOOKS HERE
> */
> + LSM_HOOK_INIT(fs_context_submount, selinux_fs_context_submount),
> LSM_HOOK_INIT(fs_context_dup, selinux_fs_context_dup),
> LSM_HOOK_INIT(fs_context_parse_param, selinux_fs_context_parse_param),
> LSM_HOOK_INIT(sb_eat_lsm_opts, selinux_sb_eat_lsm_opts),
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 6e270cf3fd30..87f7e0506f4d 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -614,6 +614,56 @@ static int smack_add_opt(int token, const char *s, void **mnt_opts)
> return -EINVAL;
> }
>
> +/**
> + * smack_fs_context_submount - Initialise security data for a filesystem context
> + * @fc: The filesystem context.
> + * @reference: Reference superblock (automount/reconfigure) or NULL
> + *
> + * Returns 0 on success or -ENOMEM on error.
> + */
> +static int smack_fs_context_submount(struct fs_context *fc,
> + struct dentry *reference)
> +{
> + struct superblock_smack *sbsp;
> + struct smack_mnt_opts *ctx;
> + struct inode_smack *isp;
> +
> + ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
> + if (!ctx)
> + return -ENOMEM;
> + fc->security = ctx;
> +
> + sbsp = smack_superblock(reference);
> + isp = smack_inode(reference->s_root->d_inode);
> +
> + if (sbsp->smk_default) {
> + ctx->fsdefault = kstrdup(sbsp->smk_default->smk_known, GFP_KERNEL);
> + if (!ctx->fsdefault)
> + return -ENOMEM;
> + }
> +
> + if (sbsp->smk_floor) {
> + ctx->fsfloor = kstrdup(sbsp->smk_floor->smk_known, GFP_KERNEL);
> + if (!ctx->fsfloor)
> + return -ENOMEM;
> + }
> +
> + if (sbsp->smk_hat) {
> + ctx->fshat = kstrdup(sbsp->smk_hat->smk_known, GFP_KERNEL);
> + if (!ctx->fshat)
> + return -ENOMEM;
> + }
> +
> + if (isp->smk_flags & SMK_INODE_TRANSMUTE) {
> + if (sbsp->smk_root) {
> + ctx->fstransmute = kstrdup(sbsp->smk_root->smk_known, GFP_KERNEL);
> + if (!ctx->fstransmute)
> + return -ENOMEM;
> + }
> + }
> + return 0;
> +}
> +
> /**
> * smack_fs_context_dup - Duplicate the security data on fs_context duplication
> * @fc: The new filesystem context.
> @@ -4876,6 +4926,7 @@ static struct security_hook_list smack_hooks[] __ro_after_init = {
> LSM_HOOK_INIT(ptrace_traceme, smack_ptrace_traceme),
> LSM_HOOK_INIT(syslog, smack_syslog),
>
> + LSM_HOOK_INIT(fs_context_submount, smack_fs_context_submount),
> LSM_HOOK_INIT(fs_context_dup, smack_fs_context_dup),
> LSM_HOOK_INIT(fs_context_parse_param, smack_fs_context_parse_param),
>

--
Jeff Layton <jlayton@xxxxxxxxxx>

Next message: Aleksandr Nogikh: "Re: [syzbot] [fat?] possible deadlock in do_user_addr_fault"
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In reply to: Christian Brauner: "Re: [PATCH v8] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing"
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