On Mon, 2025-04-07 at 11:54 +0200, Christian Brauner wrote:

This allows the VFS to not trip over anonymous inodes and we can add asserts based on the mode into the vfs. When we report it to userspace we can simply hide the mode to avoid regressions. I've audited all direct callers of alloc_anon_inode() and only secretmen overrides i_mode and i_op inode operations but it already uses a regular file.

Fixes: af153bb63a336 ("vfs: catch invalid modes in may_open()") Cc: stable@vger.kernel.org # all LTS kernels Reported-by: syzbot+5d8e79d323a13aa0b248@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/67ed3fb3.050a0220.14623d.0009.GAE@google.com Signed-off-by: Christian Brauner brauner@kernel.org

---

fs/anon_inodes.c | 36 ++++++++++++++++++++++++++++++++++++ fs/internal.h | 3 +++ fs/libfs.c | 2 +- 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c index 583ac81669c2..42e4b9c34f89 100644 --- a/fs/anon_inodes.c +++ b/fs/anon_inodes.c @@ -24,9 +24,43 @@ #include <linux/uaccess.h> +#include "internal.h"

static struct vfsmount *anon_inode_mnt __ro_after_init; static struct inode *anon_inode_inode __ro_after_init; +/*

• • User space expects anonymous inodes to have no file type in st_mode.

Weird. Does anything actually depend on this?

ISTM that they should report a file type.

• • In particular, 'lsof' has this legacy logic:
• • type = s->st_mode & S_IFMT;
• • switch (type) {

• •  ...
    

• • case 0:

• • if (!strcmp(p, "anon_inode"))
    

• • 	Lf->ntype = Ntype = N_ANON_INODE;
    

• • to detect our old anon_inode logic.
• • Rather than mess with our internal sane inode data, just fix it
• • up here in getattr() by masking off the format bits.
• */

+int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,

•        struct kstat *stat, u32 request_mask,
  

•        unsigned int query_flags)
  

+{

• struct inode *inode = d_inode(path->dentry);
• generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
• stat->mode &= ~S_IFMT;
• return 0;

+}

+static const struct inode_operations anon_inode_operations = {

• .getattr = anon_inode_getattr,

+};

/*

• anon_inodefs_dname() is called from d_path().

*/ @@ -66,6 +100,7 @@ static struct inode *anon_inode_make_secure_inode( if (IS_ERR(inode)) return inode; inode->i_flags &= ~S_PRIVATE;

• inode->i_op = &anon_inode_operations; error = security_inode_init_security_anon(inode, &QSTR(name), context_inode); if (error) {

@@ -313,6 +348,7 @@ static int __init anon_inode_init(void) anon_inode_inode = alloc_anon_inode(anon_inode_mnt->mnt_sb); if (IS_ERR(anon_inode_inode)) panic("anon_inode_init() inode allocation failed (%ld)\n", PTR_ERR(anon_inode_inode));

• anon_inode_inode->i_op = &anon_inode_operations;

return 0; } diff --git a/fs/internal.h b/fs/internal.h index b9b3e29a73fd..717dc9eb6185 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -343,3 +343,6 @@ static inline bool path_mounted(const struct path *path) void file_f_owner_release(struct file *file); bool file_seek_cur_needs_f_lock(struct file *file); int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map); +int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,

•        struct kstat *stat, u32 request_mask,
  

•        unsigned int query_flags);
  

diff --git a/fs/libfs.c b/fs/libfs.c index 6393d7c49ee6..0ad3336f5b49 100644 --- a/fs/libfs.c +++ b/fs/libfs.c @@ -1647,7 +1647,7 @@ struct inode *alloc_anon_inode(struct super_block *s) * that it already _is_ on the dirty list. */ inode->i_state = I_DIRTY;

• inode->i_mode = S_IRUSR | S_IWUSR;

• inode->i_mode = S_IFREG | S_IRUSR | S_IWUSR; inode->i_uid = current_fsuid(); inode->i_gid = current_fsgid(); inode->i_flags |= S_PRIVATE;

On Mon 07-04-25 11:54:15, Christian Brauner wrote:

This allows the VFS to not trip over anonymous inodes and we can add asserts based on the mode into the vfs. When we report it to userspace we can simply hide the mode to avoid regressions. I've audited all direct callers of alloc_anon_inode() and only secretmen overrides i_mode and i_op inode operations but it already uses a regular file.

Fixes: af153bb63a336 ("vfs: catch invalid modes in may_open()") Cc: stable@vger.kernel.org # all LTS kernels Reported-by: syzbot+5d8e79d323a13aa0b248@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/67ed3fb3.050a0220.14623d.0009.GAE@google.com Signed-off-by: Christian Brauner brauner@kernel.org

Looks good. Feel free to add:

Reviewed-by: Jan Kara jack@suse.cz

Honza

---

fs/anon_inodes.c | 36 ++++++++++++++++++++++++++++++++++++ fs/internal.h | 3 +++ fs/libfs.c | 2 +- 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c index 583ac81669c2..42e4b9c34f89 100644 --- a/fs/anon_inodes.c +++ b/fs/anon_inodes.c @@ -24,9 +24,43 @@ #include <linux/uaccess.h> +#include "internal.h"

static struct vfsmount *anon_inode_mnt __ro_after_init; static struct inode *anon_inode_inode __ro_after_init; +/*

• • User space expects anonymous inodes to have no file type in st_mode.
• • In particular, 'lsof' has this legacy logic:
• • type = s->st_mode & S_IFMT;
• • switch (type) {

• •  ...
    

• • case 0:

• • if (!strcmp(p, "anon_inode"))
    

• • 	Lf->ntype = Ntype = N_ANON_INODE;
    

• • to detect our old anon_inode logic.
• • Rather than mess with our internal sane inode data, just fix it
• • up here in getattr() by masking off the format bits.
• */

+int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,

•        struct kstat *stat, u32 request_mask,
  

•        unsigned int query_flags)
  

+{

• struct inode *inode = d_inode(path->dentry);
• generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
• stat->mode &= ~S_IFMT;
• return 0;

+}

+static const struct inode_operations anon_inode_operations = {

• .getattr = anon_inode_getattr,

+};

/*

• anon_inodefs_dname() is called from d_path().

*/ @@ -66,6 +100,7 @@ static struct inode *anon_inode_make_secure_inode( if (IS_ERR(inode)) return inode; inode->i_flags &= ~S_PRIVATE;

• inode->i_op = &anon_inode_operations; error = security_inode_init_security_anon(inode, &QSTR(name), context_inode); if (error) {

@@ -313,6 +348,7 @@ static int __init anon_inode_init(void) anon_inode_inode = alloc_anon_inode(anon_inode_mnt->mnt_sb); if (IS_ERR(anon_inode_inode)) panic("anon_inode_init() inode allocation failed (%ld)\n", PTR_ERR(anon_inode_inode));

• anon_inode_inode->i_op = &anon_inode_operations;

return 0; } diff --git a/fs/internal.h b/fs/internal.h index b9b3e29a73fd..717dc9eb6185 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -343,3 +343,6 @@ static inline bool path_mounted(const struct path *path) void file_f_owner_release(struct file *file); bool file_seek_cur_needs_f_lock(struct file *file); int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map); +int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,

•        struct kstat *stat, u32 request_mask,
  

•        unsigned int query_flags);
  

diff --git a/fs/libfs.c b/fs/libfs.c index 6393d7c49ee6..0ad3336f5b49 100644 --- a/fs/libfs.c +++ b/fs/libfs.c @@ -1647,7 +1647,7 @@ struct inode *alloc_anon_inode(struct super_block *s) * that it already _is_ on the dirty list. */ inode->i_state = I_DIRTY;

• inode->i_mode = S_IRUSR | S_IWUSR;

• inode->i_mode = S_IFREG | S_IRUSR | S_IWUSR; inode->i_uid = current_fsuid(); inode->i_gid = current_fsgid(); inode->i_flags |= S_PRIVATE;

-- 2.47.2

On Mon 07-04-25 11:54:17, Christian Brauner wrote:

It is currently possible to change the mode and owner of the single anonymous inode in the kernel:

int main(int argc, char *argv[]) { int ret, sfd; sigset_t mask; struct signalfd_siginfo fdsi;

    sigemptyset(&mask);
    sigaddset(&mask, SIGINT);
    sigaddset(&mask, SIGQUIT);

    ret = sigprocmask(SIG_BLOCK, &mask, NULL);
    if (ret < 0)
            _exit(1);

    sfd = signalfd(-1, &mask, 0);
    if (sfd < 0)
            _exit(2);

    ret = fchown(sfd, 5555, 5555);
    if (ret < 0)
            _exit(3);

    ret = fchmod(sfd, 0777);
    if (ret < 0)
            _exit(3);

    _exit(4);

}

This is a bug. It's not really a meaningful one because anonymous inodes don't really figure into path lookup and they cannot be reopened via /proc/<pid>/fd/<nr> and can't be used for lookup itself. So they can only ever serve as direct references.

But it is still completely bogus to allow the mode and ownership or any of the properties of the anonymous inode to be changed. Block this!

Cc: stable@vger.kernel.org # all LTS kernels Signed-off-by: Christian Brauner brauner@kernel.org

Definitely. Feel free to add:

Reviewed-by: Jan Kara jack@suse.cz

Honza

---

fs/anon_inodes.c | 7 +++++++ fs/internal.h | 2 ++ 2 files changed, 9 insertions(+)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c index 42e4b9c34f89..cb51a90bece0 100644 --- a/fs/anon_inodes.c +++ b/fs/anon_inodes.c @@ -57,8 +57,15 @@ int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path, return 0; } +int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry,

•        struct iattr *attr)
  

+{

• return -EOPNOTSUPP;

+}

static const struct inode_operations anon_inode_operations = { .getattr = anon_inode_getattr,

• .setattr = anon_inode_setattr,

}; /* diff --git a/fs/internal.h b/fs/internal.h index 717dc9eb6185..f545400ce607 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -346,3 +346,5 @@ int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_ int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path, struct kstat *stat, u32 request_mask, unsigned int query_flags); +int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry,

•        struct iattr *attr);
  
  

-- 2.47.2

On Mon 07-04-25 11:54:19, Christian Brauner wrote:

It isn't possible to execute anoymous inodes because they cannot be

^^ anonymous

opened in any way after they have been created. This includes execution:

execveat(fd_anon_inode, "", NULL, NULL, AT_EMPTY_PATH)

Anonymous inodes have inode->f_op set to no_open_fops which sets no_open() which returns ENXIO. That means any call to do_dentry_open() which is the endpoint of the do_open_execat() will fail. There's no chance to execute an anonymous inode. Unless a given subsystem overrides it ofc.

Howerver, we should still harden this and raise SB_I_NODEV and

^^^ However

SB_I_NOEXEC on the superblock itself so that no one gets any creative ideas.

;)

Feel free to add:

Reviewed-by: Jan Kara jack@suse.cz

Honza

Cc: stable@vger.kernel.org # all LTS kernels Signed-off-by: Christian Brauner brauner@kernel.org

---

fs/anon_inodes.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c index cb51a90bece0..e51e7d88980a 100644 --- a/fs/anon_inodes.c +++ b/fs/anon_inodes.c @@ -86,6 +86,8 @@ static int anon_inodefs_init_fs_context(struct fs_context *fc) struct pseudo_fs_context *ctx = init_pseudo(fc, ANON_INODE_FS_MAGIC); if (!ctx) return -ENOMEM;

• fc->s_iflags |= SB_I_NOEXEC;
• fc->s_iflags |= SB_I_NODEV; ctx->dops = &anon_inodefs_dentry_operations; return 0;

}

-- 2.47.2