linux-kernel - Re: [PATCH] vfs: reorder checks in may_create_in

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Date: Fri, 21 Jun 2024 09:45:03 +0200
From: Christian Brauner <brauner@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: viro@...iv.linux.org.uk, jack@...e.cz, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: reorder checks in may_create_in_sticky

On Thu, Jun 20, 2024 at 02:03:59PM GMT, Mateusz Guzik wrote:
> The routine is called for all directories on file creation and weirdly
> postpones the check if the dir is sticky to begin with. Instead it first
> checks fifos and regular files (in that order), while avoidably pulling
> globals.
> 
> No functional changes.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> ---
>  fs/namei.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 63d1fb06da6b..b1600060ecfb 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1246,9 +1246,9 @@ static int may_create_in_sticky(struct mnt_idmap *idmap,
>  	umode_t dir_mode = nd->dir_mode;
>  	vfsuid_t dir_vfsuid = nd->dir_vfsuid;
>  
> -	if ((!sysctl_protected_fifos && S_ISFIFO(inode->i_mode)) ||
> -	    (!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
> -	    likely(!(dir_mode & S_ISVTX)) ||
> +	if (likely(!(dir_mode & S_ISVTX)) ||
> +	    (S_ISREG(inode->i_mode) && !sysctl_protected_regular) ||
> +	    (S_ISFIFO(inode->i_mode) && !sysctl_protected_fifos) ||
>  	    vfsuid_eq(i_uid_into_vfsuid(idmap, inode), dir_vfsuid) ||
>  	    vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, inode), current_fsuid()))
>  		return 0;

I think we really need to unroll this unoly mess to make it more readable?

diff --git a/fs/namei.c b/fs/namei.c
index 3e23fbb8b029..1dd2d328bae3 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1244,25 +1244,43 @@ static int may_create_in_sticky(struct mnt_idmap *idmap,
                                struct nameidata *nd, struct inode *const inode)
 {
        umode_t dir_mode = nd->dir_mode;
-       vfsuid_t dir_vfsuid = nd->dir_vfsuid;
+       vfsuid_t dir_vfsuid = nd->dir_vfsuid, i_vfsuid;
+       int ret;
+
+       if (likely(!(dir_mode & S_ISVTX)))
+               return 0;
+
+       if (S_ISREG(inode->i_mode) && !sysctl_protected_regular)
+               return 0;
+
+       if (S_ISFIFO(inode->i_mode) && !sysctl_protected_fifos)
+               return 0;
+
+       i_vfsuid = i_uid_into_vfsuid(idmap, inode);
+
+       if (vfsuid_eq(i_vfsuid, dir_vfsuid))
+               return 0;

-       if (likely(!(dir_mode & S_ISVTX)) ||
-           (S_ISREG(inode->i_mode) && !sysctl_protected_regular) ||
-           (S_ISFIFO(inode->i_mode) && !sysctl_protected_fifos) ||
-           vfsuid_eq(i_uid_into_vfsuid(idmap, inode), dir_vfsuid) ||
-           vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, inode), current_fsuid()))
+       if (vfsuid_eq_kuid(i_vfsuid, current_fsuid()))
                return 0;

-       if (likely(dir_mode & 0002) ||
-           (dir_mode & 0020 &&
-            ((sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) ||
-             (sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode))))) {
-               const char *operation = S_ISFIFO(inode->i_mode) ?
-                                       "sticky_create_fifo" :
-                                       "sticky_create_regular";
-               audit_log_path_denied(AUDIT_ANOM_CREAT, operation);
+       if (likely(dir_mode & 0002)) {
+               audit_log_path_denied(AUDIT_ANOM_CREAT, "sticky_create");
                return -EACCES;
        }
+
+       if (dir_mode & 0020) {
+               if (sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) {
+                       audit_log_path_denied(AUDIT_ANOM_CREAT, "sticky_create_fifo");
+                       return -EACCES;
+               }
+
+               if (sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode)) {
+                       audit_log_path_denied(AUDIT_ANOM_CREAT, "sticky_create_regular");
+                       return -EACCES;
+               }
+       }
+
        return 0;
 }

That gives us:

static int may_create_in_sticky(struct mnt_idmap *idmap,
				struct nameidata *nd, struct inode *const inode)
{
	umode_t dir_mode = nd->dir_mode;
	vfsuid_t dir_vfsuid = nd->dir_vfsuid, i_vfsuid;
	int ret;

	if (likely(!(dir_mode & S_ISVTX)))
		return 0;

	if (S_ISREG(inode->i_mode) && !sysctl_protected_regular)
		return 0;

	if (S_ISFIFO(inode->i_mode) && !sysctl_protected_fifos)
		return 0;

	i_vfsuid = i_uid_into_vfsuid(idmap, inode);

	if (vfsuid_eq(i_vfsuid, dir_vfsuid))
		return 0;

	if (vfsuid_eq_kuid(i_vfsuid, current_fsuid()))
		return 0;

	if (likely(dir_mode & 0002)) {
		audit_log_path_denied(AUDIT_ANOM_CREAT, "sticky_create");
		return -EACCES;
	}

	if (dir_mode & 0020) {
		if (sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) {
			audit_log_path_denied(AUDIT_ANOM_CREAT, "sticky_create_fifo");
			return -EACCES;
		}

		if (sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode)) {
			audit_log_path_denied(AUDIT_ANOM_CREAT, "sticky_create_regular");
			return -EACCES;
		}
	}

	return 0;
}