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authorSalvatore Mesoraca <s.mesoraca16@gmail.com>2018-08-23 17:00:35 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2018-08-23 18:48:43 -0700
commit30aba6656f61ed44cba445a3c0d38b296fa9e8f5 (patch)
tree6c10f9e466500d7ede78400bb7b58896dd66016a /fs/namei.c
parentdc2572791d3a41bab94400af2b6bca9d71ccd303 (diff)
downloadlinux-stericsson-30aba6656f61ed44cba445a3c0d38b296fa9e8f5.tar.gz
namei: allow restricted O_CREAT of FIFOs and regular files
Disallows open of FIFOs or regular files not owned by the user in world writable sticky directories, unless the owner is the same as that of the directory or the file is opened without the O_CREAT flag. The purpose is to make data spoofing attacks harder. This protection can be turned on and off separately for FIFOs and regular files via sysctl, just like the symlinks/hardlinks protection. This patch is based on Openwall's "HARDEN_FIFO" feature by Solar Designer. This is a brief list of old vulnerabilities that could have been prevented by this feature, some of them even allow for privilege escalation: CVE-2000-1134 CVE-2007-3852 CVE-2008-0525 CVE-2009-0416 CVE-2011-4834 CVE-2015-1838 CVE-2015-7442 CVE-2016-7489 This list is not meant to be complete. It's difficult to track down all vulnerabilities of this kind because they were often reported without any mention of this particular attack vector. In fact, before hardlinks/symlinks restrictions, fifos/regular files weren't the favorite vehicle to exploit them. [s.mesoraca16@gmail.com: fix bug reported by Dan Carpenter] Link: https://lkml.kernel.org/r/20180426081456.GA7060@mwanda Link: http://lkml.kernel.org/r/1524829819-11275-1-git-send-email-s.mesoraca16@gmail.com [keescook@chromium.org: drop pr_warn_ratelimited() in favor of audit changes in the future] [keescook@chromium.org: adjust commit subjet] Link: http://lkml.kernel.org/r/20180416175918.GA13494@beast Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com> Signed-off-by: Kees Cook <keescook@chromium.org> Suggested-by: Solar Designer <solar@openwall.com> Suggested-by: Kees Cook <keescook@chromium.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Dan Carpenter <dan.carpenter@oracle.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs/namei.c')
-rw-r--r--fs/namei.c53
1 files changed, 50 insertions, 3 deletions
diff --git a/fs/namei.c b/fs/namei.c
index ae6aa9ae757c..0cab6494978c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -887,6 +887,8 @@ static inline void put_link(struct nameidata *nd)
int sysctl_protected_symlinks __read_mostly = 0;
int sysctl_protected_hardlinks __read_mostly = 0;
+int sysctl_protected_fifos __read_mostly;
+int sysctl_protected_regular __read_mostly;
/**
* may_follow_link - Check symlink following for unsafe situations
@@ -1003,6 +1005,45 @@ static int may_linkat(struct path *link)
return -EPERM;
}
+/**
+ * may_create_in_sticky - Check whether an O_CREAT open in a sticky directory
+ * should be allowed, or not, on files that already
+ * exist.
+ * @dir: the sticky parent directory
+ * @inode: the inode of the file to open
+ *
+ * Block an O_CREAT open of a FIFO (or a regular file) when:
+ * - sysctl_protected_fifos (or sysctl_protected_regular) is enabled
+ * - the file already exists
+ * - we are in a sticky directory
+ * - we don't own the file
+ * - the owner of the directory doesn't own the file
+ * - the directory is world writable
+ * If the sysctl_protected_fifos (or sysctl_protected_regular) is set to 2
+ * the directory doesn't have to be world writable: being group writable will
+ * be enough.
+ *
+ * Returns 0 if the open is allowed, -ve on error.
+ */
+static int may_create_in_sticky(struct dentry * const dir,
+ struct inode * const inode)
+{
+ if ((!sysctl_protected_fifos && S_ISFIFO(inode->i_mode)) ||
+ (!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
+ likely(!(dir->d_inode->i_mode & S_ISVTX)) ||
+ uid_eq(inode->i_uid, dir->d_inode->i_uid) ||
+ uid_eq(current_fsuid(), inode->i_uid))
+ return 0;
+
+ if (likely(dir->d_inode->i_mode & 0002) ||
+ (dir->d_inode->i_mode & 0020 &&
+ ((sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) ||
+ (sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode))))) {
+ return -EACCES;
+ }
+ return 0;
+}
+
static __always_inline
const char *get_link(struct nameidata *nd)
{
@@ -3348,9 +3389,15 @@ finish_open:
if (error)
return error;
audit_inode(nd->name, nd->path.dentry, 0);
- error = -EISDIR;
- if ((open_flag & O_CREAT) && d_is_dir(nd->path.dentry))
- goto out;
+ if (open_flag & O_CREAT) {
+ error = -EISDIR;
+ if (d_is_dir(nd->path.dentry))
+ goto out;
+ error = may_create_in_sticky(dir,
+ d_backing_inode(nd->path.dentry));
+ if (unlikely(error))
+ goto out;
+ }
error = -ENOTDIR;
if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
goto out;