From: "Duane Griffin" Subject: Re: [PATCH] vfs: introduce helper function to safely NUL-terminate symlinks Date: Wed, 17 Dec 2008 00:26:57 +0000 Message-ID: References: <1229442720-31056-3-git-send-email-duaneg@dghda.com> <1229442720-31056-4-git-send-email-duaneg@dghda.com> <1229442720-31056-5-git-send-email-duaneg@dghda.com> <1229442720-31056-1-git-send-email-duaneg@dghda.com> <1229442720-31056-6-git-send-email-duaneg@dghda.com> <1229442720-31056-2-git-send-email-duaneg@dghda.com> <20081216234638.GX5000@webber.adilger.int> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Al Viro" , "Andrew Morton" , "Theodore Ts'o" , linux-ext4@vger.kernel.org To: "Andreas Dilger" Return-path: In-Reply-To: <20081216234638.GX5000@webber.adilger.int> Content-Disposition: inline Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 2008/12/16 Andreas Dilger : > On Dec 16, 2008 15:51 +0000, Duane Griffin wrote: >> A number of filesystems were potentially triggering kernel bugs due to >> corrupted symlink names on disk. This helper helps safely terminate the >> names. >> >> +static inline void nd_terminate_link(void *name,unsigned len,unsigned maxlen) >> +{ >> + ((char *) name)[min(len, maxlen)] = '\0'; >> +} > >> @@ -4200,9 +4201,11 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino) >> } else if (S_ISLNK(inode->i_mode)) { >> + if (ext4_inode_is_fast_symlink(inode)) { >> inode->i_op = &ext4_fast_symlink_inode_operations; >> + nd_terminate_link(ei->i_data, inode->i_size, >> + sizeof(ei->i_data)); >> + } else { >> inode->i_op = &ext4_symlink_inode_operations; >> ext4_set_aops(inode); >> } > > With sizeof(ei->i_data) = 15 * 4 = 60 bytes, this will set ei->i_data[60] > as NUL, which is writing 1 byte beyond the end of the array. Argh, you are correct, of course. > Note that in ext[234]_symlink() the check for fast symlinks is: > > l = strlen(symname)+1; > if (l > sizeof (EXT3_I(inode)->i_data)) { > inode->i_op = &ext3_symlink_inode_operations; > } else { > inode->i_op = &ext3_fast_symlink_inode_operations; > inode->i_size = l-1; > } > > so in fact the fast symlinks should always have space for a trailing NUL > character, and "sizeof(ei->i_data) - 1" is the right maxlen to use for > ext[234]. > > That might not be true for other filesystems, in which case you would > need to add a "padding" field after the symlink name in memory to hold > the trailing NUL. It is true for sysv, too. The ufs code also, but that has other issues, anyway. The generic page symlink and freevxfs patches should be fine, though. Cheers, Duane. -- "I never could learn to drink that blood and call it wine" - Bob Dylan