linux-kernel - Re: [PATCH] vfs: introduce helper function to safely NUL-terminate symlinks

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Message-ID: <e9e943910812161626i6f2cdc3eldc36c68e9a98f180@mail.gmail.com>
Date:	Wed, 17 Dec 2008 00:26:57 +0000
From:	"Duane Griffin" <duaneg@...da.com>
To:	"Andreas Dilger" <adilger@....com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	"Al Viro" <viro@...iv.linux.org.uk>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] vfs: introduce helper function to safely NUL-terminate symlinks

2008/12/16 Andreas Dilger <adilger@....com>:
> 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
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