Re: [PATCH v3a] vsprintf: Check real user/group id for %pK

[Ryan Mallon <rmallon@gmail.com>]

On 11/10/13 13:20, Eric W. Biederman wrote:
> Joe Perches <joe@perches.com> writes:
> 
>> Some setuid binaries will allow reading of files which have read
>> permission by the real user id. This is problematic with files which
>> use %pK because the file access permission is checked at open() time,
>> but the kptr_restrict setting is checked at read() time. If a setuid
>> binary opens a %pK file as an unprivileged user, and then elevates
>> permissions before reading the file, then kernel pointer values may be
>> leaked.
>>
>> This happens for example with the setuid pppd application on Ubuntu
>> 12.04:
>>
>>   $ head -1 /proc/kallsyms
>>   00000000 T startup_32
>>
>>   $ pppd file /proc/kallsyms
>>   pppd: In file /proc/kallsyms: unrecognized option 'c1000000'
>>
>> This will only leak the pointer value from the first line, but other
>> setuid binaries may leak more information.
>>
>> Fix this by adding a check that in addition to the current process
>> having CAP_SYSLOG, that effective user and group ids are equal to the
>> real ids. If a setuid binary reads the contents of a file which uses
>> %pK then the pointer values will be printed as NULL if the real user
>> is unprivileged.
>>
>> Update the sysctl documentation to reflect the changes, and also
>> correct the documentation to state the kptr_restrict=0 is the default.
> 
> Sigh.  This is all wrong.  The only correct thing to test is
> file->f_cred.  Aka the capabilities of the program that opened the
> file.
> 
> Which means that the interface to %pK in the case of kptr_restrict is
> broken as it has no way to be passed the information it needs to make
> a sensible decision.

Would it make sense to add a struct file * to struct seq_file and set
that in seq_open? Then the capability check can be done against seq->file.

~Ryan