The cache files are insufficiently validated. Even though the magic
number at the beginning of the file as well as time stamps are checked,
it is not verified if contained offsets are in legal ranges or are
even pointers.
The lack of validation allows an attacker to trigger arbitrary free()
calls, which in turn allows double free attacks and therefore arbitrary
code execution. Due to the conversion from offsets into pointers through
macros, this even allows to circumvent ASLR protections.
This attack vector allows privilege escalation when used with setuid
binaries like fbterm. A user can create ~/.fonts or any other
system-defined user-private font directory, run fc-cache and adjust
cache files in ~/.cache/fontconfig. The execution of setuid binaries will
scan these files and therefore are prone to attacks.
If it's not about code execution, an endless loop can be created by
letting linked lists become circular linked lists.
This patch verifies that:
- The file is not larger than the maximum addressable space, which
basically only affects 32 bit systems. This allows out of boundary
access into unallocated memory.
- Offsets are always positive or zero
- Offsets do not point outside file boundaries
- No pointers are allowed in cache files, every "pointer or offset"
field must be an offset or NULL
- Iterating linked lists must not take longer than the amount of elements
specified. A violation of this rule can break a possible endless loop.
If one or more of these points are violated, the cache is recreated.
This is current behaviour.
Even though this patch fixes many issues, the use of mmap() shall be
forbidden in setuid binaries. It is impossible to guarantee with these
checks that a malicious user does not change cache files after
verification. This should be handled in a different patch.
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
For Serif:
Cambria, Constantia, Elephant, MS Serif
For Sans Serif:
Arial Unicode MS, Britannic, Calibri, Candara, Century Gothic, Corbel,
Haettenschweiler, MS Sans Serif, Tahoma, Twentieth Century
For Monospace:
Consolas, Fixedsys, Terminal
Take a look at the nano second in the mtime to figure out
if the cache needs to be updated if available.
and do the mutex lock between scanning and writing a cache
to avoid the conflict.
Also we don't need to scan directories again after writing
caches. so getting rid of the related code as well.
https://bugs.freedesktop.org/show_bug.cgi?id=69845
and for reference:
https://bugzilla.redhat.com/show_bug.cgi?id=1236034
In 32ac7c75e8 the behavior of
FcConfigAppFontAddFile/Dir() were changed to return false
if not fonts were found. While this is welldefined and useful
for AddFile(), it's quite problematic for AddDir(). For example,
if the directory is empty, is that a failure or success? Worse,
the false value from AddDir() was being propagated all the way
to FcInit() returning false now. This only happened upon memory
allocation failure before, and some clients assert that FcInit()
is successful.
With this change, AddDir() is reverted back to what it was.
AddFont() change (which was actually in fcdir.c) from the original
commit is left in.
Masamichi Hosoda
Akira TAGOH
Alan Coopersmith
Tobias Stoeckmann
Erik de Castro Lopo
Petr Filipsky
Behdad Esfahbod
Patrick Haller