Date: Thu, 6 Jun 2013 13:59:55 -0700 From: Kees Cook <keescook@...omium.org> To: oss-security@...ts.openwall.com Subject: Linux kernel format string flaws I've found two issues in how Linux uses format strings: CVE-2013-2852: b43 wireless driver The b43 driver reports error strings that can be interpreted as format strings. Under normal conditions, this is not a problem, but it is possible for the "fwpostfix" module parameter to change the filenames used to fetch firmware. When such a file is not found, the filename will be processed as a format string. This flaw could potentially allow escalation from uid-0 to ring-0, so except for certain environments, it is not too serious. If b43 hardware is available, this should show itself easily. I don't have any available for testing, but it seems it would show itself like this: # rmmod b43 # modprobe b43 fwpostfix=AA%xBB ... # dmesg ... b43-0 ERROR: Firmware file "b43AAdeff80ccBB/a0g1bsinitvals5.fw" not found Using %n instead of %x would lead to exciting crashes. :) It has been fixed in the upstream wireless tree: http://git.kernel.org/cgit/linux/kernel/git/linville/wireless.git/commit/?id=9538cbaab6e8b8046039b4b2eb6c9d614dc782bd CVE-2013-2851: block layer The block layer uses the "disk_name" field as a format string in a number of places. While this is normally not a problem due to how disk names are created (statically or incrementally), there is currently at least one way to define nearly arbitrary names via md. Instead of filtering md, this should be fixed within the kernel's interfaces. This flaw could potentially allow escalation from uid-0 to ring-0, so except for certain environments, it is not too serious. The test case is trivial: # echo md_%x.%x.%x.%x > /sys/module/md_mod/parameters/new_array # ls /dev/md_* /dev/md_c12cc370.df66d800.df66d80c.c13da45b Using %n instead of %x leads to exciting crashes. :) The fix has been sent upstream: http://marc.info/?l=linux-kernel&m=137055204522556&w=2 With the above fixes, a series of additional format string related clean ups has also been sent upstream: http://marc.info/?l=linux-kernel&m=137055207522563&w=2 Thanks, -Kees -- Kees Cook Chrome OS Security
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