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CVE-2017-7889: mm: Tighten x86 /dev/mem with zeroing reads · torvalds/linux@a4866aa

The mm subsystem in the Linux kernel through 3.2 does not properly enforce the CONFIG_STRICT_DEVMEM protection mechanism, which allows local users to read or write to kernel memory locations in the first megabyte (and bypass slab-allocation access restrictions) via an application that opens the /dev/mem file, related to arch/x86/mm/init.c and drivers/char/mem.c.

7 years ago

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mm: Tighten x86 /dev/mem with zeroing reads

Under CONFIG_STRICT_DEVMEM, reading System RAM through /dev/mem is disallowed. However, on x86, the first 1MB was always allowed for BIOS and similar things, regardless of it actually being System RAM. It was possible for heap to end up getting allocated in low 1MB RAM, and then read by things like x86info or dd, which would trip hardened usercopy:

usercopy: kernel memory exposure attempt detected from ffff880000090000 (dma-kmalloc-256) (4096 bytes)

This changes the x86 exception for the low 1MB by reading back zeros for System RAM areas instead of blindly allowing them. More work is needed to extend this to mmap, but currently mmap doesn’t go through usercopy, so hardened usercopy won’t Oops the kernel.

Reported-by: Tommi Rantala [email protected] Tested-by: Tommi Rantala [email protected] Signed-off-by: Kees Cook [email protected]