Kernel space: Details of the vmsplice() exploit

A recent Linux security hole allows local users to seize the power of root. We show the Linux bugs that came together to let it happen

Both of these arrays are declared, one right after the other, in vmsplice_to_page(). A quick test suggests that the partial array will be placed below pages in memory, so, once partial is overflowed, the loop will start overwriting pages instead. So the pages array will end up containing alternating values of zero and 4096 rather than the real struct page pointers it had before. (It's worth noting that the exploit still works if the arrays are placed in the opposite order, since the overflow causes code down the line to think that pages is larger than it really is).

Once all this has happened, control returns to vmsplice_to_pipe() - the overflow is not big enough to have overwritten the return address. A call to splice_to_pipe() is supposed to finish the job, but something interesting happens there. Toward the beginning of this function, this test is made:

if (!pipe->readers) { send_sig(SIGPIPE, current, 0); if (!ret) ret = -EPIPE; break; }

Looking back at the exploit code, we see that it closes the read side of the pipe before calling vmsplice(). So splice_to_pipe() will quit almost immediately. On its way out, however, it does this:

while (page_nr < spd_pages) page_cache_release(spd->pages[page_nr++]);

The call to get_user_pages() will have locked each of the relevant pages into memory to allow the kernel to work with them; this is the cleanup code which goes back and unlocks the pages which will not be used. But remember that the pointers in the pages array have been overwritten, and are now either zero or 4096. What would normally happen here is a kernel oops, since those are not legitimate addresses. The exploit code has done something tricky, though: using some special mmap() calls, it has created some anonymous memory at the bottom of its address space.

Directly dereferencing user-space addresses while running in kernel mode is frowned upon for a number of reasons; it can blow up in a number of ways. But, if the address is valid and the relevant page is resident in memory, direct access to user-space memory will work. So, when the kernel starts to work with the addresses that it thinks are struct page pointers, it does not get any sort of fault; instead, it gets the data placed in that memory by the exploit. Needless to say, that data has been arranged carefully.

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Jonathan Corbet

LinuxWorld
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