Messages in this thread | | | Date | Fri, 10 Jan 2014 04:34:12 +0000 (UTC) | From | Mathieu Desnoyers <> | Subject | powerpc: possible access beyond TASK_SIZE in start_thread |
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Hi Rusty,
I was looking at the diff between kernel v3.12 and recent master (after 3.13-rc7), and noticed that in the following commit:
commit 94af3abf995b17f6a008b00152c94841242ec6c7 Author: Rusty Russell <rusty@rustcorp.com.au> Date: Wed Nov 20 22:15:02 2013 +1100
powerpc: ELF2 binaries launched directly.
on powerpc, those lines appear in start_thread():
+ /* start is a relocated pointer to the function + * descriptor for the elf _start routine. The first + * entry in the function descriptor is the entry + * address of _start and the second entry is the TOC + * value we need to use. + */ + __get_user(entry, (unsigned long __user *)start); + __get_user(toc, (unsigned long __user *)start+1);
Note the "__" before get_user(), which bypass any kind of validation on the addresses.
Amongst the callers, if we look at fs/binfmt_elf.c:load_elf_binary(), we see:
elf_entry = loc->elf_ex.e_entry; if (BAD_ADDR(elf_entry)) { force_sig(SIGSEGV, current); retval = -EINVAL; goto out_free_dentry; }
and the elf_entry gets passed to start_thread().
If we craft a binary with elf_entry address of
TASK_SIZE - 1 (1 byte before TASK_SIZE), then I think we could make both __get_user() calls access data beyond TASK_SIZE, because elf_entry address is verified, but there is no validation on its range AFAIU. Is it expected ? Am I missing something ?
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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