Messages in this thread Patch in this message | | | From | Rusty Russell <> | Subject | Re: powerpc: possible access beyond TASK_SIZE in start_thread | Date | Wed, 15 Jan 2014 15:26:00 +1030 |
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Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes: > 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:
Cool!
I just moved the code, so any problem exists before this, too.
> 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 ?
Yes, looks like we can read the first 15 bytes of kernel space. That's not likely to be interesting, but we should probably check anyway.
Thanks! Rusty. === Subject: powerpc: don't allow entry point to be read from kernel.
Mathieu points out that while start is checked, it could be TASK_SIZE-1, which means this code could read the first 15 bytes of kernel space. That's text for now, but let's be paranoid.
start_thread() doesn't have a return value, so easiest to use get_user: you'll get a jump to zero if it fails.
Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c index 4a96556fd2d4..b4ca298a5536 100644 --- a/arch/powerpc/kernel/process.c +++ b/arch/powerpc/kernel/process.c @@ -1113,8 +1113,8 @@ void start_thread(struct pt_regs *regs, unsigned long start, unsigned long sp) * 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); + get_user(entry, (unsigned long __user *)start); + get_user(toc, (unsigned long __user *)start+1); /* Check whether the e_entry function descriptor entries * need to be relocated before we can use them.
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