Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: access_ok macor | Date | Wed, 15 Jul 2009 12:14:52 +0200 |
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On Wednesday 15 July 2009, John Williams wrote: > On Wed, Jul 15, 2009 at 2:43 AM, Arnd Bergmann <arnd@arndb.de> wrote: > > The solution then is to handle fixups from the unaligned exception handler > > if you come from the kernel. That should fix the three text cases. > > > > I don't fully understand your exception handling there, but I think you > > also need to add code checking for __range_ok() to your unaligned handler, > > to prevent malicious user space code from accessing the kernel through > > unaligned pointers. > > > Just to try to clarify - are there any alignment rules in the ABI on > user-space pointers (which end up going to get/put_user)?
The kernel normally expects aligned input from user space, but I guess it can't hurt to handle it anyway. arch/mips/kernel/alignment.c seems to handle that case. Maybe Ralf can give some more insight.
> It seems the failure path is like this: > > 1. userspace passes unaligned pointer > 2. get_user attempts to access > 3. CPU raises unaligned exception (if only it would raise the segfault as > higher priority, before the unaligned!) > 4. unaligned exception handler attempts to simulate the unaligned access > with multiple partial read/write ops > 5. CPU raises MMU exception on the read/write by the unaligned handler > 6. kernel segfault handler looks up faulting address, it is in the unaligned > exception handler, which has no fixup. > 7. no fixup -> failure
Right.
> So, I suppose the question is - where in the sequence is the true failure?
I think in step 4. AFIACT, the kernel must do a number of checks on accesses to random pointers.
> Clearly LTP thinks it's ok to pass unaligned pointers to the kernel, > suggesting (1) is fine - thus my question about alignment rules in the ABI.
No, LTP thinks it should get a -EFAULT error code for that access. It does specify whether it expects this because of an unaligned address or because of an invalid page.
> Do we need fixups on the unaligned handler itself? This will be ugly ugly > ugly.
That's what ARM does. You don't have to do it from assembly though, implementing it in C is probably easier.
> Or, some way of tracing the segfault back through the unaligned > exception and to the root cause (the get/put-user), and call that fixup as > required?
Yes, I guess that would have to look roughly like this:
int emulate_insn(struct pt_regs *regs, unsigned long addr, unsigned long len) { /* use inline assembly with fixups here, return -EFAULT on bad addr */ }
void alignment_exception(struct pt_regs *regs, unsigned long addr, unsigned long len) { const struct exception_table_entry *fixup; int err;
if (user_mode(regs)) { if (!access_ok(addr, len)) goto segv; if (emulate_insn(regs) == -EFAULT)) goto segv; } else { if (!access_ok(addr, len)) goto fixup; if (emulate_insn(regs, addr, len) == -EFAULT)) goto fixup; return;
fixup: fixup = search_exception_tables(regs->ip); if (!fixup) goto segv;
regs->ip = fixup->fixup; return;
segv: force_sig(SIGSEGV, current)); }
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