Messages in this thread | | | Date | Wed, 15 Jul 2009 13:39:05 +0200 | From | Michal Simek <> | Subject | Re: access_ok macor |
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Arnd Bergmann wrote: > 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.
you meant unaligned.c.
> >> 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.
IMHO author of this test not expect that caused too much troubles. From that tests EFAULT should be return from copy_to_user macro not caused kernel fault. LTP should contain special testcases for testing unaligned address. I think we should add one more test with invalid aligned argument for that 3 tests + some doc. I'll send it.
M
> >> 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)); > }
-- Michal Simek, Ing. (M.Eng) w: www.monstr.eu p: +42-0-721842854
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