Messages in this thread | | | From | Jeff Xu <> | Date | Thu, 25 Apr 2024 17:12:07 -0700 | Subject | Re: [RFC PATCH v2 1/1] x86/pkeys: update PKRU to enable pkey 0 before XSAVE |
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On Thu, Apr 25, 2024 at 3:49 PM Aruna Ramakrishna <aruna.ramakrishna@oracle.com> wrote: > > > > > On Apr 25, 2024, at 2:05 PM, jeffxu@chromium.org wrote: > > > > From: Jeff Xu <jeffxu@chromium.org> > > > > On 3/21/24 14:56, Aruna Ramakrishna wrote: > >> Enabling both the non-zero pkey (for the thread) and pkey zero (in > >> userspace) will not work for us. We cannot have the alt stack writeable > >> by all - the rationale here is that the code running in that thread > >> (using a non-zero pkey) is untrusted and should not have access to the > >> alternate signal stack (that uses pkey zero), to prevent the return > >> address of a function from being changed. The expectation is that kernel > >> should be able to set up the alternate signal stack and deliver the > >> signal to the application even if pkey zero is explicitly disabled by > >> the application. The signal handler accessibility should not be dictated > >> by the PKRU value that the thread sets up. > >> > > We have a similar threat model that we don't want "untrusted threads" to > > access altstack. I think this patch need not be restricted to the > > use case of zero pkey for altstack, i.e. application can also set > > non-zero pkey to altstack and expect the same. > > Agreed. In the latest version of this patchset, this assumption has been removed. > > Link here: > https://lore.kernel.org/lkml/20240425180542.1042933-1-aruna.ramakrishna@oracle.com/T/#t > > > > >> Solution: > >> The PKRU register is managed by XSAVE, which means the sigframe contents > >> must match the register contents - which is not the case here. We want > >> the sigframe to contain the user-defined PKRU value (so that it is > >> restored correctly from sigcontext) but the actual register must be > >> reset to init_pkru so that the alt stack is accessible and the signal > >> can be delivered to the application. It seems that the proper fix here > >> would be to remove PKRU from the XSAVE framework and manage it > >> separately, which is quite complicated. As a workaround, this patch does > >> something like this: > >> > >> orig_pkru = rdpkru(); > >> wrpkru(init_pkru & orig_pkru); > >> xsave_to_user_sigframe(); > >> put_user(pkru_sigframe_addr, orig_pkru) > >> > > The default PKRU of thread [1] is set as 01 (disable access) for each PKEY > > from 1 to 15, and 00 (RW) for PKEY 0. > > > > Let's use pkey 1 as an example: > > The init_pkru is 01, if the thread has PKRU (orig_pkru) as 10 (disable write > > but have read) then new_pkru from (init_pkru & orig_pkru) is 00, which gives > > RW access to the pkey 1. > > > > When the thread has orig_pkru as 01 (disable access) or 00 (RW), new_pkru is > > unchanged from orig_pkru. > > > > Now take pkey 0: > > the init_pkru is 00, regardless what threads has, new_pkru will always be 00. > > > > This seems to work out well for pkey 1 to 15, i.e. signal handing code in > > kernel only give write access when the thread alrady has read access to the > > PKEY that is used by the altstack. The threat model interesting here is to > > prevent untrusted threads from writing to altstack, and read is probably less > > of a problem. > > > > This piece of code assumed that the init PKRU value allows writes to the alternative > signal stack. As you mentioned earlier, that may not always be true - a non-zero pkey > can be used for the altstack. > Only PKEY 0 has init PKRU as 00. So in your case, the thread doesn't have write access to pkey 0, and need the write access to pkey 0 during signal handling.
> So the new version simply does write_pkru(0) (i.e. enabled all pkeys) before XSAVE. > Is this more reasonable? > write_pkru(0) will work, but it is unnecessary in the current patch.
Consider the following case: A thread has no access to pkey 1, and use pkey 1 for its altstack.
In V3 (use write_pkru(0): Signal will be dispatched to the user, i.e. write to signal frame is OK, but it will SEGV at sigreturn.
In V2: it will SEGV earlier at dispatch stage when writing to sigframe.
I would rather that the code fails earlier for this case.
> > > > Does this meet what you want? (Note the pkey 0 is different than 1-15) > > > > Suppose someone also like to disable all access to altstack, then there is one > > more place to mind: in sigreturn(), it calls restore_altstack(), and requires > > read access to altstack. However, at the time, PKRU is already restored from > > sigframe, so SEGV will raise (the value in sigframe doesn't have read access > > to the PKEY). > > > > Without changing sigreturn, using wrpkru(0) here might not be necessary: > > the dispatch to user space works fine, only to crash at sigreturn step. > > > > [1] defined by init_pkru_value in pkeys.c > > > > Best regards, > > -Jeff > > > I see what you're saying. In rt_sigreturn(): > > if (!restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags)) <— restores PKRU, disabling access to altstack > goto badframe; > ... > if (restore_altstack(&frame->uc.uc_stack)) <— needs read access to altstack > goto badframe; > > > I’m wary about reordering anything in here. Also, this code is not aware of the altstack permissions. I’m wondering if wrpkru(0) is needed here too. > We can't change PKRU after restore_sigcontext, the calling thread would have PKRU 0, not the original PKRU from before handling the signal.
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