Messages in this thread | | | From | Aruna Ramakrishna <> | Subject | Re: [RFC PATCH v2 1/1] x86/pkeys: update PKRU to enable pkey 0 before XSAVE | Date | Thu, 25 Apr 2024 22:49:29 +0000 |
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> 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.
So the new version simply does write_pkru(0) (i.e. enabled all pkeys) before XSAVE. Is this more reasonable?
> > 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.
Thanks, Aruna
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