Re: [RFC][PATCH 0/3] arm64 relaxed ABI

[Szabolcs Nagy <Szabolcs.Nagy@arm.com>]

On 25/02/2019 16:57, Catalin Marinas wrote:
> On Tue, Feb 19, 2019 at 06:38:31PM +0000, Szabolcs Nagy wrote:
>> i think these rules work for the cases i care about, a more
>> tricky question is when/how to check for the new syscall abi
>> and when/how the TCR_EL1.TBI0 setting may be turned off.
> 
> I don't think turning TBI0 off is critical (it's handy for PAC with
> 52-bit VA but then it's short-lived if you want more security features
> like MTE).

yes, i made a mistake assuming TBI0 off is
required for (or at least compatible with) MTE.

if TBI0 needs to be on for MTE then some of my
analysis is wrong, and i expect TBI0 to be on
in the foreseeable future.

>> consider the following cases (tb == top byte):
>>
>> binary 1: user tb = any, syscall tb = 0
>>   tbi is on, "legacy binary"
>>
>> binary 2: user tb = any, syscall tb = any
>>   tbi is on, "new binary using tb"
>>   for backward compat it needs to check for new syscall abi.
>>
>> binary 3: user tb = 0, syscall tb = 0
>>   tbi can be off, "new binary",
>>   binary is marked to indicate unused tb,
>>   kernel may turn tbi off: additional pac bits.
>>
>> binary 4: user tb = mte, syscall tb = mte
>>   like binary 3, but with mte, "new binary using mte"

so this should be "like binary 2, but with mte".

>>   does it have to check for new syscall abi?
>>   or MTE HWCAP would imply it?
>>   (is it possible to use mte without new syscall abi?)
> 
> I think MTE HWCAP should imply it.
> 
>> in userspace we want most binaries to be like binary 3 and 4
>> eventually, i.e. marked as not-relying-on-tbi, if a dso is
>> loaded that is unmarked (legacy or new tb user), then either
>> the load fails (e.g. if mte is already used? or can we turn
>> mte off at runtime?) or tbi has to be enabled (prctl? does
>> this work with pac? or multi-threads?).
> 
> We could enable it via prctl. That's the plan for MTE as well (in
> addition maybe to some ELF flag).
> 
>> as for checking the new syscall abi: i don't see much semantic
>> difference between AT_HWCAP and AT_FLAGS (either way, the user
>> has to check a feature flag before using the feature of the
>> underlying system and it does not matter much if it's a syscall
>> abi feature or cpu feature), but i don't see anything wrong
>> with AT_FLAGS if the kernel prefers that.
> 
> The AT_FLAGS is aimed at capturing binary 2 case above, i.e. the
> relaxation of the syscall ABI to accept tb = any. The MTE support will
> have its own AT_HWCAP, likely in addition to AT_FLAGS. Arguably,
> AT_FLAGS is either redundant here if MTE implies it (and no harm in
> keeping it around) or the meaning is different: a tb != 0 may be checked
> by the kernel against the allocation tag (i.e. get_user() could fail,
> the tag is not entirely ignored).
> 
>> the discussion here was mostly about binary 2,
> 
> That's because passing tb != 0 into the syscall ABI is the main blocker
> here that needs clearing out before merging the MTE support. There is,
> of course, a variation of binary 1 for MTE:
> 
> binary 5: user tb = mte, syscall tb = 0
> 
> but this requires a lot of C lib changes to support properly.

yes, i don't think we want to do that.

but it's ok to have both syscall tbi AT_FLAGS and MTE HWCAP.

>> but for
>> me the open question is if we can make binary 3/4 work.
>> (which requires some elf binary marking, that is recognised
>> by the kernel and dynamic loader, and efficient handling of
>> the TBI0 bit, ..if it's not possible, then i don't see how
>> mte will be deployed).
> 
> If we ignore binary 3, we can keep TBI0 = 1 permanently, whether we have
> MTE or not.
> 
>> and i guess on the kernel side the open question is if the
>> rules 1/2/3/4 can be made to work in corner cases e.g. when
>> pointers embedded into structs are passed down in ioctl.
> 
> We've been trying to track these down since last summer and we came to
> the conclusion that it should be (mostly) fine for the non-weird memory
> described above.

i think an interesting case is when userspace passes
a pointer to the kernel and later gets it back,
which is why i proposed rule 4 (kernel has to keep
the tag then).

but i wonder what's the right thing to do for sp
(user can malloc thread/sigalt/makecontext stack
which will be mte tagged in practice with mte on)
does tagged sp work? should userspace untag the
stack memory before setting it up as a stack?
(but then user pointers to that allocation may get
broken..)
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