On 25/02/2019 18:02, Szabolcs Nagy wrote:
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

Tagged SP does work, and it is actually a good idea (it avoids using the default tag for the stack). It would be quite easy for the kernel to tag the initial SP and the stack on execve(). For other stacks, it is up to userspace, as you say, and would be made easier by making it possible to choose how a mapping should be tagged by the kernel via a new mmap() flag. Some software that makes too many assumptions on the address of stack variables will be disturbed by a tagged SP, but this should be fairly rare.

In any case, I don't think this impacts this ABI proposal (beyond the fact that passing tagged pointers to the stack needs to be allowed).


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