[llvm-dev] [MTE] Globals Tagging - Discussion

[David Spickett via llvm-dev]

Hi Mitch,

In the intro you say:
> It would also allow attackers with a semilinear RW primitive to trivially attack global variables if the offset is controllable. Dynamic global tags are required to provide the same MTE mitigation guarantees that are afforded to stack and heap memory.

Then later:
> b) Hidden Symbols (static int g; or -fvisibility=hidden)
> Materialization of hidden symbols now fetch and insert the memory tag via. `ldg`. On aarch64, this means non PC-relative loads/stores/address-taken (*g = 7;) generates:
>  adrp x0, g;
>  ldg x0, [x0, :lo12:g]; // new instruction
>  mov x1, #7;
>  str x1, [x0, :lo12:g];

Does the tagging of these hidden symbols only protect against RW
primitives without a similar ldg? If I knew the address of the hidden
symbol I could presumably use the same sequence, but I think I'm
stretching what memory tagging is supposed to protect against. Mostly
wanted to check I understood.

Speaking of understanding...

> Introduce a TAGGED_RELATIVE relocation - in order to solve the problem where the tag derivation shouldn't be from the relocation result, e.g.
> static int array[16] = {};
> // array_end must have the same tag as array[]. array_end is out of
> // bounds w.r.t. array, and may point to a completely different global.
> int *array_end = &array[16];

Does this mean that the value of array_end must have the same tag as
array[]. Then &array_end would have a different tag since it's a
different global?

For example you might assign tag 1 to array, then tag 2 to array_end.
Which means that array_end has a tag of 2 and so does array[16].
(assuming they're sequential)
|            array            | array_end/array[16] |
| < 1> <1> <1> <1>  |            <2>               |

So if we just did a RELATIVE relocation then array_end's value would
have a tag of 2, so you couldn't do:
for (int* ptr=array; ptr != array_end; ++ptr)
Since it's always != due to the tags.

Do I have that right?

Also, if you have this same example but the array got rounded up to
the nearest granule e.g. (4 byte ints, 16 byte granules)
int array[3]; // rounded up to be array[4]
int* array_end = array[3];

Would you emit a normal RELATIVE relocation for array_end, because
it's within the bounds of the rounded up array. Or a TAGGED_RELATIVE
relocation because it's out of bounds of the original size of the
array?
(I don't think doing the former is a problem but I'm not a linker expert)

Thanks,
David Spickett.

On Thu, 17 Sep 2020 at 23:05, Mitch Phillips via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi folks,
>
>
> ARM v8.5 introduces the Memory Tagging Extension (MTE), a hardware that allows for detection of memory safety bugs (buffer overflows, use-after-free, etc) with low overhead. So far, MTE support is implemented in the Scudo hardened allocator (compiler-rt/lib/scudo/standalone) for heap, and stack allocation is implemented in LLVM/Clang behind -fsanitize=memtag.
>
>
> As part of a holistic MTE implementation, global memory should also be properly tagged. HWASan (a software-only implementation of MTE) has a schema that uses static tags, however these can be trivially determined by an attacker with access to the ELF file. This would allow attackers with arbitrary read/write to trivially attack global variables. It would also allow attackers with a semilinear RW primitive to trivially attack global variables if the offset is controllable. Dynamic global tags are required to provide the same MTE mitigation guarantees that are afforded to stack and heap memory.
>
>
> We've got a plan in mind about how to do MTE globals with fully dynamic tags, but we'd love to get feedback from the community. In particular - we'd like to try and align implementation details with GCC as the scheme requires cooperation from the compiler, linker, and loader.
>
>
> Our current ideas are outlined below. All the compiler features (including realignment, etc.) would be guarded behind -fsanitize=memtag. Protection of read-only globals would be enabled-by-default, but can be disabled at compile time behind a flag (likely -f(no)sanitize-memtag-ro-globals).
>
>
> a) Dynamic symbols (int f; extern int f;)
>
> Mark all tagged global data symbols in the dynamic symbol table as st_other.STO_TAGGED.
>
> Teach the loader to read the symbol table at load time (and dlopen()) prior to relocations, and apply random memory tags (via. `irg -> stg`) to each STO_TAGGED carrying global.
>
> b) Hidden Symbols (static int g; or -fvisibility=hidden)
>
> Have the compiler mark hidden tagged globals in the symbol table as st_other.STO_TAGGED.
>
> Have the linker read the symbol table and create a table of { unrelocated virtual address, size } pairs for each STO_TAGGED carrying hidden global, storing this in a new section (.mteglobtab).
>
> Create a new dynamic entry "DT_MTEGLOBTAB" that points to this segment, along with "DT_MTEGLOBENT" for the size of each entry and "DT_MTEGLOBSZ" for the size (in bytes) of the table.
>
> Similar to dynamic symbols, teach the loader to read this table and apply random memory tags to each global prior to relocations.
>
> Materialization of hidden symbols now fetch and insert the memory tag via. `ldg`. On aarch64, this means non PC-relative loads/stores/address-taken (*g = 7;) generates:
>   adrp x0, g;
>   ldg x0, [x0, :lo12:g]; // new instruction
>   mov x1, #7;
>   str x1, [x0, :lo12:g];
>
> Note that this materialization sequence means that executables built with MTE globals are not able to run on non-MTE hardware.
>
> Note: Some dynamic symbols can be transformed at link time into hidden symbols if:
>
> The symbol is in an object file that is statically linked into an executable and is not referenced in any shared libraries, or
>
> The symbol has its visibility changed with a version script.
>
> These globals always have their addresses derived from a GOT entry, and thus have their address tag materialized through the RELATIVE relocation of the GOT entry. Due to the lack of dynamic symbol table entry however, the memory would go untagged. The linker must ensure it creates an MTEGLOBTAB entry for all hidden MTE-globals, including those that are transformed from external to hidden. DSO's linked with -Bsymbolic retain their dynamic symbol table entries, and thus require no special handling.
>
>
> c) All symbols
>
> Realign to granule size (16 bytes), resize to multiple of granule size (e.g. 40B -> 48B).
>
> Ban data folding (except where contents and size are same, no tail merging).
>
> In the loader, ensure writable segments (and possibly .rodata, see next dot point) are mapped MAP_ANONYMOUS and PROT_MTE (with the contents of the mappings filled from the file), as file-based mappings aren't necessarily backed by tag-capable memory. It also requires in-place remapping of data segments from the program image (as they're already mapped by the kernel before PT_INTERP invokes the loader).
>
> Make .rodata protection optional. When read-only protection is in use, the .rodata section should be moved into a separate segment. For Bionic libc, the rodata section takes up 20% of its ALLOC | READ segment, and we'd like to be able to maintain page sharing for the remaining 189KiB of other read-only data in this segment.
>
> d) Relocations
>
> GLOB_DAT, ABS64, and RELATIVE relocations change semantics - they would be required to retrieve and insert the memory tag of the symbol into the relocated value. For example, the ABS64 relocation becomes:
>   sym_addr = get_symbol_address() // sym_addr = 0x1008
>   sym_addr |= get_tag(sym_addr & 0xf) // get_tag(0x1008 & 0xf == 0x1000)
>   *r_offset = sym_addr + r_addend;
>
> Introduce a TAGGED_RELATIVE relocation - in order to solve the problem where the tag derivation shouldn't be from the relocation result, e.g.
> static int array[16] = {};
> // array_end must have the same tag as array[]. array_end is out of
> // bounds w.r.t. array, and may point to a completely different global.
> int *array_end = &array[16];
>
> TAGGED_RELATIVE stores the untagged symbol value in the place (*r_offset == &array[16]), and keeps the address where the tag should be derived in the addend (RELA-only r_addend == &array[0]).
>
> For derived symbols where the granule-aligned address is in-bounds of the tag (e.g. array_end = &array[7] implies the tag can be derived from (&array[0] & 0xf)), we can use a normal RELATIVE relocation.
>
> The TAGGED_RELATIVE operand looks like:
>   *r_offset |= get_tag(r_addend & ~0xf);
>
> ABS64, RELATIVE, and TAGGED_RELATIVE relocations need a slight tweak to grab the place's memory tag before use, as the place itself may be tagged. So, for example, the TAGGED_RELATIVE operation above actually becomes:
>   r_offset = ldg(r_offset);
>   *r_offset |= get_tag(r_addend & ~0xf);
>
> Introduce an R_AARCH64_LDG_LO9_SHORT_NC relocation for relocating the 9-bit immediate for the LDG instruction. This isn't MTE-globals specific, we just seem to be missing the relocation to encode the 9-bit immediate for LDG at bits [12..20]. This would save us an additional ADD instruction in the inline-LDG sequence for hidden symbols.
>
> We considered a few other schemes, including:
>
> Creating a dynamic symbol table entry for all hidden globals and giving them the same st_other.STO_TAGGED treatment. These entries would not require symbol names, but Elf(Sym) entries are 24 bytes (in comparison to 8 bytes for the MTEGLOBTAB schema under the small code model). For an AOSP build, using dynamic symbol entries instead of MTEGLOBTAB results in a 2.3MiB code size increase across all DSO's.
>
> Making all hidden symbol accesses go through a local-GOT. Requires an extra indirection for all local symbols - resulting in increased cache pressure (and thus decreased performance) over a simple `ldg` of the tag (as the dcache and tag-cache are going to be warmed anyway for the load/store). Unlike the MTEGLOBTAG scheme however, this scheme is backwards compatible, allowing MTE-globals built binaries to run on old ARM64 hardware (as no incompatible instructions are emitted), the same as heap tagging. Stack tagging requires a new ABI - and we expect the MTE globals scheme to be enabled in partnership with stack tagging, thus we are unconcerned about the ABI requirement for the MTEGLOBTAG scheme.
>
>
> Please let us know any feedback you have. We're currently working on an experimental version and will update with any more details as they arise.
>
>
> Thanks,
>
> Mitch.
>
>
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