Hi Luke,
Thanks a lot for nailing these bugs.
On Wed, 6 May 2020 18:05:01 -0700
Luke Nelson <lukenels@xxxxxxxxxxxxxxxxx> wrote:
> This patch fixes two issues present in the current function for encoding
> arm64 logical immediates when using the 32-bit variants of instructions.
>
> First, the code does not correctly reject an all-ones 32-bit immediate
> and returns an undefined instruction encoding, which can crash the kernel.
You make it sound more dramatic than it needs to be! ;-) As you pointed
out below, nothing in the kernel calls this code to encode a 32bit
immediate, so triggering a crash is not possible (unless you manage to
exploit something else to call into this code). It definitely needs
fixing though!
> The fix is to add a check for this case.
>
> Second, the code incorrectly rejects some 32-bit immediates that are
> actually encodable as logical immediates. The root cause is that the code
> uses a default mask of 64-bit all-ones, even for 32-bit immediates. This
> causes an issue later on when the mask is used to fill the top bits of
> the immediate with ones, shown here:
>
> /*
> * Pattern: 0..01..10..01..1
> *
> * Fill the unused top bits with ones, and check if
> * the result is a valid immediate (all ones with a
> * contiguous ranges of zeroes).
> */
> imm |= ~mask;
> if (!range_of_ones(~imm))
> return AARCH64_BREAK_FAULT;
>
> To see the problem, consider an immediate of the form 0..01..10..01..1,
> where the upper 32 bits are zero, such as 0x80000001. The code checks
> if ~(imm | ~mask) contains a range of ones: the incorrect mask yields
> 1..10..01..10..0, which fails the check; the correct mask yields
> 0..01..10..0, which succeeds.
>
> The fix is to use a 32-bit all-ones default mask for 32-bit immediates.
Paging this thing back in is really hard (I only had one coffee, more
needed). Yes, I see what you mean. Duh! I think this only happens if
mask hasn't been adjusted by the "pattern spotting" code the first place
though.
>
> Currently, the only user of this function is in
> arch/arm64/kvm/va_layout.c, which uses 64-bit immediates and won't
> trigger these bugs.
>
> We tested the new code against llvm-mc with all 1,302 encodable 32-bit
> logical immediates and all 5,334 encodable 64-bit logical immediates.
That, on its own, is awesome information. Do you have any pointer on
how to set this up?
>
> Fixes: ef3935eeebff ("arm64: insn: Add encoder for bitwise operations using literals")
> Co-developed-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>
> ---
> arch/arm64/kernel/insn.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
> index 4a9e773a177f..42fad79546bb 100644
> --- a/arch/arm64/kernel/insn.c
> +++ b/arch/arm64/kernel/insn.c
> @@ -1535,7 +1535,7 @@ static u32 aarch64_encode_immediate(u64 imm,
> u32 insn)
> {
> unsigned int immr, imms, n, ones, ror, esz, tmp;
> - u64 mask = ~0UL;
> + u64 mask;
>
> /* Can't encode full zeroes or full ones */
> if (!imm || !~imm)
> @@ -1543,13 +1543,15 @@ static u32 aarch64_encode_immediate(u64 imm,
>
> switch (variant) {
> case AARCH64_INSN_VARIANT_32BIT:
> - if (upper_32_bits(imm))
> + if (upper_32_bits(imm) || imm == 0xffffffffUL)
nit: I don't like the fact that this create a small dissymmetry in the
way we check things (we start by checking !~imm, which is not relevant
to 32bit constants).
> return AARCH64_BREAK_FAULT;
> esz = 32;
> + mask = 0xffffffffUL;
> break;
> case AARCH64_INSN_VARIANT_64BIT:
> insn |= AARCH64_INSN_SF_BIT;
> esz = 64;
> + mask = ~0UL;
I'd rather we generate the mask in a programmatic way, which is pretty
easy to do since we have the initial element size.
> break;
> default:
> pr_err("%s: unknown variant encoding %d\n", __func__, variant);
To account for the above remarks, I came up with the following patch:
diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
index 4a9e773a177f..422bf9a79ed6 100644
--- a/arch/arm64/kernel/insn.c
+++ b/arch/arm64/kernel/insn.c
@@ -1535,11 +1535,7 @@ static u32 aarch64_encode_immediate(u64 imm,
u32 insn)
{
unsigned int immr, imms, n, ones, ror, esz, tmp;
- u64 mask = ~0UL;
-
- /* Can't encode full zeroes or full ones */
- if (!imm || !~imm)
- return AARCH64_BREAK_FAULT;
+ u64 mask;
switch (variant) {
case AARCH64_INSN_VARIANT_32BIT:
@@ -1556,6 +1552,11 @@ static u32 aarch64_encode_immediate(u64 imm,
return AARCH64_BREAK_FAULT;
}
+ /* Can't encode full zeroes or full ones */
+ mask = GENMASK_ULL(esz - 1, 0);
+ if (!imm || !(~imm & mask))
+ return AARCH64_BREAK_FAULT;
+
/*
* Inverse of Replicate(). Try to spot a repeating pattern
* with a pow2 stride.
which is of course completely untested (it does compile though).
Thoughts?
M.
--
Jazz is not dead. It just smells funny...
