Re: [RFC PATCH bpf-next 1/3] arm64: insn: Fix two bugs in encoding 32-bit logical immediates

[Will Deacon]

Hi Luke,

Thanks for the patches.

On Wed, May 06, 2020 at 06:05:01PM -0700, Luke Nelson 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.
> 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.
> 
> 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.

Ah, so this isn't a fix or a bpf patch ;)

I can queue it via arm64 for 5.8, along with the bpf patches since there
are some other small changes pending in the arm64 bpf backend for BTI.

> 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.
> 
> 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)

It's a bit grotty spreading the checks out now. How about we tweak things
slightly along the lines of:


diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
index 4a9e773a177f..60ec788eaf33 100644
--- a/arch/arm64/kernel/insn.c
+++ b/arch/arm64/kernel/insn.c
@@ -1535,16 +1535,10 @@ 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:
-		if (upper_32_bits(imm))
-			return AARCH64_BREAK_FAULT;
 		esz = 32;
 		break;
 	case AARCH64_INSN_VARIANT_64BIT:
@@ -1556,6 +1550,12 @@ static u32 aarch64_encode_immediate(u64 imm,
 		return AARCH64_BREAK_FAULT;
 	}
 
+	mask = GENMASK(esz - 1, 0);
+
+	/* Can't encode full zeroes or full ones */
+	if (imm & ~mask || !imm || imm == mask)
+		return AARCH64_BREAK_FAULT;
+
 	/*
 	 * Inverse of Replicate(). Try to spot a repeating pattern
 	 * with a pow2 stride.


What do you think?

Will