[PATCH bpf-next v2 1/3] arm64: insn: Fix two bugs in encoding 32-bit logical immediates

[Luke Nelson <lukenels@cs.washington.edu>]

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.

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 default 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 for both issues is to generate a correct mask based on the
instruction immediate size, and use the mask to check for all-ones,
all-zeroes, and values wider than the mask.

Currently, arch/arm64/kvm/va_layout.c is the only user of this function,
which uses 64-bit immediates and therefore 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.

Fixes: ef3935eeebff ("arm64: insn: Add encoder for bitwise operations using literals")
Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Suggested-by: Will Deacon <will@kernel.org>
---
 arch/arm64/kernel/insn.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
index 4a9e773a177f..cc2f3d901c91 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, full ones, or value wider than the mask */
+	if (!imm || imm == mask || imm & ~mask)
+		return AARCH64_BREAK_FAULT;
+
 	/*
 	 * Inverse of Replicate(). Try to spot a repeating pattern
 	 * with a pow2 stride.
-- 
2.17.1