[RFC PATCH bpf-next 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, 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.

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>
---
 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)
 			return AARCH64_BREAK_FAULT;
 		esz = 32;
+		mask = 0xffffffffUL;
 		break;
 	case AARCH64_INSN_VARIANT_64BIT:
 		insn |= AARCH64_INSN_SF_BIT;
 		esz = 64;
+		mask = ~0UL;
 		break;
 	default:
 		pr_err("%s: unknown variant encoding %d\n", __func__, variant);
-- 
2.17.1