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.w...@gmail.com>
Signed-off-by: Xi Wang <xi.w...@gmail.com>
Signed-off-by: Luke Nelson <luke.r.n...@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
