ARM64 Instruction Set – Pointer authentication

Signing a pointer with a key, checking it, and branching or loading through it

Introduction

Pointer authentication stores a Pointer Authentication Code, or PAC, in the unused top bits of a pointer. To sign a pointer, an instruction calculates the PAC from the pointer, a modifier and a secret key, then inserts it. To authenticate, an instruction calculates the PAC again and compares. If they match, the upper bits of the pointer are restored so that it can be used. If not, the outcome depends on the version of the architecture: the pointer is corrupted so that it will fault when used, or the instruction faults at once.

There are five keys. The mnemonic names the key:

  • key A or B for instruction addresses, ia and ib;
  • key A or B for data addresses, da and db;
  • the generic key, ga, used by pacga alone.

A z in the name means the modifier is zero. The usual pattern at the start of a function is to sign the return address in x30, with sp as the modifier, and to authenticate it before returning.

Base instructions of this kind come with FEAT_PAuth. The forms with sppc or 171615 come with FEAT_PAuth_LR and add a second modifier. None of these instructions changes the condition flags.

Some forms, including paciasp and autiasp, are hints that run as a no-operation on processors without the feature. They are listed with the other hints on the System instructions page.

The instructions

Signing

MnemonicSyntaxWhat it doesFlags
pacia, pacibpacia Xd, XnSigns an instruction address in Xd, with the modifier in Xn or sp. Key A or B.–
pacda, pacdbpacda Xd, XnSigns a data address.–
paciza, pacizbpaciza XdSigns an instruction address with a modifier of zero.–
pacdza, pacdzbpacdza XdSigns a data address with a modifier of zero.–
pacgapacga Xd, Xn, XmCalculates a PAC for the 64-bit value in Xn, with the modifier in Xm or sp, using the generic key. The code goes in the top 32 bits of Xd and the low 32 bits are 0.–

Authenticating and stripping

MnemonicSyntaxWhat it doesFlags
autia, autibautia Xd, XnAuthenticates an instruction address in Xd, with the modifier in Xn or sp.–
autda, autdbautda Xd, XnAuthenticates a data address.–
autiza, autizbautiza XdAuthenticates an instruction address with a modifier of zero.–
autdza, autdzbautdza XdAuthenticates a data address with a modifier of zero.–
xpaci, xpacdxpaci XdRemoves a PAC without checking it, for an instruction address or a data address.–

Branches, returns and loads

These authenticate and act in one instruction. The authenticated address is not written back to the register, except for the pre-index form of the loads, and there it is written without the PAC.

MnemonicSyntaxWhat it doesFlags
braa, brabbraa Xn, XmAuthenticates Xn, with the modifier in Xm or sp, then branches to it.–
braaz, brabzbraaz XnThe same with a modifier of zero.–
blraa, blrabblraa Xn, XmAuthenticates Xn, then calls it, setting x30.–
blraaz, blrabzblraaz XnThe same with a modifier of zero.–
retaa, retabretaaAuthenticates x30 with sp as the modifier and returns.–
eretaa, eretaberetaaAuthenticates the address in ELR_ELx with sp as the modifier, restores PSTATE from the SPSR, and returns from the exception. Undefined at EL0.NZCV, restored from the SPSR
ldraa, ldrabldraa Xt, [Xn, #imm]; ldraa Xt, [Xn, #imm]!Authenticates the base register with a modifier of zero, adds the offset, and loads a doubleword.–

Return addresses with a second modifier (FEAT_PAuth_LR)

These use x30 and sp, with the address of the instruction as a second modifier, so that a signature is bound to one place in the code. The immediate forms name a label, and autiasppc and retaasppc take the second modifier from it. The register forms hold the value in Xn. The 171615 forms use x17 for the address and x16 and x15 for the two modifiers.

MnemonicSyntaxWhat it doesFlags
paciasppc, pacibsppcpaciasppcSigns the address in x30 using sp and the program counter as modifiers. It includes an implicit bti.–
pacnbiasppc, pacnbibsppcpacnbiasppcSigns in the same way without the implicit bti, for code that is not a branch target.–
autiasppc, autibsppcautiasppc labelAuthenticates x30 using sp and the address of the label.–
autiasppcr, autibsppcrautiasppcr XnAuthenticates x30 using sp and the value in Xn.–
retaasppc, retabsppcretaasppc labelAuthenticates x30 with sp and the label, then returns.–
retaasppcr, retabsppcrretaasppcr XnAuthenticates x30 with sp and the value in Xn, then returns.–
pacia171615, pacib171615pacia171615Signs x17 using x16 and x15 as modifiers.–
autia171615, autib171615autia171615Authenticates x17 using x16 and x15 as modifiers.–

With FEAT_PAuth_LR, pacm sets PSTATE.PACM. While PSTATE.PACM is 1, some earlier forms such as retaa also take a second modifier, in x16.

Links

Reference