Commits on Sep 6, 2024

1. aarch64: enable PAC/BTI …

   Enable Pointer Authentication Codes (PAC) and Branch Target
   Identification (BTI) support for ARM 64 targets.
   
   PAC works by signing the LR with either an A key or B key and verifying
   the return address. Since the assembly code does not push and pop the
   link register to the stack, and it remains in the register file, their
   is no need to sign the LR, so PAC is essentially just adding it to the
   GNU notes section for auditing purposes.
   
   BTI works by marking all call and jump positions with bti c and bti
   j instructions. If execution control transfers via an indirect branch
   or call to an instruction other than a BTI instruction, the execution
   is killed via SIGILL.
   
   For BTI to work, all object files linked for a unit of execution,
   whether an executable or a library must have the GNU Notes section of
   the ELF file marked to indicate BTI support. This is so loader/linkers
   can apply the proper permission bits (PROT_BRI) on the memory region.
   
   PAC can also be annotated in the GNU ELF notes section, but it's not
   required for enablement, as interleaved PAC and non-pac code works as
   expected since it's the callee that performs all the checking.
   
   Becuase the aarch64 assembly code does not make use of pushing the LR to
   the stack, only BTI targets were needed to be instrumented and the GNU
   notes section indicating support for BTU. Thus for PAC the only
   requirement was to mark the GNU notes section as supporting PAC.
   
   Testing was done under the following CFLAGS and CXXFLAGS for all
   combinations:
   1. -mbranch-protection=none
   2. -mbranch-protection=standard
   3. -mbranch-protection=pac-ret
   4. -mbranch-protection=pac-ret+b-key
   5. -mbranch-protection=bti
   
   Signed-off-by: Bill Roberts <[email protected]>

   

   billatarm committed Sep 6, 2024
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